Tuesday, March 6, 2012

March 5: Surprise Snowstorm Narrowly Misses Washington-Baltimore

Perhaps unbeknownst to some, the Washington-Baltimore region narrowly escaped a major snowstorm on Monday morning, March 5.    Had the track of the parent low pressure system jogged 100 miles further north (a minor deviation given the large size of these weather systems) our region could have been blanketed with a fast 6”-10” of dry, powdery snow…in addition to hours of severely reduced visibilities on the highways.
The NWS correctly predicted that a fast-hitting “clipper” snowstorm would track south of DC, dropping up to 2”-4” inches across western and central Virginia.   The appropriate winter weather advisories were issued.   Then, in the midst of the mid-morning snowfall, a small, nearly-stationary band of heavy snow set up across several central Virginia counties.   Snowfall rates intensified.   The NWS rushed out a winter storm warning for the affected counties.  Not long after the warnings were posted, the heavy snow came to an end.    To be fair to NWS, the very latest model predictions that morning hinted that a heavy snow band might set up across central VA.   As soon as it became clear that mother nature was “cooperating with the models” (radar trends indicating an increase in snow rates), NWS acted as quickly as it could to elevate the level of warning.
Below is the Monday morning synoptic chart showing the low-pressure clipper system tracking across Virginia:
Here are the warnings issued by NWS (notice how small of a geographical area was impacted):
  And here are the snow reports:
Even with fairly sophisticated forecast models, we still struggle to predict smaller-scale atmospheric processes, such as those that set up narrow bands of intense snow. 
So…what happened?   The regional analysis (below) shows the details.   This is an analysis at the 5,000 foot level, where the atmosphere manufactures snow.   The solid black lines are isobars of pressure.  The large dip across the Mid Atlantic is a trough of low pressure, causing vigorous ascent of cold, moist air over central VA.  The narrow band of solid green (oriented west-east across central VA) is the heavy, nearly-stationary snow band identified on radar.  This band formed along a warm frontal boundary stretching across the southern part of the state.   But the key here is that this front was rapidly intensifying, meaning it was lifting cold, moist air more vigorously through the morning.   The technical terms is “frontogenesis”, and the three purple contours surrounding the snow band show where the rate of frontal intensification was greatest.   In other words, the snow band set up within this small bulls-eye of frontal strengthening.
  In closing, it is rare that a fast-moving clipper system such as this produces as much as 10” of snow.   It seems that this storm, for a brief few hours, was able to pull in additional moist air off the Atlantic as it tracked east of the Blue Ridge.   Combined with a strengthening front, and sufficiently cold air during the early morning hours, nature assembled a winning combination of ingredients for big snow.

Thursday, March 1, 2012

Feb 29, 2012: A Day of Unexpected Flooding in the Mid Atlantic

The same potent weather system that brought severe weather to the central Plains on Feb 29 (including the Branson, MO tornado) created a very rainy day in the Mid Atlantic.   Periods of moderate to heavy rain lead to localized flooding in the Baltimore region and prompted the NWS to issue several flash flood warnings during the afternoon.  Thunderstorms were even reported in our region during the late afternoon.
The weather map below portrays a well-developed wave cyclone (extratropical cyclone) over the Great Lakes, approaching the Mid Atlantic.   Three distinct air masses are brought into juxtaposition around this low:  (1) cool, humid air streaming from the northeast off the North Atlantic ocean (the air mass that lay over Baltimore on the 29th);  (2) a “warm sector” of warm, humid air streaming toward the north from the Gulf of Mexico, blanketing the entire southeast U.S., ahead of the cold front;  and (3) a cold, dry Canadian air mass that swept from the northwest around the back side of the low.
The warm sector spawned widespread severe activity over the eastern U.S.  Lines of severe thunderstorms generated tornadoes and damaging straight line wind across the central Plains early on Feb 29.    Severe thunderstorm watch boxes (red) and tornado watches (yellow) are shown on the map above (7 pm Feb 29),  covering most of Appalachia and the mid-South. 
Over Baltimore, batches of heavy rain swept repeatedly from west to east through the day.   This type of “training” was facilitated by the warm front (shown in red in the chart above).  Along this front, warm and humid air from the warm sector streamed over cooler air at the surface.   Continuous uplift of humid air established a corridor along the front,  oriented west to east across our region.   Winds aloft were blowing from the west, parallel to this boundary, pushing new batches of heavy rain toward Baltimore from the west.
This chart shows the rain accumulation from yesterday, derived from rainfall estimates.   You can get a sense for the west-to-east corridor of heavy rain stretching from WV across VA, MD and PA to the Eastern Shore.  There were widespread 1”-2” amounts with some pockets in excess of 2” just southwest of Baltimore.