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 29^th);  (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.