Wintry-mix, cooler air head for D.C. area

WASHINGTON — The ingredients are coming together for yet another mixed bag of precipitation in the region Sunday, which could mean snow and sleet over the weekend.

The action began Thursday as spring-like warmth mixed with rain and colder air began to move into the area. Patchy fog rolled in along with a morning drizzle, cloudy conditions and highs in the 60s. A light mist took us through the evening.

A rainy cold front will scoot by late Friday, stalling just south of the area on Saturday. Friday will see areas of rain and temperatures that will go from the 60s to the 40s.

Behind the front, a cold air mass will unleash temperatures as low as the 30s during the day Saturday.

The taste of winter will come Sunday. As the forecast stands, the precipitation will begin as a period of snow/sleet across the region. The start time is still a little difficult to pin down, but it may be from after midnight to daybreak. The change from snow to freezing rain will be seen east of the mountains and late morning along I-81.

Although ground temperatures are fairly warm, light accumulations of snow are possible before a change to a mix freezing rain/sleet/snow. Temperatures will struggle to get out of the lower- to mid-30s through the day on Sunday. By the evening hours, the warm air begins to win over the column of cold air resulting in plain rain.

The change in the air will lead to rain Monday. Rain will continue through the overnight hours and through the day on Monday.

The best thing to do is make plans to stay inside on Sunday. And if you are headed to the Redskins vs. Chiefs game, just keep an eye on the the wintry forecast.

Below is a map showing temperatures at Reagan National Airport. The temperature will increase through the overnight hours and through the day Monday.

Temps

Below is a surface map showing that cold front knocking on our door Friday night.

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An area of low pressure will develop in the Deep South (Mississippi-Louisiana vicinity) and continue to ride along the stalled front, tracking to the west of the D.C. area Monday, bringing a swath of warm, moist air with it.

A ridge of high pressure will move in from the north on Saturday, nudging south before being pushed north by the approaching low from the Deep South.

That high will set us up for a cold air damming situation — something we often see during the winter months.

Since the winds around an area of high pressure swing clockwise, an easterly wind with the high sitting to the north will bring moisture off the Atlantic. This feeds cold air in the southern lower levels that bank against the east side of the Appalachians.

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Cold high pressure and surface wind flow typical of cold air damming situations.

As the low temps move north, the warm air will pump toward the low-level cold air locked in place along and east of the Appalachians, eventually overriding it. Once the warm air moves over the cold air dome, it squeezes out precipitation and warms the “cold air dome,” trying to erode it.

Eroding the cold dome takes time and that’s where we find the major problem. It’s quite a task to erode the cold dome because as precipitation falls into a relatively dry air mass, evaporation causes it to cool even more (at first) until it becomes saturated. Considering the pesky low-level feed of cold and dry air from the northern high, it will help to reinforce the cooling as the mass above releases precipitation into the air.

When we break down that cold air mass we can expect the rain to change in the region.

What to Expect

The latest information from the Global Forecasting System model shows a period of sleet (orange) beginning Sunday morning.

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This will gradually change to a mixed bag of precipitation by the early afternoon with a mix of snow, sleet and freezing rain.

By Sunday night, freezing rain should take hold with plain rain slowly creeping in from the south and the east before a changeover to rain late Sunday night with freezing rain holding in the Shenandoah Valley.

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The low heads toward the Great Lakes by Monday morning after the switch-over to rain and all areas in the D.C. region will be in the warm sector.

Rain will continue to fall through the afternoon hours on Monday.

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Of course, this is likely to change as the actual weather event closes in, but I agree with the Global Forecasting System. We could see a few areas of snow initially, but I believe this will be a freezing rain and sleet only event that could create problems heading into Sunday afternoon. Specifically for those fans coming up for the Washington Redskins vs. Kansas City Chiefs game on Sunday afternoon.

Not only does the game time look a little dicey, but traveling there and back could be a major headache with the way the forecast currently stands.

Rain vs. freezing rain vs. sleet vs. snow

Below is a great graphic showing the difference in the precipitation types. All precipitation begins as snow in the upper atmosphere. For snow, the entire column of air is at or below 32 degrees. For sleet, the snow falls into warm air, turns to rain and then refreezes and falls as sleet or ice pellets.

Freezing rain is almost the same process as sleet but it doesn’t have time to refreeze into ice pellets since the column of warm air is just slightly larger. The raindrops freeze on contact with the earth’s surface or with power lines, trees, sidewalks, cars, etc., if the surface is at or below freezing.

For rain, the column of air is warm the entire way through.

With this wintry mix coming off a very mild week, it will take some time to overcome the warm ground temperatures. Many questions still remain surrounding this event. The main question is: How much of this wintry mix will fall before the low level cold air erodes?

That question will dictate how many problems we could have Sunday evening before the eventual switch to rain.

Here’s to another wintry mix storm!

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