Tuesday, January 27, 2009



UPDATE: TUE JAN 27 - 10:45 PM. If you have not already heard, the Sterling VA NWS extended and expanded Winter Storm Warnings to include the entire Baltimore-DC Metro Area until 12 PM / 6 PM tomorrow depending on your location. Southern Pennsylvania is experiencing a similar outcome, albeit more snow than ice for tonight and Wednesday. For the areas shaded above in "danger pink" the result will be a quarter to a third an inch of ice accretion across the Maryland into the morning hours. This will effectively seal the slam dunk for schools and colleges in Maryland / central & northern Virginia. Prepare your favorite radio station next to the bed, and get ready to hear a long list. Scheduled inservice events at Anne Arundel County and elsewhere will be canceled and offices placed on liberal leave.

STORM ANALYSIS: A rundown of the main factors that led to this expansion of the storm's impacts includes the following data:

1. The upper level low at 850 millibars (and on up to 500, about 35,000 feet) has intensified to a deeper core pressure. It is acting like a "bowling ball" as it moves north and east, forcing the "pins" in the flow to be shunted northeast, and the high offshore has helped setup a pressure gradient, which amplifies the flow ever further. This produces a strong fetch of moisture from the Gulf of Mexico forced into a fast upper level current riddled with shortwaves.

1-28 850 00z shortwaves

2. The "turning" of the air currents in a northeasterly direction is what has introduced warmer air changed snow over to sleet from central Maryland on south. As the surface and upper level lows approach from the Tennessee valley overnight, the atmospheric instability increases, in effect squeezing out more moisture as the air has less room to manuver, and hence greater precip amounts. A warm front will eventually cross Maryland from south to north, but not until at least noon on Wednesday. Result: slam dunk and clean sweep.

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