Sunday, January 22, 2017

Rare and dangerous high risk of tornadoes in GA and FL

I haven't seen tornado outbreak environments like this in some years.  The latest Storm Prediction Center (SPC) outlook still has a high risk for severe storms including long-track significant tornadoes for portions of south Georgia into north Florida.  The last time that a high risk was issued by the SPC was almost three years ago according to Skip Talbot's Facebook post, and possibly no high risks have been forecast into the Florida peninsula.  Now storms are starting to form along and ahead of a cold front in the western FL panhandle and north along the GA, AL border.  Newer storms are firing up along the cold front south into the Gulf.  These should be of interest to anyone concerned about their safety which should include especially the high risk zone.

Later, more isolated storms will fire to the south and threaten the Florida peninsula.  While they may be more isolated, the environment will also support the potential for strong tornadoes.  The risk may not be high for Tampa, Orlando and Melbourne, but if you're unlucky enough to be in the path of a potentially tornadic storm, assume it'll produce significant tornadoes putting you at risk.

Areas north of the high risk may not see an obvious environment supportive of tornadoes because of the widespread rain in southern Georgia.  However this system is unusually far to the south, and our collective experience, limited.  Thus I suspect that even western to central Georgia may see a tornadic threat as the surface low deepens dramatically to something rarely seen in central GA - up to five standard deviations below normal for this time of year.  Outside of hurricanes, the sea level pressures will be very low down into FL as well.  As a result low-level winds will be strong and that means that if you're experiencing a cloudy, cool rainy atmosphere now, that may change quickly to one favorable for severe weather very quickly.   Residents in the Huntsville, AL area on the super tornado outbreak day of 2011 can relate to that.  Temperatures were in the 50's all afternoon and then in the last hour, jumped to near 70 deg F quickly followed by a mile-wide long-tracked tornado.

Furthermore, the probability that any one supercell will produce a significant tornado currently stands in the 15% range according to research by Smith and Thompson and Marsh of SPC in the last few years.  Get used to those numbers being extremely high.  As cases are gathered and return intervals calculated, you may see them as rather unusually high.  More importantly is that these numbers will go up from here as the day progresses.  The key thing to consider is that area hodographs feature large storm-relative helicity, very humid atmosphere (in an absolute and relative sense) and lots of buoyancy for thunderstorms to grow uninhibited, as seen from this sounding from the HRRR in the FL Panhandle ahead of the storms.

Bottom line, if your sheltering location is dusty, or cluttered, clear it now!

Thursday, January 12, 2017

What will roads be like Friday-Saturday central OK?

The next winter storm is upon us one week after the cold snow we experienced.  This time it's ice that's in the forecast and one big question is what the roads will be like.  After a nearly record warm day on Wednesday with temperatures near 80 deg F, the ground temperatures are at least 10 deg F warmer than right before the snow storm and with temperatures expected to drop to just a couple degrees below freezing, it'll be tough to cool the ground surface to below freezing.
But we're talking about freezing rain, right?  It makes all the difference in the world and this time the trend will be to keep ground temperatures warm.  As opposed to already frozen precipitation where upon landing and melting, extracts heat from the ground, rain deposits energy into the ground upon freezing.  If the ground, or the road surface, were to freeze, the energy will have to be extracted by another mechanism.  A continually fresh and deepening source of arctic air could accomplish this task.  However forecasts from all numerical guidance and the NWS paint a scenario where the near surface air barely remains below the melting point throughout Friday and into Saturday morning, early.  This is hardly the needed reservoir of cold required to cool the ground below the melting point in the face of all the latent heat to be added as the rain attempts to freeze.

Consider also that the rain will be falling from a layer nearly at 60 deg F a few thousand feet above ground and you are asking a lot of barely subfreezing air to cool the rain drops while also extracting heat from the ground and successfully fighting off the latent heat added by any attempts at freezing.

All this points to road surfaces remaining wet in central OK throughout the duration of the freezing rain event.

Now the exposure of elevated roads paint a different story.  The reservoir of heat will be eroded from multiple sides, allowing the surface to potentially reach a little below the melting point and allowing the potential for falling rain to freeze.  Bridges and overpasses could become slick if untreated.  But this event is well-forecast and hopefully the OKDOT attacks elevated surfaces before precipitation starts.  Since the NWS forecasts the potential for hazards to occur, they can't depend on knowing for sure what our efforts of mitigation may entail and thus pay heed to these graphics below.

The bottom line is that elevated surfaces may become slick, if untreated in central OK while colder air could be sufficient for all untreated roads in NW OK.

Ice will accumulate on all trees, power lines in central and NW OK.  However NW OK is most likely to bear the brunt of the heaviest rainfall.     Three quarter to one inch accumulation of ice will cause power outages there, and into Kansas and adjacent Missouri.  While this storm will not likely be equivalent to the catastrophic ice events of the past 16 years, it'll be bad enough.

Thursday, January 5, 2017

Fast moving snow system to hit central OK tonight

Our first accumulating snow event of the season is looking more likely tonight.  I've been monitoring the forecasts over the past several days and only by two days ago did the forecasts have enough confidence that snow would indeed happen.  The impacts will be quite clear.  Confidence is high that a period of accumulating snow will occur between midnight and 6 am, with the heaviest rates occurring within a couple hours of 3 am.  The timing and accumulations are pretty well represented by the 09 UTC short range ensemble forecast (SREF) of snow depth, to which the NWS forecast office agrees.  However, the SREF forecasts a large uncertainty in snow fall, anywhere from 0.5 to 6"!  If you're the cautious type, you may want to consider planning for a 4+ inch event with much of it coming down in a hurry just before the morning rush hour begins tomorrow.

But you may want to ask how much of that snow will stick to the roads.  Considering the possibility that no road crews go out tonight to pre-treat the roads with salt, an apt possibility for neighborhoods and minor roads, then all of the falling snow will remain frozen after landing.  We've had nearly 36 continuous hours of subfreezing air, broken only by a few hours of temperatures a little above freezing yesterday afternoon.  The Oklahoma mesonet soil temperatures 2-5 cm below ground in east OKC responded to yesterday's warm temperatures by rising to 40 deg F but since have fallen below 35 deg F this morning.  Other central OK mesonet soil temperature sites have responded similarly.  With plenty of arctic air feeding in from the north, all forecast 2 meter temperatures remain well below freezing, with a falling trend, through tonight and into tomorrow morning.  The ground temperatures should easily fall below freezing before the first snow falls.  Certainly all elevated roads will be even colder.

The impact means all untreated roads will be snow covered by 5 am tomorrow.  These include neighborhood roads and minor streets, or in other words, where most people will attempt to depart.  On the other hand, I would expect main snow routes, highways and bridges to be treated in advance of the snow.  Once they're treated the chemicals should easily melt a 2" snow cover within a couple hours of the snow ending which should be in time for the majority of a morning commute as a majority of forecast guidance anticipates snow tapering off by 6 am.  I would expect an hour or two delay should be sufficient to allow any remaining snow on treated routes to melt.

Now to be cautious, there are a few possibilities that may cause further delays for you.  The temperatures tomorrow will remain well below freezing and so all unplowed snow in shaded roads will remain and there's a good chance enough clouds will remain that even normally sunny road surfaces will stay below freezing.  Another issue could be if the DOT is delayed treating roads until the morning.  You'll know quickly about that if you come upon a normally treated road and it's white.  Or there is a third possibility of snow covered roads, even in treated areas, if the snow fall exceeds forecasts or if the heaviest falling snow gets delayed into the morning commute.

What is the possibility of getting more snow or the heaviest snow occurring after 6 am?  I expect the possibility of both to be there.  To understand why, consider the plots below.  Our event will come from the intersection of an upper level storm system, marked by the 'L' in northern California this morning, and the elevated frontal boundary marking the top of the arctic air dome shown by the strong temperature gradient at the 700 mb level seen below.  This type of upper-level storm provides forcing for rising motion well above the ground usually in the shape of a wave  and the front provides the assist a little lower in the form of bands parallel to the temperature contours.  The relative humidity pattern at the level of the frontal boundary aloft, also at 700 mb, indicates the air is already saturated and likely lifting along bands.   When the upper-level system begins to arrive late tonight one or more bands along the front may intensify and drop heavy snow, heavier than forecast.  The higher resolution models show some indications of bands, however the southern one doesn't last long appears to be more a reflection of the upper-level system.  Yet a stationary band across the metro area is in a realm of possibilties and it could last into the morning commute resulting in plowable snow accumulations and more snow covered main routes.

One final interesting thing to consider with this snow event is whether or not we'll get the beautiful six-sided dendrite crystals.  Consider that to get this kind of snow, we want to see strong rising motion occurring in the -12 to -18 deg C temperature layer aloft.  Consider a forecast sounding below from the morning NAM model.  The cold air at low levels is forecast to become cold enough to generate dendrites but there's not much in the way of rising motion.  However thanks to warm air above the arctic air layer, we extend the optimal temperatures for dendrites into the layer where strong rising motion is forecast to occur.  So with the cold surface temperatures, we should wake up to lots of dendrites.  That's in theory.  In reality, lots of factors may interrupt the processes to create spectacular dendrites but at least the possibility is there.

Thursday, January 21, 2016

If anything, the snow fall could be even bigger in DC

There's nothing more to add from tonight's guidance that wasn't considered in last night's.  The uncertainty remains about the northern edge of the big snows as the SREF suite still insists NYC to Boston will get a major dump while the GFS keeps its southern solution.

The NAM forecast of total snowfall for a 10 to 1 snow ratio from this evening emulates the narrative the earlier SREF depicts of heavy snow fall well into New England.  New York City to Boston would join in on the fun.  Courtesy Pivotal Weather

Meanwhile the GFS denies an epic snow fall for New York City to Boston.  There is no scenario that denies the DC area an epic snowstorm.  Courtesy of Pivotal Weather

The big snow fall rates of 2-4"/hr will happen tomorrow pm in southwest VA, then spread into the DC to Shenandoah Friday night, finally into southeast PA and adjacent NJ Saturday afternoon.  I favor this because it follows the motion of the 700mb WAA pattern, and perhaps the trough of warm air aloft as the upper-level low expands in size.  However there's evidence that the DC area could be in the pivot region allowing more persistent snow well into Saturday night while at the same time the main system shifts more to the east.

Composite reflectivity and precip type from the high resolution NAM valid 03Z Saturday morning.  The heaviest snow rates will be at the latitude of the DC area.

By 15 Z Saturday the heavy band is forecast to shift north into Southern New England to southeast Pennsylvania.  Snowfall remains in the DC area courtesy of the wrapping elevated and saturated warm air advection.

The snow fall rates alone will be sufficient to overwhelm all but the most frequently plowed highways.  Considering the advertised monster snow on a Friday night, I'm anticipating most roads to be clear of traffic allowing whatever plowing planned to be unhindered.  If you are daring to go out, you may be in light company.

I anticipate this snow to be equivalent to what's experienced in a heavy lake Ontario long axis snow band.  I've experienced this and the following is a quick synopsis of what you would experience.  Basically I will assume you have enough ground clearance to handle up to a foot of snow on the highway though any more than that and you may need a hummer.  Expect the <1/4 to 1/16 mile visibility in large dendrites clusters to rimed snowballs to slow you down to the point that snow will quickly clog up your wind shield wipers.  You will need to stop frequently to give them a whack before starting up again.  Clicking on the high beams will give you the feeling of the Millennium Falcon engaging hyperdrive.  No doubt the sensation will be distracting to say the least.

I'm expecting at some point the excessive mess-alpha scale vertical motion field will sufficient vertical instability to generate lightning, momentarily blinding you in a sheet of brilliant orchid purple.  No doubt you'll expect to hear a blast of thunder but with the insulating effect of billions of frozen hydrometeors the sound will be surprisingly muffled.  If there is to be lightning, the snow may wind up falling as soft graupel the size of nickels to quarters.  Such intense convective frozen precipitation has been documented by Picca et al. (2014) in another epic snowband in Long Island. With temperatures approaching the melting point aloft, some of the grapple may appear refrozen, however the convective nature of the heaviest part of the band will prevent pure sleet.  You'd have to be outside the band to see sleet, especially southeast of the district.  One thing that may happen is that the band will be sufficiently electrified to charge the snow flakes and soft graupel to the point that it will stick to everything.  It's quite likely signs will become caked with snow.  Perhaps your car may as well requiring even more frequent wiping with your glove on the highway shoulder, should you find it.  With the bad visibility and snow-caked highway signs, you'd better have a GPS enabled map to make sure you don't miss the exit should you wish to bail.  If you do bail, make sure you've got 4-wheel drive or chains.  There are plenty of hills that will offer up a free uncontrolled ride into something you don't want to hit.

Forecast sounding for Dulles International Airport from the 00 Z 4km NAM (1/22) valid for 09 Z 1/23.  Note layers of conditional instability in the midlevels sufficient for generating lightning perhaps.

One parameter I don't expect will be a serious problem for your Friday night journey will be gobs of blowing snow in the immediate DC area.  The winds will be probably 10-20 mph in tree-covered residential areas, enough for blowing snow to temporarily blind you.  However you should still be able to drive 15 mph or so without needing a spotter ahead of you to let you know where the edge of the highway happens to be.  In serious lake effect bands the winds can be over 40 mph and that's when I've needed a spotter.  By Saturday morning sunrise, the real pressure gradient will kick in and that's when you'll see the mass aerial migration of accumulated snow on trees, rooftops, and clearings.  Serious whiteouts, of the lake effect kind, will commence and your journey will be accompanied by frequent intermissions while you attempt to figure out where the road went.

Ten meter wind forecast from the 00Z 4km NAM valid for 03 Z Saturday morning.  The winds in the DC residential and urban areas will be < 20 mph for the most part.

Ten meter wind forecast from the 00Z 4km NAM valid for 15 Z Saturday morning.  The winds in the DC residential and urban areas will be > 20 mph for the most part.

At this point, you don't want to just drive around a relatively tame, marginal blizzard.  You want to see true gale force winds resulting in blinding amounts of blowing snow.  For that, you'll need to head closer to the coast.  The problem is that most coastal areas will likely experience ice pellets and thus all you'd experience is the sound of high-velocity peppering of BB's against your windshield.  Instead, you may want to head to coastal NJ, preferably the northern half, south of Barnegat Light, to perhaps  Avalon.  This may be the sweet spot where the easterly low-level jet will smack unimpeded into the coast and then some distance inland while intersecting the cold enough temperatures and intense vertical velocity.   This is the area that I'd expect true blizzard conditions.  However, some of that action may get down to the northern Deleware and Chesapeake bays too.  That's good because if you're meandering around the DC area in the snow, you will not make NJ for a loooong time.  

probability of blizzard from the WPC winter storm impact graphics.  Notice that coastal NJ, northern Deleware are really the only places to be hit.  Coastal New England and Long Island may get blizzard but the SREF may be wrong about the northern extent of the snow.
This is probably a good thing anyway because you may wind up being a little too adventurous and decide to drive into the path of boardwalk-breaching waves or perhaps a falling power pole.  The surge will be impressive anywhere along the Deleware to New Jersey coasts.  With the full moon, the morning high tide will send water up to 4 ft above normal water level at that time which will offer the remnants of the 20' waves offshore to nail you.

Waterlevel forecasts (normal tide in solid, forecast water level in dashed and confidence interval shaded) for Barnegat Light, NJ courtesy of SUNY Stonybrook.

Wednesday, January 20, 2016

The oncoming MidAtlantic blizzard still on track and my pick for the most miserable weather

The latest forecasts for the MidAtlantic blizzard are on track for an extremely heavy snowfall event starting in the DC area by afternoon on Friday.   With no melting expected after the snow flakes land on the ground, I'm expecting roads will quickly become snow-covered within an hour of first snow except where roads have been treated.  And I'd expect more roads to be treated than with todays light snow, big impact event.  But look at how quickly the SREF members pile on the snow within a couple hours of onset.  The heaviest rates appear to occur with the strong surge of elevated warm air advection along the front end of this system Friday night.  I don't imagine any road treatment, except for aggressive plowing, will be able to stay ahead of the accumulating snow.  Considering that everyone will be expecting impassible conditions Friday night, I don't expect big traffic jams and numerous trapped people and thus plowing will not be seriously inhibited.

Forecast SREF snowfall accumulation at Washington DC.

At least the winds will not be strong Friday night and visibilities will be restricted mainly by snow to 1/8 mi.  Don't expect them to stay light by Saturday, however.  While snowfall rates will diminish, roads exposed to fields could experience local whiteout conditions.

The 4km NAM does show one area where strong winds may begin earlier - the Shenandoah valley.  Downslope or gap winds could enhance blowing snow and drop visibilities pretty quickly Friday night in some areas.  Would this impact I-81?  It would be interesting to find out.

Forecast 10 m winds from the NAM 4km model.

By Saturday the storm will be at its peak across the DC area and into Philadelphia to the New Jersey Coast.  What I find impressive is the continuing variability in the thickness and precipitation fields from the short range ensemble from late this afternoon.  The precipitation fields are quite varied amongst the SREF members, with the front precip edge ranging from southern to northern Pennsylvania.  Likewise the snowfall accumulation forecasts are equally variable, more so from points along the northern fringe of the expected precipitation.  Just like last year, I'll be thinking of the challenge facing NWS forecasters from State College to New York City when they have to come up with a snowfall forecast.
SREF forecast of 3 hour precipitation and sea level pressure  for Saturday morning from the Wednesday afternoon run.

If I picked a spot where conditions would be most horrid, it may not be with the purely heaviest snowfall rates that may fall from the Shenandoah to northwest Maryland but instead, the northern New Jersey Coast.  This area on Saturday will likely face the highest chance of blizzard conditions while at the same time the NOAA storm surge model depicts water level departures of more than 4 ft and waves over 20 ft high.

Forecast likelihood of blizzard conditions and >2"/hour snowfall rates for Saturday morning.

Storm surge forecast by Saturday evening Jan 24.

Wave height forecasts from the Wavewatch model Saturday night

High tide at Barnegat, NJ Saturday afternoon could feature wave spillover into houses and boardwalks while wet wind driven snow plasters the sides of houses and covers roads with a gluey mess where the surge fails to reach at the same time bringing down power lines.  Doesn't that sound wonderful?

Waterlevel forecast from SUNY Stonybrook for Barnegat, NJ

Friday, April 3, 2015

Moore damage survey of March 25, 2015

The Moore tornado of March 2015 provided a challenge to damage surveyors to derive an accurate rating.  Initially the NWS Norman office gave a preliminary rating of EF1 which led to some heartache by some of the media and yet allowed the survey teams to reassess the tornado strength.  Eventually the members of the four survey teams met to discuss whether or not the tornado should be upgraded.  There were several areas that were under consideration for an upgrade and the teams decided that the damage to several structures was enough to assign the tornado an EF2 rating and others that did not make the cut. This post provides some of the thoughts that went into deciding what to rate the tornado based on these structures.  Before going on, you may wish to visit the NWS Moore tornado page for detail on the tornado path and also this post about the meteorological aspects of the tornado.  I also highly recommend this blog entry by Robin Tanamachi for a well written story of what what it was like to be surveying this tornado.  This blog post is a more detailed perspective from someone who is a meteorologist trying to understand the point of view of an engineer and a forest ecologist when faced with damage to structures and trees.  I try my best but both the engineer and the ecologist have a much more in-depth understanding of the terms I attempt to use and the application to their fields.  You may find some edits to this post once they begin to read it.  :^)

The Moore tornado began at 2335 UTC on March 25 near Southwest 119th St. and May Ave. in southwest Oklahoma City and ending near Northeast 36th Ave and Indian Hills Road.   As the tornado matured it moved east and then took a turn to the southeast.  The track was surveyed by four teams:  The first covered the beginning quarter of the track (Chris Spannagle, Steve Mullens, Alyssa Bates and Matt Taraldsen), The second looking over the wide portion of the track just west of I-35 included Doug Speheger, Tiffany Meyer and Robin Tanamachi, The third covered the area from I-35 and east for three miles involved Dan Dawson, Robert Prentice and I, and then a team of one, Greg Stumpf, surveyed the final few miles of the track.
Figure 1.  A map of the Moore tornado of March 2015 including damage tracks and also a faded image of the rotation tracks where red indicates azimuthal vorticity of .01 /s or greater.   The three boxes represent areas of 

The first area of interest included the only areas of damage determined to be EF2.  These were in the vicinity of the Southgate elementary school area and then a half mile to the southeast.  Both team one and team 2 surveyed these areas but Robert Prentice and I did a more detailed survey of the Southgate elementary school (labeled '2' below).  The area of interest to me was the house with the highest degree of damage found in the entire tornado track and that is labeled '1' in figure 2.
Figure 2.  An intermediate zoomed view of the most intense portion of the tornado with EF scale contours and individual damage points where blue is EF0, green is EF1 and yellow, EF2.

The Southgate school area contained the most intense portion of the tornado with the maximum width and intensity.  Much of the tornado was dominated by a northwesterly inflow jet from the RFD with occasional corner flow vortices.  The strongest of these vortices hit just west of Southgate elementary and then to the southeast.  I’ll focus on the first one that included Southgate and a couple houses to the west (labeled 1 and 2 in figure 3).
Figure 3.  The highest zoomed view of the first area of interest including the EF2 house (1) and the Southgate elementary school (2a-c).  
 The house in the images below lost several walls once the roof was lifted away (figure 4).  According to the teams surveying this house, there were anchor bolts driven every 8’ to a depth of about 2” along the bottom sill plates into the foundation.  Otherwise there were shot nails driven into the foundation.  Not much is known about the roof-to-wall connections, however the house appears in Google’s street view product and some assessment can be made about the shape of the structure (figure 6).  While the house was embedded in shrubs a gable roof can be identified both on the west and south ends.   Since the debris in the picture facing NW appears to have blown eastward, the west gable was likely exposed to the strongest winds.  It is possible the weak links in the house were the gabled west side of the roof and the garage door seen in the second picture (figure 5).  The expected wind speed for collapsed exterior walls was 132 mph but team 2 lowered the wind speed down to the lower bound 113 mph.  Both lie within the EF2 rating.  The Cleveland County Assessors site indicates that the house was built in 1963. 

The next house to the north also lost its roof though it preserved all of its exterior walls.  The surveyors lowered the estimated wind speed to 107 mph, which is EF1, based on their similar appraisal of weaker than typical construction quality.
Not many other DIs exist in close proximity of these two houses, except for other upstream and downstream houses.  The trees in the pictures were located outside the core region of the tornado thus leaving only the telephone poles as potentially representative confirming damage indicators.  All of them were replaced after the tornado, however a piece of one lay against one of the collapsed walls in the first picture (figure 4).  Broken wooden poles also yields an EF2 rating with an expected wind of 118 mph.

Figure 4.  A photograph of the EF2 house with roof gone and some walls collapsed.   
Figure 5.  A photograph of the same house from its west side facing east.  The garage door blown inward was below a gabled roof end.  The house on the left also lost its entire roof but all walls remained standing.
Figure 6.  A Google street view picture of the house from the same view as the first image of the house above.  

 The Southgate elementary school, built in 1963, lost part of its roof on the southwest end of the building.  This image in figure 6 shows some of the debris that landed to the west of the school consisting of insulation boards and roof membrane.  

Figure 6.  The Southgate elementary school viewed from the west (labeled 2a in figure 3).
The southwest side of the school featured a peculiar roof uplift forming an arch, upon which the roof soffit was tilted into the vertical and may have subsequently acted as a vertical load support (figure 7).  Whether this support prevented the collapse of the roof deck is possible though unknown at this point.

Figure 7.  The school viewed from the southwest (labeled 2b in figure 3).
 The interior view of the roof uplift revealed in figure 8 shows an unreinforced CMU block wall.  In this hall the weak link was the grouting one course below what appears to be a very thin bond beam.  In the next room to the east the bond beam separated from the steel web joists.  The interior also shows the soffit that tilted vertically and helped to possibly keep the roof supported through its studs and plywood sheathing.  It’s not known here whether the uplift pressure was from  Bernoulli forces above the roof edge or interior pressure from breached windows in adjacent classrooms.  More likely a combination of the two forces pushed the roof upward.  The roof covering was still intact in this picture and shows insulation boards attached to the web joists.  I was surprised to see no metal roof decking.  Thus I considered this roof to be lower than expected quality for a wind speed estimate.

Figure 8.  An interior view of the uplifted roof over the entrance visible in figure 7.
Two rooms to the northwest a classroom lost all the roof decking, most likely contributing to the debris field to the west of the school.  No metal roof decking was visible in the debris field (figure 9).  Though here the bond beam appears to be a metal girder, stronger than the masonry bond beam further southeast, I considered this roof to be lower than expected quality here as well due to the lack of significant roof decking.  I rated this portion of the school as uplift of roof decking and significant loss of roof covering (>20%) with an expected wind speed of 100 mph.  I consequently lowered the wind speed to 95 mph.  This estimate still yields an EF1 rating.

Figure 9.   An interior view of a class room on the west side of the school.
A second area just east of I-35 had a single story house that lost the entire roof (figure 10).  The team in which I was on inspected this house to determine which side of the EF1/EF2 boundary the house should be rated.  Every house in the expected damage track was rated but only those appearing with colored triangular icons exhibited damage.  Thus the vortex was mostly too weak to cause damage except to a few houses where a brief damaging subvortex may have occurred. 

Figure 10.  An overhead map of the area of interest near Broadway Ave. just east of I-35.  The inset left points to the same area as the white outlined box in the main map.  The circle in the inset refers to the house in question.

This image in figure 11 shows the roof debris blown to the south side of the house consisting of composite shingles, 1by4” decking boards (likely tongue and groove) and 2X4” rafters spaced 24” apart.  Note the adjacent house to the east exhibited no apparent damage.   Adjacent houses to the west and north also showed no visible damage.

Figure 11.  A view of house labeled photo 2 in figure 10 facing northwest.   Figures 12 and 13 were taken at the label '1a and 1b' in the photograph. 
 This image on the east side of the house shows the part of the gable wall top plate and 2 nails (6d?) separated every 18” approximately (figure 12). 

Figure 12. A view of the top end of the collapsed gable roof including the gable wall top plate.  

The rest of the gable showed two nails between each rafter and the gable top plate approximately every 18”  (figure 13).  The gable fell nearly intact, however the orientation of this side was nearly parallel to the strongest wind, as determined by the direction of debris. It appeared though that the gable was poorly connected and exhibited no bracing.  While the lack of bracing appears to be common, the lack of a strong connection also appeared to be less than typical quality construction.

Figure 13.  A view of the rest of the gable.

Perhaps more significant than the low construction quality of the deroofed house was the lack of damage outside that caused by the debris.  These surrounding houses to the southwest showed little to no damage and even the fences remained standing outside the debris zone (figure 14).  The backyard shed was tipped over but it appeared to have no connection to the foundation other than gravity.  The light poles also were undisturbed except where the wires intercepted blown debris.  The only damage in the immediate vicinity occurred from impacts from debris.  As the debris from the roof and a car port blew to the southeast, it struck the fences and another house to the southeast resulting in the loss of exterior brick fa├žade and several windows (figure 15).  
Figure 14.  A view to the southeast from the house.  

Figure 15.  Two panoramas where the top one faces the house from the south end of its backyard.  The bottom panorama was taken from the backyard to the southeast of the house labeled '2' in figure 10.

Why did only this house suffer a loss of roof with the surrounding houses showing negligible damage?  One possibility is that this is the only house to have a car port in the vicinity (figure 16).  So there are two possibilities that affect our decision-making:  1)  The car port debris weakened the roof upon impact causing its failure. 2) A subvortex was small enough to only affect this house to the exclusion of all others.   Both possibilities yield a lower than expected wind speed because either collateral damage could’ve lifted off the roof at relatively low wind speeds or the duration of the subvortex over any spot would've led to a less than 3 seconds.   Consider that the vortex signature on the WSR-88D was moving between 40 and 45 mph and its diameter of maximum winds would be only 50 ft, the maximum winds would not have lasted the standard three seconds defined in the EF-scale.   This vortex would’ve needed to have spun up immediately northwest of the house because there was no visible damage to the houses across the street to the northwest.  However one house to the southeast (not the one with collateral damage) suffered loss of roof deck panels, suggesting the subvortex continued on to the east-southeast.

The degree of damage was labeled as large sections of roof removed, walls remain standing but the three second wind speed was lowered to roughly 105 mph or EF1.  The most likely scenario of what happened here is relatively weak connections between the top wall plate and the roof, along with possible impact from the car port allowed the subvortex to remove the roof from the house.
Figure 16.  A pre- and post-tornado view of the deroofed house as viewed from the northeast.  The pre-tornado view courtesy of Google Streetmaps.
 The final damage indicator that straddled the EF1/2 boundary was in east Moore where a 3600 sq ft house was directly hit by the core of the tornado (figure 17). 

Figure 17.  An overhead view of the final area under consideration, ShadyCreek St. just north of 19th in east Moore.  The inset shows the photo locations in subsequent figures including a wide panorama.
The house is a large single story wood framed structure with a hip roof design, brick and stone veneer and a composition shingle roof on OSB decking.  All the walls were constructed with 2X6” pine studs and roof rafters.  The owner built the house in 2014 and applied updated buildings codes that he promoted to the Moore City council following the May 20, 2013 EF5 tornado.  He mentioned that the house was built with anchor bolts and metal clips from the sill plate up to the top plate. 

The house experienced significant loss (>20%) of roofing above the complex roof-to-wall connections including the decking and numerous rafters (figure 18 and 19).  However there was no damage to windows or the garage doors on the north side of the house.  The adjacent garage shop west of the house lost shingles (DOD=1) and underlay on the south side while the neighboring house lost some shingles (DOD=1) on its west side (figure 18).

Figure 18.  A view of the three buildings including the house with the damaged roof.  Two insets indicate a pre- and post tornado view of the house.  

Figure 19.  A zoomed in view of the west side of the house.

 The tornado passed through the shelterbelt west of the house felling two large multiple trunk Elm trees with trunk diameters approaching 2’ (figure 20).  The smaller trees on either side were not affected thus supporting the premise that larger trees suffer disproportionately from severe winds even in an unfoliated state.   The tree damage indicators in the EF-scale were difficult to apply when vastly different tree response occurs within a just a few meters.  I used the standard wind speed of 100 mph based on either an uprooted or snapped tree trunk and assumed the tornado was of similar strength as at the house.  A narrow section of felled trees also could be seen in the woods opposite the field to the northwest.  The narrow width of felled trees strongly supports the idea of a narrow vortex progressing across the field and over the most affected house.  Due to its small diameter the buildings on either side escaped with relatively little damage.

Figure 20.  A panorama (labeled 'pano' in figure 17) including the houses and the shelter belt to the west.
 With the rafters clipped to the top plate and the lack of damage in the lower roof, why was the upper roof as damaged as it was?  A closer inspection of the roof near the ridge board revealed that the OSB decking was stapled to the rafters at irregular intervals (figure 21).  We could not measure the interval between staples but they were almost certainly spaced at greater than 6” intervals.  Since the building envelope was not breached it’s likely external pressure forcing overwhelmed the limited attachments of the roof decking.  The standard composition shingles offered no more than standard resistance to the wind.  This is below the quality of typical construction to give an expected wind speed and thus the wind speed estimate required to do this damage fell short of an EF2 rating.   Had the decking been clipped to the rafters this roof could’ve survived and EF2 intact and thus similar damage would’ve resulted in a much larger wind speed estimate.  Upon talking to the owner at the time, he admitted that he paid a lot for this flaw in his design.  If he used the same materials and techniques used to construct the porch overhang in framing the roof, he may have saved himself a lot of grief (see figure 22). 

Figure 21.  A zoomed view of the roof ridgeline  labeled '3,4' in figure 17 showing staples used to fasten OSB roof decking to the rafters.  

Figure 22.  A view of the east side of the house.

This survey though is about more than a single rating for the tornado.  With the advent of the Damage Assessment Toolkit (or DAT), we were able to create a high resolution survey from which the whole track could be contoured by EF scale rating.  The map that Doug Speheger of the NWS office in OUN, and others, created from this survey easily shows how small an area the EF2 winds occupied compared to the whole area that the tornado covered (figure 23).   By far the majority of the area covered by this tornado was only EF0 in strength and thus the energy that this tornado exacted upon the landscape was not as big as its EF2 rating suggests it may be.  

This is where high resolution surveys provide their biggest benefit.  Now we can see what fraction of the tornado path is occupied by winds of various strengths instead of making assumptions that has been standard practice based on a small sample of  well surveyed tornadoes.  Now the NWS has the opportunity to vastly increase that sample size and possibility to eliminate the need for more assumptions.  This information is more useful to NWS partners than many of us think.   Considering the users that include insurance industry, the Nuclear Regulatory Commission, risk analysts, FEMA, state and local governments and more and you may begin to see the value that comes from this level of detail in this NWS product.  It doesn't take a team of 12 people for every 12 miles to create them either.  It can be streamlined to be done quickly.  I'll reserve another post for that.  In the meantime, keep them coming!
Figure 23.  The map of the tornado path contoured by EF scale  published by NWS Norman.


To clarify, the media didn't force a reassessment of the tornado rating. Rather the initial rating of the EF1 allowed the NWS time to make a better assessment while still giving the public a preliminary assessment.