The evolution of a monster D.C. area snowstorm
Dec. 17-20, 2009: Simulations, satellites, radars, and maps
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.
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.
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.
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.
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