A chance of “conversational” snow in Washington, D.C. Wednesday afternoon

Yes, I said it, it *might* snow in D.C. Wednesday afternoon – right as everyone is trying to get out of town.

BUT, the chance of 1 inch of snow in the immediate area is just 10 percent.

Percent chance of 1 inch of snow across region according to National Weather Service - we agree with this

Percent chance of 1 inch of snow across region according to National Weather Service – we agree with this

If it snows, it’s most likely “conversational snow” (and/or sleet) that may whiten grassy areas and car tops rather than sticking on roads and snarling traffic.  Having said that, we cannot rule out some slick travel in our colder north and west suburbs mid-to-late Wednesday afternoon (30 percent chance).  And, if somehow a heavy snow band develops, there’s a chance things get “interesting” even in the immediate metro area (10-20 percent chance).

[Speaking of 10 percent snow probabilities - we plan to debut a revised Snow Potential Index tomorrow.  In the past, it indicated the chance of measurable snow (more than trace amounts) over the next week on a 0-10 scale.  This winter, we're revising the criteria to make it stricter (and hopefully more meaningful).  It will now represent the chance of at least 1 inch of snow in the next week on a 0-10 scale.]

A difficult forecast

We have high confidence cold air will stream into the region tonight and tomorrow, but we’re not sure whether enough cold air will arrive soon enough at high altitudes for rain to change to snow before the storm system pulls away and the region begins to dry out.

In many cases, snow forecasts on the backside of storms do not work out.  But, when they do work out, it’s usually when a strong front comes through AND there is a pronounced area of upper level energy to generate lift in the atmosphere.  Such lift keeps the precipitation process going, even as the atmosphere is drying out (due to the cold intrusion).

It appears we will have sufficient cold air at high altitudes arriving between 11 a.m. and 3 p.m. for rain to change to sleet and/or snow from west to east.

Simulation of temperatures at altitude of 5,000 feet - a rough approximation of rain/snow line - Wednesday from the high-resolution NAM model (WeatherBell.com)

Simulation of temperatures at altitude of 5,000 feet (in degrees celsius) – a rough approximation of rain/snow line – Wednesday from the high-resolution NAM model (WeatherBell.com)

And, there are a couple packets of upper level energy (spin centers) which will try to keep the precipitation going.

Simulation of upper level vorticity (spin centers/enhanced energy) from NAM model Wednesday afternoon and evening. (StormVistaWxModels.com)

Simulation of upper level vorticity (spin centers/enhanced energy) from NAM model Wednesday afternoon and evening. (StormVistaWxModels.com)

If we see snow, it will probably be in intermittent narrow bands pivoting through the region, with the greatest concentration to the north and west.

Snow could begin along the I-81 corridor before noon, in DC’s western suburbs by 1 p.m., and along and east of I-95, between 1 and 3 p.m.  Any transition from rain to snow may involve an intermediate period of sleet.

The bottom line

The prospects of snow accumulation across the area Wednesday afternoon and evening are low, but they increase as you go north and west.

High-resolution NAM model simulation of snowfall Wednesday. Note this is just one simulation to be illustrative - NOT  a forecast. (WeatherBell.com)

High-resolution NAM model simulation of snowfall Wednesday. Note this is just one simulation to be illustrative – NOT a forecast. (WeatherBell.com)

In the immediate D.C. area, there’s about a 50 percent chance of “conversational” snow.

Chance of at least a trace (conversational) snow Wednesday afternoon from National Weather Service - we agree with this

Chance of at least a trace (conversational) snow Wednesday afternoon from National Weather Service – we agree with this

Due to relatively warm ground temperatures and the limited window of time for snow to fall, accumulation on anything but grassy areas is a long shot.

Also on Capital Weather Gang

A deluge and wild temperature swings tonight in D.C.