For the past several days we’ve been tracking the potential for three wintry threats this weekend and during the coming week. The first two, today and tomorrow, are turning into mostly nonevents, although a wintry mix could have minor impact tomorrow evening in our north and west suburbs. However the third, Tuesday night into Wednesday, could bring a significant winter storm to the area. The quick summary for all three systems:
- Today’s system, as we predicted Friday, has missed the D.C. area. There is a light coating of wintry mix down around Richmond.
- Sunday’s system should bring a wintry mix of rain, sleet and, maybe, a bit of snow during the afternoon into Sunday evening, with a light accumulation on grassy areas possible. Most spots should stay warm enough to prevent accumulation on pavement. But areas north and west of the Beltway may dip close enough to freezing for a time Sunday evening to cause patches of ice on untreated surfaces.
- Tuesday night into Wednesday continues to show the potential for a significant winter storm, with school delays and closings possible Wednesday. But while our confidence is increasing for a period of accumulating snow Tuesday night into Wednesday morning, it is too early to get into specific accumulations, and much uncertainty remains around how slow or fast the snow would change to a wintry mix and eventually rain on Wednesday afternoon and into the night.
Detailed discussion on Tuesday night-Wednesday storm
Not much has changed since Friday regarding the potential for a winter storm Tuesday night and Wednesday. The stage is set for a wintry mess as high pressure to our north locks in the cold air at the surface, while low pressure with plenty of moisture approaches from the southwest. The Wednesday morning commute probably will be affected by accumulating snow, possibly mixed with sleet.
Models predict the precipitation will start as snow very late Tuesday or early Wednesday morning, then transition to sleet, freezing rain and eventually rain by Wednesday night. The timing of this transition is still uncertain and will depend on how quickly the warm air comes in at the middle levels of the atmosphere, and how fast the cold air near the surface is scoured out. The latter is a tricky question, since low-level cold air can linger longer than forecast, especially in our northern and western suburbs.
Why do we think the precipitation will change from snow to sleet, freezing rain and rain? Because the storm’s low-pressure center probably will track to our west, helping to feed a warm flow of mid-level air, which suggests temperatures at that height will warm above freezing. How quickly this warm layer of air pushes into our region, and how heavy the initial push of precipitation, will determine how much snow we get before a changeover.
The models differ on critical details. Last night’s European model forecast several inches of snow before a changeover to sleet and freezing rain late Wednesday morning or in the early afternoon. This morning’s European model suggested a little bit less, but still more than the American model, which predicts closer to 1 to 3 inches, as it is slower with the initial onset of heavier accumulating snow.
Similar patterns rarely produce more than four or five inches, and even that much snow would be unusual given the overall setup. The different runs of the American model, known as its “ensemble members,” provide a range of snow forecasts from a dusting to 4.6 inches, with an average of two inches. At the moment, we would lean toward these lower amounts — but that could change.
The models also differ on how long the sleet and freezing rain will last. Both suggest that snow and sleet could last into the early afternoon hours near and just north of Washington. The European model suggests that the freezing rain could hold on in our northern and western suburbs into Wednesday night, and today’s Canadian model keeps surface temperatures near freezing in our western suburbs through Wednesday night. Colder air often does hang around as long, and sometimes longer, in our northern and western suburbs than the models predict.
Temperatures are expected to climb above freezing everywhere late Wednesday night into Thursday morning as a second low-pressure system forms off the Mid-Atlantic Coast and moves to our north, switching our low-level flow to a westerly direction. That should lead to warming and eventually some drying behind the storm.
Bottom line: We’re increasingly confident that wintry weather will affect the Wednesday morning commute, and that the snow and wintry mix will transition to rain. This transition should happen by Wednesday afternoon around the city and points south and east, but we’re unsure how late into the afternoon or night freezing rain might last as you head north and west of the District. As we get closer, we should have a better idea on snow accumulations and how much of an effect any freezing rain might have on roads and power lines.