Winter Storm Watch Posted

Beautiful water vapor image of a potentially ugly storm

Mid week winter storm on Gulf Coast with 1009 mb central pressure

Severe Storms Prediction Center slight risk for Florida

As our mid week winter storm continues to develop on the gulf coast, the National Weather Service has posted a Winter Storm Watch for Wednesday afternoon and night.  The “Watch” means there is a “potential” for significant winter weather within the next 48 hours.  It is interesting that the potential for significant winter weather is increasing even though compared to yesterday’s frigid arctic blast, temperatures have warmed considerably today as the arctic high pressure moves off the Maine coast.  The storm on the Louisiana coast has a central pressure of 1009 millibars and is showing signs of strengthening, kicking up thunderstorms over the gulf waters and prompting the Severe Storms Prediction Center to outlook most of Florida with a “slight risk of severe weather”.

Gulf Coast storm forecast to track up the eastern seaboard

As the storm tracks northeast tonight and tomorrow, our big forecasting dilemma will be trying to figure out how cold the atmosphere will be at various critical heights above the ground.  It seems archaic, but even in the 21st century we still rely on the data gathered by weather balloons launched around the world a couple times each day to help us determine just how warm or cold the atmosphere is.  Around here, the folks up at the Aberdeen Proving Ground will frequently launch a balloon in the early morning, but our closest regular balloon launch is done just outside of Dulles airport in Sterling, Va.  The balloons launched this morning at these two sites revealed temperatures just under freezing (-1.5c at APG and -0.9c at IAD) at the rain/snow critical level of 4500 to 5000 feet above the ground.  In the mid Atlantic region temperatures usually need to stay below freezing at this level for some snow and sleet to fall.  Warmer temperatures at this level usually produce rain.

Freezing line at critical upper level 7:00 a.m. Wednesday

Freezing line at critical level 10:00 a.m. Wednesday

Freezing line at critical upper level 7:00 p.m. Wednesday

So, how cold will this critical level be as the gulf coast storm arrives late tonight and early Wednesday?  The latest run of our in-house RPM model shows the precipitation reaching central Maryland between 2:00 a.m. and 4:00 a.m.  At that time most of Maryland north of the Bay Bridge is forecast to be cold enough for at least some snow and sleet to be in the precipitation mix.  By 10:00 a.m., however, the freezing line at that critical level has pushed farther north and west, meaning rain, or the dreaded freezing rain (rain turning to ice on contact with colder surfaces), will be pushing into areas roughly south and east of I-95.  As the storm intensifies and begins to pull away, the freezing line at 4500-5000 feet above the surface surges back to the south and east and the mixed precipitation, for most areas west of the bay, will likely change to all snow by 7:00 p.m. Wednesday, perhaps continuing into early Thursday morning.

RPM snowfall forecast ending 1:00 p.m. Thursday

So, with multiple precipitation types and a couple of rain/sleet/snow change-overs, it will be very difficult to determine how much of the total precipitation ends up as accumulating snow.  The 12z RPM is painting a stripe of 2-7″ of snow along and to the west of I-95 by early Thursday afternoon.  Right now it looks like the heaviest snow around here (about 7″) will fall in a band extending from the Catoctin Mountains, perhaps clipping northwest Carroll County, and stretching southwest into the Shenandoah Mountains of northern Virginia  where the RPM is forecasting some isolated 10-12″ totals.  For Kent and Cecil Counties on the upper eastern shore the model is showing 2-4″ of snow by Thursday afternoon with just 1-2″ for the rest of the eastern shore from Queen Anne’s County south, and 1-2″ for the western shore south of BWI-Marshall.

This is an early estimate of the snowfall possible with this storm, but due to the “messy” nature of the expected precipitation the forecast will probably need to be tweaked as the system continues to organize and make its approach from the south.

Tom Tasselmyer

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