January 16, 2013: Winter Storm Iago Coming

10 PM UPDATE - MODEL TREND IS FOR SLIGHTLY LESS SNOW ACCUMULATION

The Weather Channel will likely name a fast-developing, and fast-moving coastal low to impact the Mid Atlantic tomorrow, Winter Storm Iago. 

The storm will spread a swath of snow across the Central Appalachians, West Virginia, Virginia, Maryland and Delaware, as it moves out of the Tennessee Valley and offshore late tomorrow night.

Based on the current model storm track, the center of the action is expected to stay south of the Washington-Baltimore region.  However, with sub-freezing air in place near the surface, and abundant moisture pulled into the storm, accumulating snow is likely to reach as far north as Baltimore.

The first figure, based on a computer model forecast for tomorrow evening, shows the surface low (1007 mb) over the North Carolina-South Carolina coastal border.   Colors represent precipitation amounts.  D.C. - Baltimore is on the far northern fringe, with 1/4" to 1/2" precipitation totals.  The precipitation bullseye - painted red, to the tune of 2" - is poised over the Central Appalachians, hundreds of miles to our southwest.

Surface Weather Forecast, Thursday evening (Unisys Corp)

The precipitation amounts are so high, partly because impressive jet stream dynamics are energizing this system - including a vigorous and intensifying wave (trough) in the jet stream, and a fast-moving pocket of air called a jet streak.

Below is another computer model forecast of snow amounts for tomorrow-tomorrow night:

Forecast Snow Amounts for Thursday-Friday (Weatherbell Analytics)

That's over a foot of heavy, wet snow in southwest Virginia.  D.C.-Baltimore, again, are on the northern fringe of the snow shield.   Snow that falls on the northern edge in drier, colder air will likely be dry and powdery.  

Finally, the NWS in Sterling has now issued a swath of Winter Storm Watches, including D.C., counties to the immediate south and north.  The city of Baltimore and Baltimore County are NOT included at this time:

Winter Storm Watch for Thursday (NWS)

Here is the latest snow accumulation guidance from NWS Sterling - note the large north-south gradient in expected snow totals, consistent with our reasoning on this storm:

10 PM - Forecast Snow Totals for Thursday(NWS)

It's likely that Baltimore will get placed in a Winter Weather Advisory, once the storm track becomes better refined.  Regardless, timing of this event is expected to be very bad - with a period of moderate snow coinciding with the hours of tomorrow's evening rush.

Of great interest, tomorrow we will be watching the radar evolution of this system very closely.  A narrow band of heavy snow, with snow rates up to 2" per hour, is expected to set up somewhere across Central VA.   Depending on the exact track of the storm...that snowband could shift north, as there has been a tendency in the models for the storm to track closer to D.C.-Baltimore.