Tuesday, February 12, 2013

Feb 12, 2013: Nuisance Snow Event Wednesday Night

An area of low pressure beginning to form along the Gulf Coast, today, will intensity and track northeast tomorrow (Wednesday), passing south of Baltimore Wednesday night.   This afternoon's weather map shows the low organizing a sizeable shield of precipitation across the southern Plains and southeast U.S.:

The forecast centers on two main questions:  (1) How far to our south will the heaviest precipitation track; and (2) what form will the precipitation take?  The latest model runs suggest we will pick up between 1/4"-1/2" precipitation:

Forecast map, 1 AM Thursday, Feb 14 (Unisys Corp)
The form of the precipitation - liquid vs. solid - depends on the thermal profile in the atmosphere's lowest 5,000 feet.  Most scenarios suggest enough warm air initially present to start as rain, but with sub-freezing air pulled in on the backside of the system, the rain changes to snow.  Assuming an 8:1 to 10:1 snow:liquid equivalent, we're talking 1"-2" in general.  The rain-soaked ground will probably melt much of this on contact.  If roads are adequately pre-treated, and treated during the event, they may remain wet or lightly slushed.   So this event should mainly be a light, grass-surface snowfall.   The system is not very intense, and it moves through our region briskly - both factors that will also limit accumulation.

Here is the official NWS snow accumulation forecast:

The snow amounts are higher to the north and west of the metro, for two reasons:  (1)  these regions will remain in the subfreezing air for a longer time;  and (2) the air will be sub-freezing at higher elevations in the mountains.   A shift in the forecast track can change this pattern, and if a small-scale snowband sets up - as some models are suggesting - snow amounts may be locally higher by several inches (often these bands are not well forecast).

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