The culprit, as is typical for this region and this time of year, was a compact and powerful extratropical cyclone that developed and amplified along a stationary front draped through the Mid Atlantic.
|Synoptic surface chart at 7 pm, December 7 (unisys.com)|
Yes, that is a severe thunderstorm watch box (yellow quadrilateral) over southeast Virginia. Although no tornadoes developed, there were about 40 reports of wind damage due to severe thunderstorms.
The storm had a very sharply demarcated warm and cold side. Washington-Baltimore was fortunate to remain in the warm sector of this system throughout the duration. The flip side is that rainfall in excess of three inches fell across most of our region, setting new all-time December 1-day rainfall records at the area's three airports. Despite the threat of 1"-3" of heavy wet snow, the moisture and energy exited our region faster than sub-freezing air could arrive from the northwest. Heavy rain totals were due to five factors: (1) the overall slow movement of the stationary front; (2) training (repeated passage) of storm cells over the same locations; (3) high precipitable water content (abundant Atlantic moisture feeding into the storm); (4) very energetic conditions in the upper atmosphere that favored vigorous ascent of air; and (5) strongly "frontogenetic" conditions i.e. a rapidly intensifying front as the wave of low pressure deepened.
|Radar view of the storm, showing a plume of heavy rain extending across the region. Embedded reds indicate thunderstorms. Blue colors indicate moderate snowfall (weathertap.com)|
|Radar-derived rain accumulation for the December 7 storm (NOAA)|
|Pattern of isobars (black solid lines) - notice the extremely large pressure gradient over western Maryland and West Virginia. Red solid lines indicate regions where the surface pressure is abruptly rising (NWS).|
The final image (below) shows that great variety of weather watches and warnings that were posted for our region, at the height of the storm. It is quite clear that Washington-Baltimore was on the thin dividing line between cold and warm weather hazards. Whenever you see lots of different colors such as this, look out!