Sunday, March 3, 2013

Snow bow echo?

How many times has there been a bow echo in an environment cold enough for snow to reach the ground within the confines of its heavy precipitation shield?  The answer for my experience is – never! The closest for me was a snow thunderstorm and shelf cloud over Santa Fe ski area in a March event years ago.  But that was not a bow echo like this one.   This storm produced wind damage around Billings Montana in the late afternoon including snapped poles and an overturned tractor trailer.  The radar loop from IA State below shows the onset of the bow over Billings and then its progression east.  The local NWS office was on the situation and quickly issued a severe thunderstorm warning, later expanded to capture the growth of the bowing system.  

Later after passing east of Billings, I snagged a few radar images of the bow.  The bow echo appeared to show a velocity peak just south of Crow Agency though the area of high velocities was small.  The Differential reflectivity showed only heightened values east of the high reflectivity.  That's somewhat unusual for bow echoes of warmer environments where the heaviest rain and highest Differential reflectivities are colocated.  Differential reflectivity in light rain is typically small but in heavy rain the values are inflated up above 2 dB.  With high reflectivity and low Differential reflectivity, I am thinking that a large amount of ice makes up the precipitation type. The bottom right panel of figure 3 also shows the Kdp indicating lack of liquid.  Normally the Kdp would indicate elevated values in a bow echo in a warmer bow echo. But then the bottom right picture shows high Correlation Coefficient.  In a summer bow echo, any high reflectivity associated with low Differential reflectivity would mean falling hail and the Correlation Coefficient would be depressed.  Here it is not depressed.  My best guess is that this bow echo contained mostly ice in the form of snow and graupel.  

Figure 2.  Reflectivity (left panel, base velocity (middle panel), and Differential Reflectivity (right panel) from the Billings, MT WSR-88D lowest elevation scan at 2013-03-03 2246 UTC.

Figure 3.  Reflectivity (left panel, Correlation Coefficient (middle panel), and KDP (right panel) from the Billings, MT WSR-88D lowest elevation scan at 2013-03-03 2246 UTC.

A closer look at the Billings area shows that temperatures quickly fell from near 50 deg F to near freezing as a cold front swept through, quickly followed by the bow (fig. 4).  The webcam in figure 5 shows depressed visibility with heavy snow falling, and new snowcover on the ground, however the image was shot 30 minutes after the bow echo's arrival.  Winds were already strong immediately following the cold front which arrived at the Billings airport at 2129 UTC (fig. 6).  The bow arrived roughly 10 minutes later with a mix of rain, snow and graupel.  The precipitation changed to completely frozen forms only seven minutes later while the winds were still gusting to 50 kts.  Occasional lightning accompanied the heavy frozen precipitation.  This progression of precipitation agrees quite well with the Dual-polarization radar data in figures 2 and 3.

Figure 4.  A surface plot with a webcam image overlaid from near 2146 UTC courtesy of

Figure 5.  An image taken at 2216 UTC 2012-03-03 courtesy of StormTeam webcam.  
KBIL 032147Z 31033G51KT 1/4SM R10L/3000VP6000FT -TSPLGSSN FG SCT007 BKN032CB OVC070 01/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34E47PLB39GSB42E43B47SNB43 TSB43 PRESRR OCNL LTGICCG TS OHD MOV E P0000 I1002 $
KBIL 032143Z 30036G51KT 1/2SM R10L/3000VP6000FT -TSRAPLSN SCT006 BKN032CB OVC070 01/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34PLB39GSB42E43SNB43 TSB43 PRESRR OCNL LTGIC TS OHD MOV E P0000 I1002 T00111011 $
KBIL 032140Z 30034G51KT 1SM R10L/5000VP6000FT -RAPL BKN020 BKN049 OVC080 02/M01 A2956 RMK AO2 PK WND 31051/2135 WSHFT 2114 RAB34PLB39 PRESRR VIS 12V1 1/2 P0000 I1001 T00221006 $
KBIL 032129Z 33027G43KT 10SM FEW030 SCT050 BKN080 09/M01 A2952 RMK AO2 PK WND 34043/2127 WSHFT 2114 VIRGA ALQDS VCSH W-N T00941006
KBIL 032053Z 08008KT 10SM SCT075 BKN110 12/M01 A2952 RMK AO2 SLP994 T01221006 58029
Figure 6.  A meteogram fro KBIL on 2012-03-03 including an annotation of the thunderstorm from 2140 - 2230 UTC.

The cold front (fig. 7) that forced the bow echo's formation was part of a very strong upper-level short-wave trough and accompanying jet streak passing through Montana.  Convective activity was occurring all along the front ahead of a dry slot.  Note that the majority of the frontal forcing was occurring behind the surface frontal location, a characteristic of anafronts.  This same system is forecasted to track to the southeast, merge with another southern stream wave and give the Mid-Atlantic states a late season snowstorm.

Figure 7.  Surface plot by UCAR/RAP overlaid on top of a GOES visible image taken at 2145 UTC 2013-03-03.  The blue curve represents the cold front in which the thunderstorm symbol marks the location of the bow.  
 With all of the convective activity, I would've expected to see some instability in the form of CAPE.  Finding some CAPE proved elusive.  According to the SPC mesoanalysis, the nearest surface-based CAPE could only be found in northern Wyoming (fig. 8).  However steep lapse rates in the lowest 3 km of the atmosphere pointed to the possibility that with a little more moisture than analyzed, CAPE could be found (fig. 9).  Even without much CAPE, the front appears to have been quite strong with a steep interface.  One could argue that the cold front was strong enough alone to cause a convective-like line, also called a narrow cold frontal rain band.  But if that's the case then why wasn't there one?  And why did a bow echo form?  Instability had to be there to produce this event.  However, the instability still led to relatively small amounts of CAPE, and I cannot ignore the strong vertical forcing of this front.  It is interesting that the MCS maintenance parameter painted high probabilities in the vicinity (fig. 10).  I suppose that any convective convective parameter that doesn't include CAPE would be ironically effective.

This bow echo was likely not very intense.  On its own, I doubt that the bow could've generated severe winds, or anything remotely close.  However, the cold front was very strong, and the overall system, dynamic.  Only a small convective supplement would be needed to push the winds into severe thresholds.

Figure 8.  Surface-based CAPE, low-level winds and mosaic reflectivity from the SPC mesoanalysis page.  

Figure 9.  0-3 km lapse rate analysis from the SPC mesanalysis page.

Figure 10.  The Mesoscale Convective System (MCS) maintenance parameter available on the SPC mesoanalysis page.

Monday, February 25, 2013

Incredible snow forecast for Woodward, OK — or is it?

We're in of what could be the biggest snowfall in Oklahoma history if you believe some of the models.  On 00 UTC Feb 25, the NAM put out a nearly outrageous forecast of 30" or more for Woodward, OK.  No wonder that was the case because the storm system coming across New Mexico is expected to close off and then pivot around an axis in Northwest Oklahoma allowing the precipitation band to remain firmly entrenched for hours.

Even earlier, the 21 UTC SREF (Short Range Ensemble Forecasts) snowfall chart shows a mean dump of over 25" in Woodward.  In the image below, all of the ensemble members are plotted on the same timeline from the SPC website (time increases to the right).  Some of the forecasts bury Woodward in over three feet of snow! To make a comparison, the big nor'easter in New England dumped a record 36-40" of snow in Connecticut.  If the upper end of these forecasts came true, this storm would be even more unusual.
But wait, there's one subtle feature here that can make or break this monster snow forecast and that is the forecast snow ratio and compaction.    First, snow ratio is incredibly complex to measure, or to even have consensus as to how to measure.  Before the snow even hits the ground, there are a multitude of factors within and below the cloud that can affect the density of snow flakes.  Crystal shapes can change quickly from relatively compact plates to the more classic dendrites just by changing the supersaturation of the cloud ever so slightly.  Some research indicates that supersaturation increases when the vertical velocity increases.  But supersaturation can also depend on how fast the liquid and gaseous water is being scavenged out by the crystals themselves.  That's a feedback loop that can gunk up our initial guess.  Then when a crystal falls into warmer saturated air, it can accrete other crystals, grow new ones right from water vapor, or directly intercept liquid cloud droplets.  The rate at which these processes happen again depend on the vertical velocity, liquid and vapor content in the cloud and the number of ice crystals competing in the same space for available water.  The end result of all these processes is a flake of snow with a certain density.  This is a process that cannot be directly observed by operational forecasters.

However, we attempt to make some assumptions about the density of the falling snow flakes as a function various simpler processes and then see what happens to the forecast snow to liquid ratio (a simplistic estimate of falling snow density).  The most simple estimate is to just apply a climatological average snow ratio.  One is available here created by Dr. Martin Baxter.  Let's assume a 12:1 ratio and we get a timeline of snow accumulation (called a plume diagram) for an ensemble member near the mean snow fall. The time now increases right to left and the appropriate axis is labeled in inches in the far right.  The blue line below shows the 12:1 ratio and the accumulation peaks just over 20", a respectable snow storm.  

But there are other techniques.  A maximum temp in profile technique assumes the snow ratio increases as the maximum temperature in a vertical column decreases.  The thinking here is that the density of falling snow decreases as the maximum temperature in the warm layer aloft decreases.  There may be some merit to that if that warm layer is saturated since the maximum liquid cloud water content available for riming decreases as temperature decreases.  Notice here the forecasted snow ratio for max temp in profile slowly increases as the air cools aloft.

Meanwhile there is the Zone omega technique (colloquially called the Cobb 05 technique) where snow density decreases if the strongest ascent occurs in the dendrite production zone (-12 to -18 C).  I talked about this a couple years ago before our drought when we had a much colder snow storm.  This is a horribly difficult method to verify and this method is completely statistical.  The Woodward forecast below also shows the extreme volatility of the snow ratio.  The snow fall winds up being pretty high (25-30") because this technique allows for snow ratios exceeding 40:1 if the vertical motion spikes in the dendrite production zone.  Many times this technique overestimates the ratios (underestimates falling snow density) based on the experience of forecasters.

Due to the errors, an alternate version of the technique cuts the snow ratios for each temperature down by almost a factor of two.  Now the snow fall is around 23", or similar to that of the first two techniques. 

What we discussed so far only represents our best attempt at predicting the density of falling snow.  What happens after the snow hits the ground before we go to measure it is a completely different matter.  Snow begins to compact immediately after the flakes hit the ground and accumulate.  Every one of the graphics above initiates a compaction routine based on an time-dependent exponential decay function.  That's why the forecast snow accumulations decrease with time.  If we removed that function, the purple line shows the snow depth forecast and now you can see values in excess of 35". 

However, the exponential decay function is static, and therefore presents an unrealistic display of the processes that affect snow compaction.  Perhaps the only realistic component of this is that the compaction continues with time and thus presents an idea of how much snow depth loss (density increase) occurs before someone measures the snow.  But the rate of compaction can change according to the wind. The stronger the wind, the more blowing and drifting of snow causes crystal breakup and compaction.  A strong wind like what Woodward is expecting today could cause drifts compact enough to support someone walking on them.  If so, that kind of density is going to be associated with very small snow ratios, maybe 3 to 4:1!  But let's assume a flat, representative surface for measuring snow.  If that's the case then there's a nifty neural net (called the Roebber technique) located here that allows you to enter in the QPF (in liquid equivalent) and the expected wind speed.  It will estimate the snow ratio for you.  I entered in 2" of QPF and a 25 kt wind, certainly reasonable numbers for today.  The output snow ratio falls to 9:1.  That would yield less than 20".  The Roebber technique also accounts for temperature related compaction.  Certainly some of that occurred since Woodward was well above freezing yesterday.
All of this of course depends on an accurate QPF.  Fortunately Woodward is in an area where the SREF had a high probability of > 2" of QPF and therefore a high confidence of forecasting if this snow will be recordbreaking or not.  For those less fortunate areas where the QPF uncertainty is greater, the errors in snow ratio may not matter so much.  

Monday, February 11, 2013

A sun pillar caught me off guard

Our drought seemed to have caught me unprepared for this evening where the clouds actually consisted of liquid water and a sun pillar formed.  A field of altocumulus clouds spread overhead from the west marking creating another spectacular sunset, one that I hopefully adequately captured from the bottom of Lake Thunderbird's Jim Blue bay.

An altocumulus-filled sunset from Lake Thunderbird 2013-02-11 2354 UTC.
This cloud layer occasionally sported small trails of snow precipitating out from each individual altocumulus.  The altocumulus clouds were cold, but how cold?  To answer this question, I referred to a vertical sounding taken at the National Weather Center.  The balloon usually launches around 23 to 23:15 UTC, or up to 50 minutes before this shot was taken.  The altocumulus clouds were overhead at launch time and so I'm pretty confident that the thin layer of moisture at 600 mb in the sounding below represented the altocumulus layer.  If so then they were centered around -10 to -15 deg C, right about the temperature where snow crystals like to form fairly quickly.

The vertical sounding taken at Norman where the balloon likely passed through the altocumulus layer at about 2330 UTC.
A little later, a spectacular sun pillar formed to our west as the sun fell just below the horizon.  Unfortunately I was caught off guard eating dinner after shooting the early pictures, and so my only picture was taken through a nest of Post oak branches.  Other folks in town were more fortunate and grabbed some really nice unobstructed shots.

A sun pillar caught through the branches of our trees at home taken 2013-02-12 0018 UTC.

I should've been more prepared to shoot this sun pillar at a better location because they are rare in this part of the country, more rare than sun dogs and possibly rainbows.  Sun pillars like to form from sunlight reflecting off the top and bottom faces of plate crystals that are tilted upward toward the sun, especially a few minutes after the sun has set.  This site explains the process very nicely.  Given that supercooled altocumulus clouds were present, the pillar seemed to show a presence of a pretty widespread field of ice crystals which were probably dominated by plates.   The roughly -10 to -15 C temperature of this layer can easily produce plates as long as the supersaturation is low.  Take a look at the morphology diagram like this one from

I doubt the supersaturation was high otherwise we'd be looking at stronger updrafts and more beefy looking altocumulus castellanous, or a continuous sheet of nimbostratus if there was widespread lift.  Then forget about seeing the sunlight.  No, these little altocumulus were barely able to condense liquid water.  They also formed quickly, as expected, before the pictures above were taken and then moved northeast overhead (see the satellite image below).  Why was that expected?  Because the rapid ice crystal formation would quickly scavenge the clouds of their water supply eventually converting all the altocumulus to ice trails (virga).  

The satellite image also shows that this field appears to have been connected to the larger shield of warm advection clouds forming above a polar airmass from a departing surface high to the east.  The 850 mb and 700 mb plots below show the gulf moisture sliding over the lower level cold air and perhaps our altocumulus cloud deck formed from this same warm air advection but in a very thin layer.

While the warm air advection happens relatively frequently (except when we're in drought) it is rare to get such a thin layer of lift to condense a small amount of moisture into altocumulus clouds at temperatures that support plates (for the best pillars) and yet have not had time for the liquid water to completely be scavenged out by the forming ice crystals.  How many times a year do I see these conditions get met at sunrise or sunset without intervening cloud layers to dim the sun?  Not too often.  Next time I'll be prepared for a sun pillar shot but I suspect I'll see a lot of tornadoes before then.
Visible satellite loop from 2013-02-12 2002 to 2315 UTC.

Surface analysis from NCEP/HPC for 2013-02-12 00 UTC.

Thursday, February 7, 2013

One day snow forecast for NYC 2 - 20" ??

The epic nor'easter is only one day away from New York City with precipitation already spreading up to Virginia and the pressures are falling fast just offshore and yet the model guidance cannot give us any clue whether or not they'll get 2 or 20" of snow.  Twenty one members of model guidance shows an absolutely huge spread in possible snowfall amounts for JFK airport.  In the timeline plot below, the grayish curves show the snow fall rapidly increasing and then reaching a peak before the snow pack settles down as the time increases to the left.  But some of the model solutions show almost no snow at all.

A time trace of 21 model members of the Short Range Ensemble Forecast (SREF) system for JFK airport from the run starting at 2013 Feb 07 21 UTC.  Time runs from right to left.  The grey traces represent snow whose amounts can be determined from the right vertical axis.  The green to blue traces represent the liquid equivalent amount forecasts.  The horizontal blue line represents a static snow to liquid ratio of 12:1.
What in the world is giving this amazingly huge uncertainty for being only one day out?  One reason could be the wide spread in precipitation forecasted by the forecast ensembles.  The blue traces above also show a huge spread ranging 0.6" to nearly 4.0" of liquid equivalent.  The total precipitation map below shows how some model solutions almost leave New York City high and dry.

The total liquid equivalent precipitation forecast from each SREF member in the small squares and the mean in the large square.  The model run was at the same time as the figure above and valid for 2013-Feb-08 21 UTC.  Note some of the members forecast the heavy precipitation to fall south of NYC.  Image courtesy of the Penn State e-wall.

However that's not the full story.  Not only is the amount of precipitation unusually uncertain for NYC but the model members are unsure what side of the freezing line the temperatures should be.  Here is another multi-panel image from the Penn State e-wall site showing the expected precipitation type and the location of the surface and 850 mb freezing line for the afternoon on Feb 08 (21 UTC).

Same SREF model run time and forecast hour as above but now I show the expected precipitation type (blue for snow). The blue contours show the surface freezing line while the black contour shows the freezing line at 850 mb.  Notice the uncertainty of the freezing line around NYC.

Visualized another way is this SREF sounding plot below showing all the members plotted.  All the members show that surface temperatures should be cold and likely from 30 to 40 F.  That's unfortunately centered around freezing and the results are dramatic with respect to what kind of precipitation is expected to fall.  The left panel shows a huge splatter of expected precipitation types in the late afternoon based on a partial thickness technique.  Unfortunately changing the technique won't improve the uncertainty what will fall in the late afternoon for NYC.

Forecast SREF profile of temperature (red) and dew point (green) for JFK valid for 2013 Feb 08 21 UTC.  The left panel shows the expected precipitation type for each model run (red circles) based on the partial thickness technique.  

This forecast dilemma is not going to improve until we actually see the observations in the morning and see which side of the freezing line New York City will happen to reside.  I'm going to be paying special attention to the AMDAR aircraft sounding profiles as they take off from the NYC area airports.  Unfortunately I hear that flights are being canceled left and right in the NYC area just when we need frequent observations of temperature aloft.  That's why frequent radiosonde launches will be so critical for the NYC area tomorrow.  It's the only platform that can successfully get us the data we need in bad weather.  That is unless some group can fly foul weather drones.

Now for the Boston area, there is more confidence of higher liquid equivalent precipitation though a couple model members leave Boston almost as dry as New York City.   As for snowfall, there is also a similar uncertainty in temperatures but all the runs "safely" keep Boston below freezing throughout the atmosphere and snow, and lots of it, is a virtual certainty.

Same kind of time trace for the SREF as above but for Boston, MA.  Note the precipitation uncertainty is similar as fo

Same forecast model sounding for the SREF as above but for Boston, MA.  The low-level temperature uncertainty appears just as high for BOS as for JFK but the soundingis colder and all members forecast snow.

Tuesday, January 22, 2013

More 'Sandy-like' storms? What do we do about them?

Truly remarkable storm occurred over the West Pacific last week that reminded me that there are storms like the landfalling hurricane Sandy traveling across the oceans on a more frequent basis than we may want to hear about.  It had a very low pressure of 932 mb at a latitude of only 40 deg north as it went explosive deepening just off the coast of Japan.  This pressure is actually lower than Sandy's 936 mb value as it transitioned toward an extratropical state.

An Ocean Prediction Center analysis of the west Pacific cyclone at near peak intensity.

The wind field that was hurricane force over a wide area, mostly west and south of the low center in a famously typical area for rapidly deepening extratropical cyclones.

An ECMWF 3 hour forecast of the 10 m winds around the west Pacific storm (courtesy of  the Weather Underground).

With the intense, wide wind field, the NOAA's WaveWatch model forecasted a significant wave height of 63'!  This is in itself an amazingly bold forecast considering that the highest scientifically measured significant wave height as of 2006 was 61' off the coast of Scotland by a British Oceanographic Vessel RRS Discovery (see Holliday et al. 2006).  Since the significant wave height represents the 66th percentile of wave heights, a wave exceeding 100' may not even have been considered a rogue wave (more than 4 standard deviations above the mean significant wave height).

The NOAA WaveWatch model 48 hr forecast from 2013-01-13 - 06 UTC courtesy of the Ocean Prediction Center.
The analysis later on had a more 'modest' significant wave height of 49'.
The Ocean Prediction Center analysis of significant wave height

This even came about by phasing of two shortwave troughs, much like what happens often in the eastern US.

500 mb (left) heights and anomalies and sea-level pressure (right) from 2013-01-13 - 00 UTC to 2013-01-15 18 UTC (courtesy of Penn State e-wall).  The west Pacific storm evolves in the upper-left of each panel.

The IR imagery confirms the  spectacular coil so often associated with the intense oceanic cyclones.  An old technique of using this kind of imagery for estimating the central pressure of oceanic cyclones comes to my mind.  Called the Smigielsky, Mogil and Burt technique (SMB; Smigielsky and Mogil, 1995), it's empirical in a similar way to the concept of the Dvorak technique for estimating tropical cyclone strength. The SMB applies a workflow where once a baroclinic developing system is established, one compares the pattern of the mid- and high level clouds.  The more spiraling exhibited by the storm, the deeper the central pressure is likely to be.  Comparing this imagery and the technique's flowchart for north Pacific storms seems to show a degree of spiral that exceeds the lowest pressure option.

A GMS infrared satellite loop of the west Pacific cyclone (courtesy of the Penn State e-wall).
The SMB flowchart for estimating central pressure of extratropical cyclones in the west Pacific.

Why was it so reminiscent of 'Sandy'?  Both of these storms were warm core at their respective lowest pressures.  At its peak intensity, the west Pacific storm showed a region of higher 1000-500 mb thickness as seen in the analysis and forecasts of the GFS, ECMWF and even Canadian models.  Here is an example from the ECMWF model analysis near the time of the storm's lowest pressure.  The thickness represents the mean temperature from the 1000 to 500 mb layer and it shows a bubble surrounding the surface low.  Outside, the thickness gradient shows a ridge indicating the occluded front.
ECMWF sealevel pressure and 1000-500 mb thickness 33 hour forecast from 2013-01-16 00 UTC (courtesy of the Weather Underground).  
This structure is not new.  It has been well documented going back decades.  One of the most incredibly intense extratropical cyclones in recorded history at relatively low latitudes was captured within the domain of project ERICA (Experiment of the Rapidly Intensifying Cyclones over the Atlantic).  The storm's central pressure dropped to below 930 mb at only 40 deg N latitude!  And it also exhibited a warm core as analyzed by Neiman and Shapiro (1993) that closely resembles the west Pacific storm.  The authors noted that this storm was the most intense extratropical cyclone that they recollected in this part of the western Atlantic, south of Newfoundland.  I believe their assessment holds to this day even if Sandy was considered extratropical.

An analysis of an intense oceanic extratropical storm in project ERICA at 1989-01-05 00 UTC.  On the left is 850 mb temperature analysis with plotted wind observations (left) while on the right is the analysis of se-level pressure, fronts and observations.  See Neiman and Shapiro (1993) for more details.

Even though Sandy wasn't extratropical,  her structure also agrees quite well with the west Pacific storm.   The warm bubble surrounding the surface center of Sandy shows up embedded within a cold front to the south and a semblence of a warm frontal baroclinic zone on the other side.  The only differences here are that the warm bubble around Sandy has a higher thickness value (570 dam vs 540) and the cold/warm frontal bands were rotated counter clockwise about 90 deg.

Comparison of the ECMWF 33 hr sea-level pressure and 1000-500 mb thickness forecast (yes it's mislabeled) of the west Pacific storm (upper-left) to the 30 hr GFS forecast of Sandy (upper right) and the 114 hr GFS forecast of Sandy (lower right).  The shading in the ECMWF forecast represents 6 hour QPF.

How did these similarities come about? This question can be answered by viewing the structure of these storms in the form of a cyclone phase space diagram conceived of by Hart (2003).  In the picture below the west Pacific storm produced its warm core quite a bit differently.  The phase space diagram shows the cyclone began as a traditional deep cold core extratropical baroclinic system (point A lower left) as the thermal wind indicated the typical increasing values with height from near the surface to 300 mb (hPa) around the surface low center.  As the storm exploded in intensity, it entrained warm air from its warm sector and wrapped it around the center, similar to that described by Shapiro and Keyser (1990).  They have a classic conceptual model of what's been named the 'warm seclusion'.  Schultz and Vaughan (2011) later explained that this kind of process should occur with occlusions by wrapping up the air along the warm front around the surface low (see below).  At it's lowest pressure, the west Pacific storm evolved into a deep warm core system (lower left) with a high degree of symmetry (upper left).  Sandy, on the other hand, began as a traditional tropical cyclone warm core, symmetric system (point A lower right) and then evolved into a slightly asymmetric, deep warm core system nearing landfall in New Jersey.    But by the time the respective cyclones were at their lowest pressure, their thermal structures are not very different.  If anything, the west Pacific storm had a slightly more symmetric warm core than Sandy, if you can believe it.  But the vagaries of model analysis and forecasts suggest being careful in interpreting these differences too literally.  The bottom line is that both storms were deep warm core within 500 km of the low-level centers.

A cyclone phase space diagram of the west Pacific extratropical storm (left panels) and hurricane Sandy (right panels).  The top row represents the degree of thermal symmetry exhibited by the cyclones and the lower panels represent the depth of the thermal core (warm or cold).  A detailed explanation of these diagrams is available at Hart (2003) or here.

A conceptual model of the Shapiro and Keyser (1990) warm seclusion process modified by Schultz and Vaughan (2011).  The top row represents sea-level pressure and fronts in four stages temporally separated by 6-24 hours.  The bottom row of contours represent the lower tropospheric temperatures for each stage.
The most intense winds typically are located along the exterior of the warm core, just outside of the bent back occlusion rearward and right of the surface low.  This is the most dangerous area that both Sandy, the west Pacific storm, and the project ERICA storm shared.   Most of the time, the east coasts of continents are spared this area and don't experience what these parts of the storms can offer in terms of wind damage, coastal flooding, and general suffering.  The west coasts of continents do experience these on occasion though perhaps not the worst either since many times systems have finished bombing out by the time they hit the west coast, especially that of North America.  Yet some big storms have hit Europe such as the great Braer storm of 1993, arguably the lowest nontropical/nontornadic sea level pressure on record (<911 mb).  This storm actually helped to disperse an oil spell east of the Shetland islands.  Of course it was responsible for grounding the ship that spilled the oil in the first place.  The west coast of Alaska can get nailed by intense Aleutian lows that may have exhibit warm seclusions and were responsible for severe coastal flooding in western Alaska as noted by Christopher Burt in his blog post.   And there was the great Columbus day blowdown in the Pacific Northwest caused by an intensifying 962 mb low.   But imagine a scenario where the ERICA storm of 1989 or the west Pacific storm last week was caught in a blocking situation where they turned the worst of their winds toward their respective coasts in regions unaccustomed to them (in other words, the eastern coasts of major continents).  Despite their completely extratropical origins, I cannot imagine any difference in impact from that of Sandy.

Several blog posts have stressed that hurricane warnings should have been maintained for Sandy as a result of it's maintaining a warm core right up till landfall (see Norcross, 2012, and others).  Perhaps that may have been the solution that would've offered the least trouble with Sandy.  But perhaps it's not the best general solution.  As we've seen, Sandy was not so different than other intense extratropical cyclones. The only difference is that Sandy turned westward.  In some near future event, a more traditional extratropical storm may turn in a similar direction, with a similar sealevel pressure and wind field.  Certainly in such a situation nobody I know would think hurricane warnings would be the answer.  But the end result would be the same.  And how different would the result be from a more traditional hurricane?  Given similar winds and pressure distribution, I don't think the impacts to people would be any different either.  Yes, deep convective hurricanes have the capability of generating much stronger pressure gradients and higher winds, but a building or structure doesn't care what generated the wind, only that the wind is not exerting a potentially damaging force.  Likewise the sea doesn't care whether it's a hurricane or extratropical storm when the water's being pushed upward toward the coast.  Perhaps it's time for warnings to reflect this view.

Now upon finishing this reflection, I see the latest numerical guidance shows three sub-950 mb cyclones expected to form in the north Atlantic in the next five days, two of them are forecasted to fall below 940 mb, one below 930 mb.  All of them will have warm seclusions and likely exhibit warm core behavior.

GFS forecasts of intense north Atlantic lows this week.

The ECMWF forecast of a north Atlantic low on 2013-01-26 with a warm seclusion, hurricane force winds, near calm eye and a sea level pressure of < 930 mb.

Brian Norcross, 2012:

Stu Ostro, 2012:

Recount of record-breaking oceanic cyclones by Christopher Burt

Recount of the Braer storm of 1993

the great Columbus day blow of 1962:

Doswell, C. A., III, 1984: A kinematic analysis of frontogenesis associated with a nondivergent vortex. J. Atmos. Sci., 41, 1242–1248.

HartR. E., 2003A cyclone phase space derived from thermal wind and thermal asymmetry. Mon. Wea. Rev131585616.

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