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Posted at 10:15 AM ET, 12/22/2009

The evolution of a monster D.C. area snowstorm

By Ian Livingston

Dec. 17-20, 2009: Simulations, satellites, radars, and maps

* Full forecast thru X-mas | How did this happen? | Locals react & stats *
* View CWG-user snowfall map | NWS snowfall reports (view map) *

Historic nor'easter along the East Coast at 12:30 p.m Sat., December 19, 2009. Heavy snow was falling across the metro area at the time. Image courtesy: College of DuPage Weather. Also see, amazing visible loop for Dec. 19. By calamity, created from NASA imagery.

Seasoned weather forecasters know snowstorms simulated in computer models days ahead of time should be viewed with healthy skepticism. But two days before the weekend storm struck, I was shocked and awed by a particular storm simulation (from the North American Model, NAM). Sensing a special storm might be in the works, I started archiving some of the readily available imagery around the Web to document the evolution of what would become the Snowpocalypse. The view from space is now a white one, and that's just one of many images that tells the story of the storm.

Continue on for more imagery related to the historic 2009 nor'easter.

Early on, it wasn't clear if the storm would take the right track to give us snow. We needed the northern (polar) and southern (sub-tropical) jet streams to combine in the right place at the right time. Small differences in this process could swing the outcome from a big hit to a big bust. Some model guidance three and four days before the storm struck had it going out to sea. By 2-3 days out, the picture became clearer that the storm would come up the coast, socking us with snow. And by the day before, there was actually quite good agreement that a historic event was about to unfold.

Surface pressure forecast made at 2 p.m. Thursday, for late Saturday (or 60 hours out). Image courtesy: Hydrometeorological Prediction Center.

Model guidance midday Friday on precipitation totals and low placement. At left, 12z Global Forecast System ensemble mean surface low pressure and precipitation depictions. At right, 15z Short Range Ensemble Forecast quantitative precipitation forecast. Images and data courtesy: National Centers for Environmental Prediction.

In the immediate lead-up to the snowstorm, CWG mentioned that our forecasts for a major event were backed by pretty high confidence. Large-scale pattern signals, combined with increasingly "bullish" guidance and historically similar events, portrayed the potential for a memorable one. It's hard to predict a record breaker, but our final forecasts did, and they verified.

A low pressure system in the Gulf of Mexico gathers moisture late Thursday night. Image courtesy: University of Wisconsin at Madison.

Radar showing abundant moisture moving out of the Gulf of Mexico Friday morning. Image courtesy: WSI Corporation.

By late Thursday and early Friday it was increasingly apparent that the storm system forming in the Gulf of Mexico would have no shortage of moisture. Satellite imagery Thursday night showed a band of convection headed well south into the tropics, as is the case with many major East Coast storms (see past examples: 1983, 1993, 1996). Radar the next morning indicated a huge swath of precipitation making its way toward the area.

Snow overspreads the area around midnight Friday and extends well to the southwest. Image courtesy: AccuWeather.

Local radar from late-morning Saturday into early afternoon shows continued strong banding pushing through the area. Image courtesy: Weather Underground.

After some moderate overnight snow, mainly as a result of warm air advection ahead of the low, we started to really feel the brunt of a rapidly deepening system off the Carolina and Virginia coast throughout the late morning into the early afternoon. National Airport reported heavy snow -- with visibilities as low as 1/8 of a mile -- from 9:52 a.m. until 3:39 p.m., picking up 6" of snow during the period. Some locations in the area saw even higher rates and snowfall totals in a short window, with snow then continuing into the evening.

Surface plot showing strong low pressure off the Mid-Atlantic coast on Saturday afternoon. Image courtesy: Unisys Weather.

Historic nor'easter pulls away from New England early Sun., December 20. Clouds clear over the D.C. metro area, revealing snow on the ground. Image courtesy (click for super-high resolution): CIMSS/SSEC: Modis Today. Also see, closeup of D.C., Md. and Va. from Earth Observatory.

By Ian Livingston  | December 22, 2009; 10:15 AM ET
Categories:  Recaps, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Cold, and dry...until Christmas
Next: PM Update: Chilly today, chillier tomorrow


Totally awesome!

We need a snow storm scale. Something like 24 or more = Monster, 18-24 = Historic, 12-18 = Major, 8-12 = Significant.

That's probably not right but it's a start.

Any thoughts?

Posted by: jaybird926 | December 22, 2009 7:50 AM | Report abuse

Does that almost look like an "eye" in the first satellite photo? I heard that the pressure of the low was equivalent to a Cat 2 hurricane.

Posted by: dixiechick1 | December 22, 2009 8:12 AM | Report abuse

Nice summary, Ian. You even got in a plug for Matt's place.

Posted by: NoVaSnow | December 22, 2009 8:17 AM | Report abuse

cool images!

Posted by: madisondc | December 22, 2009 10:41 AM | Report abuse

it's funny, though, because i think that most of our snow came from the deepening low and atlantic moisture unlike some events where we win from the WAA. we really did need a strong low to produce those bands during the midday hours which ultimately gave us our high snow rates.

Posted by: swishjobs | December 22, 2009 11:18 AM | Report abuse

Ian, nice composite of information, graphics and maps of the snowstorm.

One item that is the truly eye opening is the "amazing satellite loop for Dec 19" that shows the successive development and movement north of snow bands (seen also, but not as dramatically in radar).

The banding is due to "conditional symmetric Instability (CSI) where the atmosphere is stable to vertical motion (not conducive to vertically developed thunderstorms), but unstable to the slantwise motion as air rises from low levels over sloping frontal boundaries aloft (==>warm air advection). Thundersnow can occur in the bands, but not from what we might think of as thunderstorm cells.

Posted by: SteveT-CapitalWeatherGang | December 22, 2009 11:49 AM | Report abuse

Snow flurries are falling in N. Stafford at the moment.

Posted by: david_in_stafford | December 22, 2009 11:56 AM | Report abuse

Too often the WAA is too darned warm--and we get nothing but that raw cruddy rain [which ThinkSpring seems to like so much]. This is what kept happening all through the boring strong El Nino of 1997/98. There was one huge ice storm that year but it slammed the area from New England into Quebec. We got nothing but a nasty wind-driven rainstorm out of that one.

As I posted yesterday, Saturday's storm spun up in the same place as, and traveled along roughly the same route as, the historic Knickerbocker storm of Jan., 1922. When a storm travels this route and the cold air is in place, Washington tends to get hit with a huge amount of snow.

The Washington-Jefferson storm of Jan. 28, 1772 may well have followed the same general track, and, with 3 feet of snowfall [most ever historically recorded for Northern Virginia] likely was more intense than both the "Knickerbocker" and Saturday's "Snowpocalpyse"!

Posted by: Bombo47jea | December 22, 2009 11:56 AM | Report abuse

Correction: That should be "Snowpocalypse".

Steve T's CSI post seems to confirm what I appear to have observed about thundersnow events which occur along a frontal boundary right on the cold side of the rain/snow line. Sometimes there's little or no vertical turbulence, but the horizontal mixing of water droplets and ice/snow crystals in violent low-level turbulence right behind a passing cold front with temperatures near zero Celsius may occasionally occur so vigorously that charge separation and lightning discharges may result. I've also observed thundersnow events which seem to result from conventional vertical convection; I believe the thundersnow of the Veterans' Day event in 1987 was a result of ordinary vertical convection. The event of Dec. 31, 2008 was also the result of a thunderstorm, in this case a relatively low-topped thunderstorm associated with a passing snow squall.

Posted by: Bombo47jea | December 22, 2009 12:22 PM | Report abuse

Nice summary. Thank you very much, as I was driving south on I-95 Friday afternoon doing my best to escape back to Florida before the worst hit, so I had no time until now to recap the storm. What a drive!

I remember climbing up the steps out of the G.W. University gym when I was in college one cold day to see falling snow accompanied by thunder. Quite impressive all around and I haven't experienced it since. I wonder if anybody had thundersnow this time around?

Posted by: BoteMan | December 22, 2009 12:28 PM | Report abuse

Many thanks for that documentation. Pet peeve of mine that weather doesn't get its due, graphics-wise. I mean, it's one of the most awesome events anyone can experience -- nothing less than, anong other things, the direct effect of an entire planet pivoting slowly on its axis. And the tools are there to convey some hints of the majesty of an event like this, without sacrificing information and accuracy. Water-vapor imagery, for one, which always shows how all weather is global. But most of time all we get is little 3-hr radar loops set to bad New Age muzak on the Weather Channel. Once in a while, a little Zarathustra or Messiah is in order.

Posted by: Groff | December 22, 2009 1:19 PM | Report abuse


A great way to see the BIG picture is to use Google Earth. If you do not have it, it can be downloaded here .

Once installed and running, click "clouds" on the "layer" selection panel on left hand side.

Posted by: SteveT-CapitalWeatherGang | December 22, 2009 2:57 PM | Report abuse

The European model made an excellent forecast four days out. GooFuS had it scooting out to sea.

ECMWF 96-hr forecast and verifying analysis @


Posted by: toweringqs | December 22, 2009 3:16 PM | Report abuse

We were driving up from Florida and hoped to beat the storm. We failed. Became a 23 hour drive.

Boteman is lucky he left in the afternoon. Due to trucks stuck on the Richmond-Petersburg turnpike, traffic southbound backed up from Richmond almost to Woodbridge. We talked to one person who left Fairfax at 8pm and had only reached Ashland (near King's Dominion) at 3am.

Posted by: rrosen3 | December 22, 2009 4:59 PM | Report abuse

Thanks, Steve. Glad you covered the banding with more detail -- actually mentioned that maybe I was missing that in the post! There was probably more to include but I think this thing is well over 1mb as is. ;)

Posted by: Ian-CapitalWeatherGang | December 22, 2009 6:32 PM | Report abuse

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