Snow, ice, and/or rain possible next Sunday into Monday

The season’s first shot for accumulating frozen precipitation is appearing in long-range model simulations. As arctic air seeps into the region this coming weekend, a wave of low pressure will approach from the southwest. This combination means we have a chance of snow, ice, and/or rain next Sunday into Monday.

I’ll begin with the usual qualifier that forecasts 6-7 days out carry a lot of uncertainty and can change substantially. So, at this juncture, it’s best to look at the big picture, rather than specifics.

Here’s what we know:

* Cold air, courtesy of an arctic front, should reach the region during the day Saturday.

* A storm system – with a large area of precipitation – is likely to approach the region from the southwest Sunday.

* A large, strong area of high pressure – feeding cold air south – will exist to the north of the region – a necessary ingredient for frozen precipitation in our region most of the time.


GFS Model shows strong high pressure north of the District Sunday morning (WeatherBell.com)

* Most models simulate a mixed bag of precipitation over the region Sunday into Monday.


GFS model shows mixed precipitation over the region Sunday afternoon.  The European model, not shown, generally shows less precipitation than the GFS. (WeatherBell.com)

* In these cold air “damming” situations, where cold air drains south at low levels and warm air overrides it at high altitudes, cold air often hangs around near the surface longer than forecast.

* Temperatures in the days prior to the storm will be relatively mild, so the ground will not be all that cold.

Here’s what we don’t know:

* How heavy the precipitation will be

* The exact track the storm will take and the make-up of the precipitation. The farther to our west the storm passes, the less frozen precipitation we will see, as southerly winds ahead of the storm would change any frozen precipitation to rain. If the storm tracks close to us, a wintry mess is likely, with snow and/or ice accumulation. (If the storm passes to our south and then east, substantial snow would be possible – but models don’t really show this scenario.)

Here’s our initial thinking, overall:

The overall set up seems conducive for a period of mixed precipitation – especially for the District and points north and west, which probably changes to rain. We cannot rule out a period of snow, but don’t see this as a snowstorm.

“Even though it’s a type of storm that can produce winter weather, it’s not the kind that should produce a big snow,” says Capital Weather Gang winter weather expert Wes Junker, who will analyze this storm in a post tomorrow.

There is an outside risk that this turns into an ice storm for some part of the region (colder areas to the north and west of the District), but that would require several different ingredients to come together, which are too uncertain to forecast at this time range.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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Jason Samenow · December 2, 2013