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Posted at 12:27 PM ET, 02/21/2013

Remnants of massive Midwest storm may make for sloppy Friday PM commute in D.C.


Precipitation (liquid equivalent) over last 24 hours in Midwest - a lot of this in the form of snow, with widespread amount over 6 inches in Kansas and western Missouri. This iprecipitation is much needed given drought in the region.

An intense, sprawling storm has crippled parts of the Plains and Midwest with near-historic amounts of snow.

Thundersnow has been widespread. Between 9 and 10 a.m. (local time) alone, Kansas City picked up 3 inches and the airport is shut. Wichita, which reported thundersnow for 2.5 hours, has picked up over 10 inches and is closing in on its all-time snow record of 15 inches. (The snow is great news for the drought in that part of the country.)

This impressive, moisture rich weather system - the biggest of the winter in the Midwest - will not carry the core of its heavy snow and icy precipitation to the D.C. area. Instead, it will weaken, with the storm center lifting almost due north into Canada.

A piece of the storm, however, will survive, spreading a bit of moisture over the cold air in the Mid-Atlantic Friday afternoon. As such, we’re expecting a period of light frozen precipitation, coinciding - unfortunately - with Friday’s PM commute in Washington, D.C.


Generally light precipitation is simulated between 1 and 7 p.m. on the NAM model in the D.C. area, around 0.2” liquid equivalent but temperatures should be near freezing making for a wintry mix. (StormVistaWxModels.com)
Precipitation is likely to begin between 1 and 4 p.m., and may briefly be in the form of snow, especially north and west of the District. Based on our analysis of temperature profiles, we expect sleet to be the predominant from of precipitation with snow accumulations less than one inch.

Temperatures will be near freezing when the precipitation falls, cold enough for a light coating of frozen precipitation on roads if some moderate to heavy areas of sleet develop. Generally, though, we expect precipitation intensity to be light. As such, we do not expect this to be a high impact winter weather event, but rather low to moderate (we mention moderate only because the precipitation will coincide with the P.M. commute...otherwise, it would be a low impact event).

The precipitation should taper off between 7 and 10 p.m., and may transition from sleet to freezing drizzle or just plain drizzle (from the District and south and east).

Another round of precipitation may develop Saturday morning, but by that time, temperatures will have warmed sufficiently for mainly plain rain, except a chance of some light freezing rain well north and west of the District towards I-81.

Why sleet?

The reason we’re expecting mostly sleet as opposed to snow or freezing rain, is due to a warm layer about 5,000-6,000 feet above the surface. Snowflakes originating at cloud level will melt in this layer, but then refreeze into ice pellets, aka sleet, beneath that warm layer before landing (and bouncing) on the ground. See the forecast temperature profile below....


Illustrated temperature profile from the NAM model shows warmer layer above 850 mb (or 5,000 feet) which will likely melt snow, but cold layer beneath it will refreeze melted snow into sleet. (TwisterData.com, adapted by CWG)

The warm layer comes courtesy of a warm front associated with the weakening Midwest storm, pushing northward. While this warm layer will initially be thin (and at a high altitude), it will eventually widen towards the ground such that the refreezing process making sleet will stop and we’ll have rain (or freezing rain, as long as surface temperatures hover near freezing.)

By  |  12:27 PM ET, 02/21/2013

 
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