2011-11-07 tornadic supercell

It seemed that everything happened in Oklahoma on 07 November.  We had flooding rains, a tornado outbreak and a 4.7 magnitude earthquake. The supercell responsible for most of the tornadoes was the highlight of all the activities in Oklahoma that day generating one spectacular tornado after another.  Fortune was on our side that these tornadoes missed towns and damaged only a few buildings.  

The primary tornado tracks associated with the Frederick to Ft. Cobb supercell.  This image doesn't include the more minor tornadoes surrounding the major events.  Image courtesy of NWS Norman.

The setup

The severe weather outbreak was pretty well anticipated by all forecast models days in advance as a large trough amplified in the intermountain west.  For once, the southern Plains was the focus of this trough with broadly diffluent flow aloft transitioning from cyclonic to anticyclonic flow.  This was a pattern that didn't occur too often until this fall.  This long wave trough had more than one short-wave trough pass through.  The last one, in Ontario at 12 UTC, did the classic duty of removing the low-level ridge in the western Gulf so that this wave could quickly surge the moisture northward.

What impressed me about this setup was the lack of an excessive warm layer at 800 - 700 mb.  The 12 UTC 700 mb temperature at Midland was a modest 5 deg C.  Yet there was a steep enough low-level lapse rate to allow for pretty good potential instability, especially considering the deep moisture in the Ft. Worth and OKC soundings.

So it was not surprising to see SPC hit this area pretty hard with their outlooks.  The probabilistic outlooks didn't waver from day 3 till that morning.  This was likely a testament to the high predictability of the short-wave trough as it dove southeastward from the Gulf of Alaska.  By the day 1 outlook, Daphne and I decided we'd go for a storm chase and bring Dylan along so that he could see for himself what tornadoes looked like since he was old enough to remember.  The general target of southwest Oklahoma was a hard target.  But there were details to consider that would help us refine our target.

When we left, we targeted the east end of the field of enhanced cumulus bubbling up between the dryline and a subtle boundary further east where the convective inhibition increased a little, the LCLs were a bit lower and the low-level winds were somewhat backed.  We didn't want to be anywhere near the cold front because any storms forming on it would likely either go elevated as they moved over the cold air or they'd quickly go linear.  The outflow boundary in central OK was a little too far from the most likely initiation east of the dry line.

As we departed we thought we'd have to go to Altus to Quanah.  Considering the expected northeast storm motion, Altus seemed like a good place.

On our way down we could monitor the radar and surface map - something I don't take for granted considering the old days.  Once west of Lawton, we could see that storms were already going up and we were presented with several possibilities.  The more mature ones were well west and in our original target area.  But there was a new patch of convection initiating further east that grabbed my attention  (D below).  These storms were immature but they had the advantage of unimpeded low-level flow with somewhat lower LCLs and subtly more backed flow than the more mature storms near Quanah, Hollis and Altus (A, B, C below).  So we decided to take a chance and set up south of Snyder to take time lapse of the storms approaching.

The Storm

We arrived 1 mi south of Manitou around 2018 UTC (yellow circle below) and decided not to go any further.  Even from our spot, we could see the base of the primary storm while it was still in Texas because the land was so flat and the sky devoid of haze.  The storm already showed a wall cloud near the horizon and it looked very good embedded within a wide updraft base.  Daphne and I have seen a similar look to a pretornadic base from the Briscoe, TX tornadic supercell on 28 March 2007.  That one produced a 30 minute beautifully contrasted tornado.  This situation felt similar to us.  We could've intercepted the storm at closer range but I wanted to take a long duration time lapse from one spot while the storm approached.  At 2030 UTC, we were filming and we could see how fast the storm was turning into a strong supercell with already strong low-level convergence.

Within 15 minutes, the low-level convergence strengthened and now we had visual on a large lowering, half of it extending below the horizon.  But we could see strong rotation already.  At our distance we couldn't verify a tornado though the Frederick 88D showed one that was likely there.

Just a few minutes later, there was no doubt we had a large tornado with half of the funnel cloud below the horizon.  For the past few minutes we were hobbled by forward flank rain and strong easterly winds. I had my SLR tripoded taking 5 second shots while Daphne was running the Camcorder inside the car on a tripod.  Unfortunately we had to run the wipers for the camcorder while I held an umbrella against the wind to keep the SLR dry.  Dylan had been enjoying hanging out by the dirt road outside now had to watch the tornado in the car.

The tornado was approaching and getting somewhat closer when we realized another funnel descended in front and a little to the right.  The second tornado never did wrap around the primary.  Instead it persisted a bit and moved roughly parallel to the big one.  I was thinking at the time that it's behavior was somewhat odd and I never considered it anything but a satellite tornado.  But now in hindsight I should've realized it was an anticyclonic tornado acting as one pole to its cyclonic counterpart.  Neither pole would rotate around each other and a merger would be extremely unlikely.    I certainly didn't complain at the time for this extra benefit.

The rain was subsiding too allowing us even better visibility in this haze-free air.  Even with the rain we could see everything we needed to quite clearly.  This was not hook echo rain that can easily obscure visibility.

By 2100 UTC we realized that this tornado was moving to the left of the parent supercell.  Not only that but we could see a belt of lowered cloud base to the left and much closer to us.  We figured that was going to be mesocyclone #2 at some point but at this time there was no low-level rotation.  Right about this time or perhaps a few minutes before, the tornado struck an Agronomy research house a few miles south of Tipton.  The damage to that site was enough for the NWS to classify this tornado as an EF4.  I'm not surprised considering the strength of the rotation that I saw.

Five minutes later the main tornado kept moving more to the north and we realized that this was as close as we were going to get to it.  I would've liked to have been closer to the tornado but that would've meant losing this awesome vantage point to watch the evolution of the new mesocylone.  And we had a pretty good view of the primary tornado as it reached our latitude anyway.  The new mesocyclone was really starting to come together at this time with some weak rotation visible and now an inflow tail.  Clearly the radar showed stronger convergence than before.

At 2110 UTC we finally saw the writing on the wall that the primary tornado was in its demise stage.  The cloud base around the vortex was scouring away and cloud tags on the outer edge of the tornado were descending.  The tornado was also narrowing.  But the motion was quite violent, perhaps more so than when it was to our west (above).  Meanwhile, the new mesocyclone to our west really ramped up in rotation and even produced a small funnel (visible below).  At this time we were confident about tornadogenisis.

The entire tornado sequence speed up 8X from the car camcorder.


The second mesocyclone did produce a tornado shortly after 2110 UTC just northwest of Manitou, OK.  This tornado became spectacular displaying all sorts of shapes highly visible from just about any direction.  It started out as multi vortex and then consolidated into one cone funnel with a debris ring around its base.  The shot I took below highlighted the prolonged cone shape accompanied by a horizontal eddy that managed to entrain some cloud making for a spectacular scene.

Tornado north of Manitou, OK at 3:21 CST with accompanying horizontal swirl.  Photograph by Jim LaDue.

KFDR reflectivity (left) and velocity (right) for the 0.9 deg elevation scan.  The circles show where the photograph above was taken.

We were surprised to see the second mesocyclone outlasting the Manitou tornado.  But that's what it did as the tornado simply lost most of its funnel revealing an inner thin funnel and then dissipating shortly afterwards.  There was no rope out stage at all.  The same mesocylone became long lasting and produced at least two more tornadoes, one just east of Snyder and then a large tornado that passed through the western Wichita mountains and through  theBlue Canyon Wind Farm to its north.  We were slowed down by an active RFD gust front producing small tornadoes on highway 62 as the Snyder tornado formed to our north and as a result we were on the unfavorable west side of the Wichita mountains tornado as it formed and then crossed Rt 54 and we were limited to low contrast views of that tornado. The mesocyclone and tornado became very large as it entered the mountains but perhaps not with the intensity of the Tipton tornado.  Finally the mesocyclone produced two more tornadoes, the last one being the Ft. Cobb tornado.  

We didn't see the last two tornadoes because Dylan was ready to explore the Wichitas including the Prairie Dog town.  So it was time to shift gears a bit.  I was happy to oblige as my expectations were completely surpassed on this day, especially considering that all these tornadoes occurred in full daylight in November!  

The NSSL rotation tracks composite supports the idea that the second mesocyclone lasted from Manitou to near Ft. Cobb.  All of the tornado tracks in this area paralleled the parent storm motion.  Also, none of the tornadoes in this track before Ft. Cobb appeared to go through the classic rope out stage as Tipton did.  Perhaps it is possible that there was a rope out with the short-lived tornado near Alden though I haven't seen an image to show that. In addition, Wichita mountains tornado appeared to end much in the same way as the Manitou tornado.  I believe Dan Dawson's video showed the tornado on its last legs in the Blue Canyon Wind Farm while the broad mesocyclone continued embedded in plenty of updraft.  Back in 03 May 1999, the Chickasha tornado dissipated in much the same way and the parent mesocyclone went on to produce the big Bridge Creek to Moore F5. I really wish we had a VORTEX2 style armada to document these interesting ways that tornadoes dissipate.

NSSL rotation tracks image from 07 November 2011 18-2359 UTC.  Rotation increases as the colors range from yellow to red to white to blue.  The tornado tracks are shown in white and are not mapped precisely.  The blue trail represents our route for the day.  The rotation tracks show the demise of the mesocyclone for the Tipton tornado but the continuous mesocyclone for the next four or five tornadoes. 

November 15 update:  

Owing to an excellent account from Roger Edwards, there appears to be two, possibly three more tornadoes between the demise of the Wichita Mountains tornado and the Ft. Cobb tornado.  In addition, Roger described the handover from one mesocyclone to another two times in this stretch of the supercell's track.  Therefore, I added mesocyclone paths and the tornadoes he described in his summarized spreadsheet.  I also split the mesocyclone into two just east of Snyder on the account of Don Burgess.  None of the tornadoes have appeared to go through the classic rope out stage  in a similar process as the Tipton tornado and parent mesocyclone.

As above but with the addition of tornadoes that Roger Edwards described in his chase account between the Wichita Mountains and Ft. Cobb.

How did the thousand year rain in Washington DC happen?

In an earlier post, I documented an incredible three hour rainfall that fell on Ft. Belvoir, VA on 08 September that had a return interval of 1000 years.  Other places during the same time of day also experienced similar magnitudes.  All of these extreme totals were courtesy of the remains of tropical storm "Lee" as it slowly made its way up the east coast.  These rains occurred on the second day of a two day deluge.  But only during the late afternoon on the 8th did these extremely rare three hour rains occur.   Take a look at two rainfall maps (figure 1).  The rainfall 00 to 06 UTC on 08 September featured a widespread north to south swath with high amounts embedded while the rainfall from 21 to 03 UTC 08 to 09 September was confined to smaller but more intense areas.

Figure 1.  Six hourly rainfall amounts from two times showing the different character of the rainfall.  The latter rainfalls on the right showed two areas that exceeded 1000 year return intervals just south of Washington DC.  This image is courtesy of http://nmq.ou.edu/.

Why did the 1000 year three hour rains occur on the second day?  Both the 7th and the 8th had the moisture,  forcing and the instability to produce heavy rain rates, at least the kind you'd expect from convection in a tropical-like airmass that was brought north by the remnants of Tropical Storm Lee interacting with a frontal zone.  In fact the 7th had even more precipitable water (figure 2) and and low-level horizontal winds to enhance moisture transport (figure 3).  So it's not surprising to see a much larger area of very heavy rainfalls that eventually led to the record flooding amongst the rivers of the Susquehanna river basin.  Grumm (2011) summarized the synoptic conditions that led to this event as being closer to a Maddox type synoptic flooding rain category.  But the 7th didn't produce quite the extreme three hourly rainfall that occurred on the 8th.  Instead these rains occurred on a day with weakened forcing aloft as the upper-level coupled jet structure weakened as the trough west closed off (figure 4) and the low-level jet weakened. With this in mind, we continue on understanding that synoptic-scale forcing cannot provide the full answer.  We have to drill down to the convective scale to explain this.

Figure 2.  Short Range Ensemble Forecasting (SREF) system analysis of precipitable water  and standardized anomalies for 21 UTC 07 Sept on the left column and then 21 UTC 08 Sept on the right columns.  The take home message here shows up in the bottom row where the shaded contours show a smaller area of 2 standard deviations above normal values on the 8th.  Mean precipitable water is also lower on the 8th.

Figure 3.  An SREF analysis of 850 mb mean winds for 21 UTC 07 September  in the left column and one day later in the right column.  The shading in the top row represents the 850 mb U-wind standardized anomalies.  The 850 mb V-wind standardized anomalies show up on the bottom row.  The strong southerly low-level jet on 07 September shows up as nearly four standard deviations above normal over the Mid Atlantic states.

Figure 3.  An SREF analysis of 850 mb mean winds for 21 UTC 07 September  in the left column and one day later in the right column.  The shading in the top row represents the 850 mb U-wind standardized anomalies.  The 850 mb V-wind standardized anomalies show up on the bottom row.  The strong southerly low-level jet on 07 September shows up as nearly four standard deviations above normal over the Mid Atlantic states. 

Figure 4.  Upper-level analysis showing the weakening of the upper-level dynamics from 00 UTC 08 September (left column) to 00 UTC 09 September (right column).

In order to have accumulated the three hourly rainfall with a thousand year return period in the DC area, a convective core dumping  around 2.5"/hr of rain must remain over a specific area for three hours.  This equates to roughly 45-50 dBZ of echo averaged over that three hours.  Either there was an incredible rainfall rate in a shorter period of time, say 6"/hr or there was persistent heavy rain over three hours.  It turns out the latter is true as Ft. Belvoir measured approximately 2.5"/hr for two hours and then almost 2" the final hour (23 - 00 UTC).   The DC area sees that kind of rainfall rate quite frequently when a core of a thunderstorm passes overhead.  But to have that kind of rain rate persist for more than 15 to 20 minutes is unusual and extremely rare for three hours.  

How did the thunderstorms dropping that kind of rainfall rate persist over one area for so long?  Either the convective rain area must be unusually large or was moving slowly.  In the video below two backbuilding multicell thunderstorms showed up, one on the western side of the Washington DC metro area south to Ft. Belvoir and another showed up in southern St. Charles county, MD.   The individual thunderstorm cells moved to the northwest while new cells continually form on the southeastern flanks of the two multicells.  The rate of formation on the southeastern flanks almost exactly matches the individual cell motion leading to anchored multicells (video 1).  These two anchored multicells produced outflows but they were so weak that they only served to sharpen the stationary front and reinforce the mechanism to produce or intensify upwind cells over the same places.  

Video 1.  A loop of radar 0.5 deg reflectivity and base velocity from KLWX 2011-09-08 21 - 23 UTC at 10 min intervals.

This anchored multicell behavior was largely absent on 07 September as the video 2 below highlights.  Instead, the individual multicells moved just a bit east of north along with the individual cells.  Some of the multicells appeared contain a dominant supercell such as the one that passed by Upper Marlboro, MD (east of DC).  Others appeared to be more linear with a long axis parallel to the convective layer steering flow.  But none of the multicells appeared to be anchored as well as the following day.

Video 2.  Same as video 1 except for 2011-09-08 01 - 03 UTC.

Despite the slightly lower precipitable water values of 09 September 00 UTC relative to 24 hours before, the sounding remained nearly saturated for the lowest 500 mb because of cooling above the ground (figure 5).  The nearly saturated atmosphere kept the downdraft production in check and thus the cold pool was too weak to clear the convection away from the western and southern DC area.

Figure 5.  The Sterling, VA sounding for 2011-09-08 - 00UTC and 2011-09-09 00 UTC. 

Both days thus had mechanisms to sustain heavy convection over the same areas for a long duration.  On the 7th to early 8th, the stationary front was parallel to the deep convective steering layer flow (0-6 km mean wind) despite the fact that the flow was strong.  Multiple small multicells and line segments formed in Virginia and traveled up the front eventually congealing into a large rain mass that flooded Pennsylvania (Fig. 6).  No one area sustained extremely heavy rain rates for over an hour but the multiple hits by these convective rains added up to huge totals over periods greater than six hours.  The lack of a significant convectively induced cold pool meant that new convection could could easily exist in the wake of previous convection.  It also meant the cold pool didn't propagate very far away from the stationary front.

Figure 6.  An analysis of key features involved with the heavy rains on September 7th through 06 UTC on the 8th September 2011.  

Late on the 8th, the same stationary front was there in the same general location to provide low-level forcing for new convection.  However, this time, the steering layer flow was much weaker.  Convection initiated on the front, moved to the northwest over the front while new convection would initiate on the front that was slightly enhanced by the weak cold pool (Fig. 7).   The cold pool sometimes surged to the east following a brief intensification of the convection but never very far (Fig. 8).  The forcing remained in the same area but this time the slow storm motion allowed more persistent extreme rains to remain in the same area for a longer period of time than in the previous day allowing for the three hour 1000 year recurrence interval to be achieved.  However the weaker forcing limited aerial extent of those rains.

Figure 7.  Similar to figure 6 except for the afternoon of 08 September 2011.

It certainly helped that the near saturated profile through a deep layer and fairly weak lapse rates fostered the low echo centroid type of convection implying significant warm rain processes aided by some graupel production in the cold layers. There was lightning observed but only occasionally.  This echo structure was observed in the devastating Ft. Collins convective flash flood on 30 July 1997 (Peterson et al. 1999) and the Madison County, VA flood of 27 June 1995 (Pontrelli et al. 1999).  Both of the referenced events occurred as topography helped anchor the main initiation region of the multicells rather than their cold pools.  In this case, it was the stationary front that served as the primary anchor.  Otherwise all cases had weak cold pools that remained in the vicinity of the convection.

Figure 8.  KLWX reflectivity  (upper left) and velocity (lower right) cross section for 2153 UTC 08 September 2011.  

References

Grumm, R. H., 2011:  Heavy rainfall associated with frontal interactions with Tropical Storm Lee 5-8 September 2011, [Available online at http://nws.met.psu.edu/severe/2011/08Sep2011.pdf]

Peterson, W. A., and Coauthors, 1999: Mesoscale and radar observations of the Fort Collins flash flood of 28 July 1997. Bull. Amer. Meteor. Soc.,80,191–216.

Pontrelli, Michael D., George Bryan, J. M. Fritsch, 1999: The Madison County, Virginia, Flash Flood of 27 June 1995. Wea. Forecasting, 14, 384–404.

A thousand year rain in DC?

What an incredible rainfall event over the MidAtlantic states!  The remnants of tropical storm "Lee" nearly matched the ultimate rainfall producer in the 20th Century in these parts called "Agnes" in 1972.  Rainfall exceeded 20" in a few places in southern St. Charles county in Maryland while amounts exceeding 15" were reported near Harrisburg, PA and over a foot was widespread from Virginia north into New York.  Binghamton, NY actually exceeded 10" and broke their all time rainfall record for a single day. The Hydrometeorological Prediction Center has a nice compilation of the huge rain totals from "Lee".


Many areas have likely gotten rainfall that they haven't seen in decades including the Washington DC area.  What is really amazing is that a few sites saw three to six hour rainfalls that exceeding even a 1000 year return interval.  Even by tropical cyclone standards this is a pretty incredible total.

One case in point is the rainfall observed at Ft. Belvoir, VA.   This site was nearly in the middle of a huge rainfall maximum of 9 - 12" that mostly accumulated on September 8, or the second day of heavy rain in the area (figure 1).  A majority of this rain fell during the afternoon of the 8th as thunderstorms erupted in a north to south axis right across the western half of the Washington DC metro area.

Figure 1.  12 hour gauge bias-corrected radar-estimated rainfall centered over Washington DC and ending at 09 September 2011 01 UTC.   The arrow points to Ft. Belvoir (KDAA).  This data is available at http://www.srh.noaa.gov/ridge2/RFC_Precip/.

What I am really impressed with is the intensity of this deluge.  Let's take the same type of graphic above  but for the three hour rainfall (figure 2).  The rainfall exceeded 7" along I-95 south of the Beltway while more than 5" of rain extended up toward Arlington, VA.  No doubt this was big rainfall and the flash flooding impacts were huge namely because of the dense population.  

Figure 2.  Similar to figure 1 except for three hour rainfall ending at 01 UTC.

Consider the rainfall accumulation at Ft. Belvoir (KDAA) as it compares to the expected time interval in which these kinds of rains are expected to occur.  This is near the epicenter of the heaviest rainfall (figure 3).  The measured hourly rainfall was quite heavy, peaking at 2.65" ending at 22 UTC but this kind of rainfall can be expected to occur once every 20 - 25 years.  It's rare but nowhere near as rare as seeing this kind of rainfall rate persist past one hour to three hours!  But that's what happened at Ft. Belvoir.  They saw greater than 2"/hour rainfall rates last three hours yielding over seven inches!  That's where we see that rainfall entering into the 1000 year return interval.   Finally the thunderstorms let up and the rainfall rates diminished.  Notice that the 12 hour rainfall of 7.9" is around a once in 200 - 300 year event and the 24 hour total of 9.3" is expected to occur once in perhaps 150 years.

Figure 3.  A plot of rainfall vs period of rainfall accumulation for Ft. Belvoir, VA (black trace).   In addition,  this plot shows the maximum expected rainfall as a function of time for several return periods ranging from one year to 1000 years.  Notice that the three hour rainfall at Ft. Belvoir exceeded the 1000 year return period!  Rain return intervals acquired from NWS Hydrological Design Studies Center.

Consider that the nature of the land becomes accustomed to a typical rainfall pattern where the return intervals are fairly small.  By saying accustomed, I'm implying that the nature of the stream banks are shaped by frequent periods of rainfall and subsequent runoffs.  The soil accumulates in a certain way in response to normal rainfall and runoff too.  The vegetation grows in a similar way from the grass to the trees that line stream banks and other low spots.  The root systems grab on to the soil only as tightly as needed to hold on in normal rainfall and runoff patterns.  And we build our infrastructure in response to what we consider the normal range of events from the height of bridges to where we consider the boundaries of a flood zone.  Certainly in the age where we build cheaply, we're even more dependent on staying within what we consider normal events.

So can you imagine what happens in a once in a thousand year rainfall event?  Perhaps these pictures compiled from the Capital Weather Gang may provide some justice to the impacts.  Needless to say the impacts were big in a negative way with multiple high water rescues, washed out roads, flooded buildings, and transportation halted in general.   Numerous school districts were closed the following days.  Residents in some areas were isolated due to damaged roads.  And there were four fatalities.

So in summary, it's not just the rainfall amounts but also the rarity of these events that's important.   Even though a thousand year rainfall event is basically a statistically-based extrapolation, the point is made that we should expect major societal impacts when the event is rare.  The three hour rain that fell in Ft. Belvoir would even match a 100 year event along the upper Gulf Coast of the US according to the Rainfall Frequency Atlas.

That's all for now, however I'm going to look at what it takes to get a three hour rainfall of that magnitude from a meteorological point of view.

Fires and intense updrafts in east Norman

While working on a winter weather course, a colleague of mine shouted about a fire visible to the east of the National Weather Center.  Naturally this was something I had to see. This was the second time a major fire was visible from the Center, the last one being a major apartment complex fire a mile to the east.  Upon reaching the observation deck, I was treated to an impressive column of smoke to the southeast that appeared extend almost overhead.  

The smoke column extending upward from the Noble fire around 2:30 pm CDT.

Flames were easily visible at the bottom of the column as you can see from the video below.  Some of these flames were most likely over 100' tall as numerous dry Eastern Red Cedars caught fire.  Columns of darker smoke erupted as trees or groves of Cedars ignited.

This fire wasn't just one of those single acre type grass fires that we see on roadsides though it certainly started small.  No, the potential of this fire to go out of control and consume thousands of acres was real.  We were already in an exceptional long period drought and the Oklahoma Mesonet Fire danger model was showing very high values throughout eastern Cleveland county up to the edge of the Cross-Timber woods.  Groves of dense Eastern Red Cedar mixed with completely dormant grass fields combined to make a volatile combination.

The Oklahoma Fire Danger Model explained here.  The arrow points to the Noble fire location.  Notice the sharp gradient to lower values (more containable fires) in eastern Cleveland County within the Cross Timbers.

The weather was typical for this summer of record breaking heat and dryness.  Temperatures were in the low 100's F and relative humidities were in the low 20 percentage range and south winds gusting over 20 kts.  SPC already issued a nearly critical fire danger outlook and the state of Oklahoma issued a burn ban. During the early afternoon the expected conditions materialized.

The SPC day 1 fire outlook.  Central Oklahoma's on the edge of a critical risk.

The fire danger was certainly high but not unprecedented in Oklahoma.  During the winter time we'll frequently see these conditions appear and we'll get rapidly moving fires.  But what set these summer conditions apart from the winter was the huge depth of the nearly dry adiabatic lapse rates, on this day, almost 3 km.

The 00 UTC Norman sounding shortly after the fire was put out.

These conditions we had were typically found in the western mountains where the airmass was of classic desert continental tropical origin, sometimes advected eastward driven by strong synoptic forcing into the Plains States following the dryline.  But now they were prevalent all throughout the southern Plains as we've become the new source of the continental tropical airmass through months of uninterrupted baking of the ground.  Time was the only factor needed for the continuous baking to cure huge swaths of vegetation into a combustable fuel which now included even parts of the Cross-Timbers.

Could fire tornadoes or even a firestorm have developed?

Anybody unfortunate to be ahead of a big fire would be justified in calling out that they experienced a firestorm.  However I hear amongst fire specialists that they refer to a firestorm to describe a certain extreme behavior.  That is a firestorm generates a velocity structure that helps it to intensify in a positive feedback loop.  Enhanced inflow at low-levels feeds the fire Oxygen from ahead and the lateral flanks.  Vortices forming along the flanks also help to concentrate the heat at the head (downwind) end of the fire.  The updraft plume is typically deep and separated from the ground in a single column consisting of short-period pulses.  This updraft structure is effective at not just generating the internal fire-induced circulations but also launching embers in typically erratic directions but often well ahead of the fire.  These types of fires are said to be plume dominated as opposed to the wind-dominated fires that we see in central Oklahoma during a pre-dryline strong south wind events of the cool season.  Wind dominated updraft plumes tend to hug the ground.

The picture below shows the type of fire plume so reminiscent of an incipient firestorm we've heard and seen further west.  It erupted immediately off the ground in an updraft column that was roughly uninterrupted until it reached its LCL at nearly 3 km AGL as shown by the sounding above.  Fire induced updrafts often contain a bit more water vapor than implied by the sounding due to the combusted fuels and so the LCL may have been a bit lower.  Nevertheless, this 300 acre fire was capable of producing an upright updraft 3 km deep.  The video above even shows some anticylonic vorticity within the west side of the broad updraft as it interacted with the environmental shear just above ground.  I wouldn't be surprised if this fire modified the low-level flow creating a calm wake to the north and accelerated flow around the west and east flanks creating a low-level broad vortex pair as it was tilted by the updraft in a similar way to one mechanism by which a supercell forms.

A vertical fire induced updraft with pyrocumulus at the top.  

A little more intensity of the fire and we may have had significant fire vortices erupt out of the flanks of the fire to propagate downwind.  I may exaggerate but compare the pictures and video above and the time lapse by John Hart with the fire vortex simulations available on the Visualization and Enabling Technologies webpage of NCAR.  I chose one to highlight below that shows a fire with surface wind vectors, three-dimensional heating and vorticity.  The similarities are pretty striking.  

These vortices would be large, not the small ones sometimes visible within small flames.  For an example, see the fire induced vortex from a forest fire in eastern Colorado here.  The main fire updraft plume is to the left of the vortex.  However the vortex connects with the main plume aloft.  A very dynamically similar analog occurs with heated water induced updraft plume around the entry point of lava into the Pacific Ocean in the big island of Hawaii.  There are multiple videos online showing this such as here, here, and here.  Perhaps the intensity of the Noble fire is far short of that of the lava-induced plumes or the great fires out west but both heat sources had erect plumes separating from the ground.  

Were there other plume-driven fires during this outbreak that could've been better candidates to produce a firestorm with large fire vortices?  As it turned out, there was one candidate to the southwest of us in the Wichita Mountains.  The Meers, OK fire as it was called, started about the same time as the Noble fire and both plumes can be seen in the GOES-E visible imagery at 1930 UTC.  The Noble fire was at its peak and the Meers fire was just getting started perhaps both having burned similar areas.

The GOES-E visible imagery at 1930 UTC captured from the NCAR RAP website showing two fire-induced smoke plumes, one near Noble (upper right arrow) and another near Meers, OK (lower left arrow).

While the Noble fire was aggressively suppressed, the remote rough terrain around the Meers, OK fire possibly inhibited access by firefighters and the so the fire grew.  By 2302 UTC, the GOES visible imagery showed a classic wedge-shaped anvil with a singular strong updraft plume anchored on the south side.  There was even a hint of an overshooting top.  The size of this fire surely put the Noble fire to shame and yet the Noble fire also exhibited a similarly shaped smoke plume suggesting a plume-driven fire. Even more striking was the obvious anvil-layer divergence signature at the top of the fire.

The GOES-E visible image from 2302 UTC showing the anvil and intense updraft plume from the Meers, OK fire.  Two other fires to the south in TX exhibited much weaker updraft behavior.

The KTLX 2250 UTC 0.5 deg scan of the Meers, OK fire.  On the left is the reflectivity and the right, radial base velocity.  The green and red arrows highlights where you can see the anvil-layer divergence over the top of the smoke plume.

The Meers, OK fire grew to over 20,000 acres as opposed to the 380 acre spread of the Noble fire as reported by news9.com.  At any one time the Meers, OK fire was likely much more intense too.  The radar data from the Noble fire did not indicate this kind of diverging anvil and its updraft was much weaker.  So if the Noble fire plume exhibited signs of a vortex pair, I would expect that the Meers, OK fire was much more capable of producing intense vortices.  However no radar could adequately sample the lower levels of the plume where the vortices would most likely reside.

Fire intensity and Cedars.

How did the ground conditions affect the fire northeast of the Noble Highschool?  An overhead view of the area near the fire initiation pretty much shows patches of dense Eastern Red Cedars.  These trees were so close together that their crowns were touching.  Considering how dry we've been, the fuel moisture level must've been incredibly low in these trees.  But these trees can become torches even without such dry conditions.  I also saw numerous trees whether or not they were burned, that showed branches right down next to the ground.  The stage was set where a ground fire (as it started considering that a ditch digger started the fire) could easily have spread into the crowns of the trees.  The video above and from real-time media clearly showed numerous crown fires.  I'm not sure if these crown fires started to spread independently of the ground fire based on what the video above showed.  Considering the sporadic crown eruptions, I suspect the crowns torching in lockstep with the advance of ground fire and not spreading on its own.  Nevertheless, where crown fires existed, the fires were intense and everything quickly burned as the picture below so eloquently shows.

I might add that in addition to the crown fires suggestive of a front, I could see that there were numerous spot fires ahead of the main fire also sending up their own plumes.  A prominent one showed up on the pictures above.  The nature of this plume-dominated fire probably contributed to producing these spot fires.

A Google Earth overhead view of the Noble Fire initiation area (represented by the red swath).  The inset shows a red arrow where the picture below was taken.

A picture of intense burning in Eastern Red Cedars to the east of Noble Highschool.  One of the Cedar trunks showed active flames within a hollow (at center).

Finally, I'm amazed at how fast this fire was quickly brought under control considering the hostile conditions.  There were two firefighters that suffered some burns, and another with a shoulder separation.  In addition, the blackhawk helicopter crew faced a hostile male when they lowered the bucket into a nearby pond.

A Blackhawk firefighting helicopter lifting another bucket of water from a pond to dump on the remains of the Noble fire.

References to read:

Fire whirls and vortices simulation

http://www.vets.ucar.edu/vg/FIRE/shape.shtml

COMET's extreme fire behavior course
http://www.meted.ucar.edu/fire/s290/unit11/
this is part of a larger course on fire weather forecasting.

Oklahoma Fire Danger Model
http://agweather.mesonet.org/models/fire/description.html

WIldland Fire Assessment System
http://okfire.mesonet.org/public/?cat=links

A benefit of probabilistic surge forecasts - an example from Irene's impact on NYC

Here's a good site for surge predictions from SUNY Stonybrook.  The latest forecast for the Battery shows a good surge of 1-1.5 m above normal level in the early morning on Sunday.  You can see this forecast below where the dashed line is the expected forecast within a grey zone that represents the standard deviation of all the surge predictions at the Battery.  The red trace represents what the observed surge happens to be and the differences between observations and forecasts is represented in green.  Notice that the forecasts so far have been underestimating the observed water level most of the time.

The forecast surge height is one type of forecast but that doesn't tell how high the water will be relative to mean sea level.  Afterall, if the surge hits at low tide then the impact isn't as high.  So Stonybrook offers this second forecast timeseries superimposing the forecast tide on top of the surge and below is that forecast.

The timing appears bad if you don't want a flood.  The peak surge that you saw above happens to coincide with the morning high tide at the Battery yielding a water level 7 to 9 feet above mean low water.  This forecast is confirming why the New York City government placed mandatory evacuations of all low lying areas in their Zone A.  To see where Zone A exists, check out the city government site's interactive map at http://project.wnyc.org/news-maps/hurricane-zones/hurricane-zones.html

What is nice about these forecasts from Stonybrook is the grey zone of uncertainty.  It allows us to plan for reasonable worst case scenarios because forecasts have uncertainty.  But is the zone broad enough?  Will there be a surge higher than the grey zone?  The answer is quite possibly yes but the probability may be somewhat low.

Another site is available that shows the probability at which a surge is forecast to exceed some height, and also the surge heights as a function of probability.  I like the latter because it allows me to set my threshold probability for taking action and then I can see if the surge height exceeds my elevation.  Getting the forecast is easy.  Just go to http://www.nhc.noaa.gov/ and then click on the storm (Irene) and storm surge probabilities are listed above.  I choose the probability of a 5' surge and you can see the results below.  The forecast shows a 50% probability of a surge exceeding 5 feet in the New York harbour, even more around Staten Island.

  Of course the problem is how cautious do you want to be?  If you were sitting at 5 feet above the water would you move if the probability of you going under is at least 50%?  That's probably playing it a little too aggressive in my opinion.  Fortunately there's a website related to the one at  the NHC that allows you to dial your own probability.  Go to http://www.weather.gov/mdl/psurge/ and select the drop down menus until you find "20% exceedence height".  Now you see that the surge height forecasts are higher, 5 - 7 feet around the Battery to almost nine feet in western Staten Island.

So if you're a little more cautious, like me, perhaps you would dial in your personal probability threshold at 20% and you'll see that if you're below 5-7 feet above the water, you may want to move.  So to put it all together, here's what I'd do to decide for yourself if you need to move.  Note that I'm assuming you have no outside call for evacuation.

1.  Know my altitude above Mean Low Water
2.  Look at the tide forecasts closest to your site at http://tidesandcurrents.noaa.gov/gmap3/.  Click on the forecast point to get a menu and click on 'Tide Predictions'

3.  Determine the height above mean sea level (set at zero) for a set time.  Here I choose 8 am on Aug 28 and I get 5.5 feet.  That's high tide.  

4.  Then look at the probability of exceedence for at least 20%, preferably 10% from http://www.weather.gov/mdl/psurge.  We saw from above that 5-7 feet, so let's choose 7 feet.  

5.  Then add the value you got from 3 and 4.  In this forecast, 12 feet.

6.  Compare what you got from 5 with your altitude above mean low water.  If what you see from 5 is higher, then you make the decision to move.

Better yet, listen to the authorities because I do not espouse taking these steps above as the way to make a critical lifesaving decision when the authorities have so much more information at their disposal.  NYC produced a really nice evacuation map at http://project.wnyc.org/news-maps/hurricane-zones/hurricane-zones.html


BTW, the surge at the Chesapeake Bay Bridge was about four feet at 6 pm today.  That compared to a 20% exceedence of 5-7 feet.  So yes, you may have stayed dry if you were at least 8 feet above the high tide today which was 3.5 feet.  But why take chances?

The collapse of a stage at the Indiana State Fair 13 Aug 2011

Note:  I've added an addendum at the bottom of this post


Concert stage collapses from sudden wind gusts happen all to frequently and now we hear about another one occurring at the Indiana State Fair last night around 8:50 pm EDT.  The latest one was tragic with the loss of 5 people and multiple injuries, some of them life altering according to the Indianapolis Star.
To understand what happened, at about 8:50 pm, a gust front from a line of thunderstorms struck the stage just to the north of the main grandstand before the Sugarland concert was to begin.
This youtube video shows all the graphic detail of the stage collapse which unfortunately fell beyond the platform and into the VIP seating area full of people.




This tragedy was entirely preventable but a series of missteps aligned to put people in harms way. First, the stage was a 'house of cards', in other words, a flimsy metal scaffolding frame supporting a huge area of fabric facing the wind.   Second, it appears that people in charge of safety at the fair were determining when people should evacuate based on their uninformed interpretation of the meteorological data.  Mike Smith of WeatherData inc. eloquently described the issue with nonexperts serving the role as experts.  Needless to say this night proved how dangerous that can be.  Their hearts were nonetheless in the right direction as they began evacuation procedures.  This leads to a third problem.  Once they initiated evacuation procedures, not everyone was on the same page as the Daily Star reported that the WLHK program director addressed everyone with a mixed message, one that suggested to prepare for evacuation, the second that the show will go on.  Of course the very people closest to the stage would be the same ones least likely to take the first advice and most likely to stay on.

I first explore how strong the winds were likely to have been when the stage collapsed.  Well, that's not an easy question since there are no anemometers around the site.  But there are a few clues leading to a range of likely wind speeds.  One set of clues comes from what was not damaged.  I noticed a flagpole to the right and behind the stage that didn't blow over.  There were even tents nearby that remained.  Assuming a flagpole is similar in strength to a typical powerpole, typically 70 to 80 mph winds are required to initiate damage, sometimes less.  However there was no damage and so it's doubtful the wind exceeded 70-80 mph.  The same could apply to the many light standards around the fairgrounds in the video.  There were also no other reports of significant damage at the fairgrounds, a place full of large open span structures that could easily start to suffer damage at similar wind speeds (this is preliminary of course since I heard there was some minor nondescript damage scattered around the park).  I decided to take a stab at tracking identifiable plumes of dust that speed across in front of the stage platform.  The amount of time some of these plumes tracked across from one end of the platform to the other was about 1.6 to 2.5 seconds.  The stage measured 38 m long according to Google Earth (see figure 1) and that wind speed amounts to roughly 20-25 m/s or about 37-48 kts.

Figure 1.  A satellite view of the Indiana State Fairgrounds courtesy of Google Maps.  The inset shows the grandstand and the stage north of the dirt track.  The length of the stage platform is nearly 38 m and the wind direction was estimated from the WNW.

Could 37-48 kts be realistic?  Well, there is some supporting evidence.  The highest wind speed at the airport was 44 kts at 8:56 pm EDT.  In addition, to the south at the KIND WSR-88D, the highest winds at the radar site only reached to 40-45 kts (figure 2).  

Figure 2,  The KIND four panel display of reflectivity and velocity for 0056 UTC.  The half degree elevation shows up on the top row and the 2.4 deg elevation shows up on the bottom row.

Of course there are some reports that may indicate higher winds occurred in the vicinity.  One report occurred west of downtown about the same time as the stage collapse of a large tree blown down and an estimated wind speed of 70 mph attached.  Perhaps winds to 70 mph were there.  The other wind gust southwest of Indianapolis was considerably higher at a measured 77 mph. Power did go out around the town and by the next afternoon,  and about 1250 customers were still out of power.   Again, I'll wait and see if those winds occurred at the fairgrounds at various times but judging by the video, the wind speeds were considerably less and likely not even 50 kts when the stage collapsed.

figure 3,  A map showing the outline of the severe thunderstorm warning issued by the Indianapolis NWS forecast office, and the three local storm reports received during the course of the warning.

So the stage was set to put people at a high vulnerability level.  Certainly it appears this stage couldn't withstand winds any more than 45 kts and it's likely the stages that failed this year in Tulsa, OK Ottawa, ON, and last year in El Reno, OK also were similarly weak.  Too bad there was no communication as to the vulnerability of these stages from one set of event coordinators to another.

Given this vulnerability and the consequences of its failure,  the onus should be on the event handlers to provide an extra level weather awareness.  Unfortunately there was not since it appeared that the handlers were interpreting something for which they have no expertise.  In addition to in appropriate interpretation, the Indianapolis Star also reported that there was confusion as to when the threat would arrive.  The event handlers that Mike Smith mentioned,  waited till 8:45 pm to consider evacuation plans, four minutes before the stage collapse.  In addition, the Indianapolis Star quoted "According to a timeline issued today by Indiana State Police, at 8:49 p.m. -- about 25 minutes before the storm's forecasted arrival -- a strong gust of wind blew through the fairgrounds, toppling the stage setup onto the those closest to the stage.  Bursten said the early indication was that the "isolated significant wind gust" took authorities and event coordinators by surprise, since the storm itself was still about 30 minutes from arriving. They had been in contact with the National Weather Service for much of the evening." So it also appears that they were expecting a later arrival than what happened.   It's difficult to determine what the nature of state fair official's contact with the NWS had been but there is some evidence that there was some expectation that they had perhaps 25 minutes to prepare for severe weather.  What was the source of this misunderstanding?  

The NWS did issue a severe thunderstorm warning that covered the state fairgrounds and all of Indianapolis from when it was issued at 8:39 pm EDT.  As shown by the text below, the severe thunderstorm warning was issued as shown where an initial location of the threat was labeled and the entire area was covered in a polygon (figure 3 and 4).

AT 835 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
  LINE OF SEVERE THUNDERSTORMS CAPABLE OF PRODUCING QUARTER SIZE
  HAIL...AND DAMAGING WINDS IN EXCESS OF 60 MPH.  THESE STORMS WERE
  LOCATED ALONG A LINE EXTENDING FROM 9 MILES NORTH OF ZIONSVILLE TO
  GREENCASTLE...AND MOVING EAST AT 25 MPH.

 The NWS marked the line of storms given the town landmarks from nine miles north of Zionsville to Greencastle (figure 4).  However, the text of the warning did not include the position of the gust front which was considerably further east.  

Figure 4 A map depicting the location of the severe thunderstorm warning and the 8:35 pm reflectivity.  Also the convective line (red) mentioned in the text of the warning is shown at the 8:35 pm position mentioned in the 8:39 pm severe thunderstorm warning.  The blue line marks the position of the gust front at 8:35 pm.

This is often the case that NWS meteorologists track the location of the heaviest, most likely severe, portions of the storm (s).  The problem is the way the event handlers may have interpreted the warning, and the location of the threat relative to the confines of the polygon.  The threat that was of the most interest to them should've been the gust front, not necessarily the worst of the severe weather that was located and tracked by the NWS forecasters.  The warning polygon gave the event handlers a lead time of 7-8 minutes before the gust front hit assuming a minute delay from warning issuance to their reception. That's when they should've taken action.  They would've needed all of that time considering that they had poor communication protocol amongst all the players in the warning dissemination, including the WLHK program director.

Large event handlers need a special relationship with expert meteorologists in order to help provide specific warning information.  Canned products are not enough because they don't provide the information that the handlers need nor does it give a tailored product to match the exposure of the event participants.  The severe thunderstorm warning from the NWS was fine but it didn't provide the handlers the timing of the gust front.   The handlers didn't know that they needed the timing of the gust front and so they were confused when the severe weather hit 20 minutes earlier than the actual line of heavy rain which the NWS was tracking.  Lacking knowledge, the handlers should've played it safe and evacuated early.  But the concert-goers, and program directors want to push the envelope and keep running the show till the last minute.  Wouldn't an event handler want a little more detailed information to help resolve these sometimes conflicting needs?

The concert stage was a flimsy structure where upon collapse, proved deadly.  It appears that not even severe thunderstorm threshold winds were needed to collapse the structure.  Thus it's entirely feasible that this tragedy could've happened without a severe thunderstorm warning.  When considering exposure level, everyone has a different threshold at which their safety may be compromised.  Consider figure 5 below that conceptually illustrates the varying thresholds of safety depending on the type of structure or situation for which someone may reside.  Each dot represents a situation where a person's safety is at high risk ranging from being in a small boat when the wind reaches 40 mph (special marine warning) all the way up to being inside a hospital or office building (140 mph).  These values vary from one situation to another but the point is made that each of us has a different exposure level and that official products do not account for this variety.  The event handlers need to know the exposure level of those in their charge and relay that to the meteorological consultants, in this case very low thresholds.  Then the consultants can use that information to make a better warning product.

Figure 5,  A conceptual diagram of exposure level vs. threat severity where personal safety can become compromised.  The threat severity happens to be wind speed.  Several examples are provided and compared relative to the severe thunderstorm wind speed threshold.

Typically when it comes to preserving life, the event handlers can choose to stoke a relationship with the NWS, either by an incident meteorologist (called IMET) physically present, on phone, or on chat.  Or a private sector company can be hired to be the consultant.  Certainly each has its advantages, the former is already paid for while the latter can devote more time and attention.  What should be communicated is what kind of exposure the people are subjected (e.g., under a flimsy stage, outside, in tents, in water), how much time he/she needs for evacuation, and any other special criteria.  The meteorologist can take these constraints and then give a forecast of when the relevant severe weather parameter is expected to strike.  The event handler may ask about the forecaster's confidence, and a reasonable answer should be given that conveys the truth.  Then the even handler can do what he/she has been trained to do.

Let's hope this message goes somewhere.


Addendum:

15 Aug 2011 - Now that a couple days have past, the questions arise as to whether or not the fair directors should've done more to protect the fair patrons.  This article at msnbc summarizes these questions quite nicely.  The questions fall into two camps, one related to who's overseeing the construction standards of temporary buildings, the other about who's responsible for executing a response to the warning of severe weather.  For the latter, I was intrigued to hear that an outdoor concert featuring the Indianapolis Symphony Orchestra at the Conner Prairie Amphitheater was canceled well before the storms hit.  Being that this venue was entirely outdoors with no protection, the concert directors had a plan, a source of expert weather advice, and a conservative attitude.  As a result, there would've been no news about their successful evacuation if it weren't for the tragedy at the fair.

As an aside, I hear some rather intriguing meteorologically-related theories as to why the stage collapsed.  They all have to do with some extreme local wind gust, even called a fluke that nobody could've forseen by the governor Mitch Daniels.   I hope this statement doesn't deflect the need to prevent future tragedies from collapsed stages.  This wind event was not a freak event never to show up again.    What is more likely is that the stage was such a house of cards that it fell where even tents stood less than 50 m away.  I tracked the plumes of dust across the stage platform and came up with rather modest winds of ~ 50 mph (~45 kts).  Perhaps my analysis was wrong and there were higher winds though I doubt I'm far off.   As the story on CBS confirms my thought, there was no freak anomalous wind that struck the stage, what was anomalous was the weakness of the stage.