Over the past few days, we've been tracking the potential for three winter threats this weekend and next week. The first two, today and tomorrow, are mostly transformed into non-events, although a wintry mix tomorrow evening could have a small impact in our northern and western suburbs. However, the third Tuesday night through Wednesday could bring a significant winter storm to the region. The short summary for all three systems:
- Today's system as we predicted on Friday, missed the DC area in the south, with a light coating of winter mix near Richmond ] The Sunday system should bring a wintry mix of rain, sleet, and possibly some snow during the afternoon to Sunday evening, allowing for a slight grassy buildup. Most places should stay warm enough to avoid buildup on the sidewalk. However, areas north and west of the Beltway may freeze close enough for a time on Sunday evening to cause uneven frost on untreated surfaces.
- Tuesday night through Wednesday still shows the potential for a significant winter storm with school delays and closures on Wednesday. But as confidence increases in a period of snow from late Tuesday evening until Wednesday morning, it is still too early to get into certain clumps, and there remains much uncertainty as to how slowly or quickly the snow will turn into a wintry mix and finally Wednesday could rain in the afternoon afternoon.
Full Discussion on Tuesday Night – Wednesday Storm
Since Friday, the potential for a winter storm on Tuesday and Wednesday has not changed much. The stage is ready for a wintry mess, as the high pressure in our north encloses the cold air at the surface, while low pressure nears with much moisture from the southwest. Commuting on Wednesday morning is likely to be influenced by the accumulation of snow and possibly mixed with the sleet.
Models predict that precipitation will begin in the late Tuesday night or early Wednesday morning, but then sleet will pass over Wednesday night. The timing of this transition is still uncertain and will depend on how quickly the warm air enters the middle atmosphere layers and how quickly the cold air near the surface is removed. The latter is a tricky question, as low-level cold air can often last longer than predicted, especially in our northern and western suburbs.
Why do we think that the rainfall will change from snow to sleet, freezing rain and rain? As the storm's low pressure center is likely to drain to the west, a warm middle-level airflow may be supplied, indicating that temperatures are above freezing at that altitude. How quickly this warm layer of air penetrates our region and how much precipitation begins determines how much snow we will get before a conversion.
The models currently differ in important details. Last night's European model predicted a few inches of snow before switching to sleet and sleet late on Wednesday morning or early afternoon. The European model this morning suggested something less, but still more than the American model closer to 1 to 3 Inch predicts, as it is slower and initially starts to accumulate more snow.
Similar patterns rarely produce more than four or five inches, and even so much snow would be unusual given the overall configuration. The various runs of the American model, known as its cast member, offer a reasonable range of snow predictions from dust up to 4.6 inches with an average of two inches. At the moment, we would be leaning on these lower volumes, but that might change, and we should have a better idea of accumulations as we get closer.
The models also differ in how long the sleet and the ice-cold rain last. Both suggest that snow and sleet might persist into the early afternoon hours near and north of DC. The European model suggests that the freezing rain in our typical northern and western suburbs could last until Wednesday night, and today's Canadian model keeps the surface temperature near freezing in our western suburbs until Wednesday night. Chill air often hangs in our northern and western suburbs for so long and sometimes longer than the models predict.
Temperatures are expected to rise above freezing throughout late Wednesday night until Thursday morning second low pressure off the Mid-Atlantic coast and pulls into our north and westwards our low river. This should lead to warming and eventually drying behind the storm.
The bottom line is that we're increasingly confident that winter weather will affect commuter traffic on Wednesday mornings and that the snow and winter mix will eventually turn to rain. This transition should take place around the city on Wednesday afternoon, heading south and east, but we are not sure how late in the afternoon or at night freezing rain can take you moving north and west of the district. Hopefully, as we get closer, we'll have a better idea of snow accumulation and what effects a freezing rain can have on roads and power lines.