After a duo of 70-degree days this weekend, Washington’s average January temperature is 10 degrees above normal. But a shift toward colder weather arrives later this week, ushered in by a potentially nasty mix of frozen precipitation.
Snow, sleet and freezing rain are possible in the Washington region starting during the first half of Saturday before possibly transitioning to rain or drizzle later in the day as temperatures slowly rise. Some accumulation of snow and ice is possible.
However, as the storm is still five to six days away, details regarding the exact timing, types and amounts of precipitation will take a few more days to come into focus.
“It’s too early to provide details about the timing of the onset of icy weather or how long it might last but, if you are planning on traveling Saturday, monitor the local forecasts,” said Wes Junker, Capital Weather Gang’s winter-weather expert.
Weather forecasting models are fairly unified in the idea that a large Arctic high pressure zone will park itself to Washington’s north on Friday, feeding cold air south. This is a phenomenon known as cold air damming and supplies the chilly air required for many of the winter weather events in the Mid-Atlantic.
As the cold air is filtering over the region, a storm system will organize just east of the Rocky Mountains.
Precipitation will streak eastward as this storm system scoots from the Plains toward the Midwest late Friday, potentially arriving in the Mid-Atlantic predawn Saturday or Saturday morning, or even late Friday night toward the mountains.
Enough cold air may be in place for precipitation to take the form of snow when it begins before transitioning to sleet and freezing rain. As temperatures slowly rise during the second half of Saturday, precipitation should ultimately change to plain rain or drizzle. However, accumulating frozen precipitation could occur earlier.
As is customary with these wintry mix events, the most snow and ice and the longest duration of frozen precipitation should be expected in Washington’s colder suburbs north and west of the city.
Because the storm is predicted to track northwest of Washington, it will draw enough mild air northward that this will not be an all-snow event. Temperatures will slowly rise, first several thousand feet above the ground, changing snow to sleet and freezing rain, and then near the ground, changing sleet and freezing rain to liquid rain. In these cold air damming situations, however, the milder air often is slow to establish itself, which can prolong the period of frozen precipitation.
Over the next few days, we will try to get a better handle on what time the precipitation will move into the area, how heavy it will be, how much will fall and when the transitions from snow to ice to rain will occur.
Small shifts in the storm track, timing and the amount of cold air available will have important implications for how significant a winter weather event this becomes.
At the moment, the computer model consensus is that precipitation will end Saturday night, with sunny but blustery conditions on Sunday.
The cold air arriving on Friday will mark the end of a much milder-than-normal weather pattern which began just before Christmas. Every day but two since Dec. 23 has seen above average temperatures (18 of 20 days).
Near normal to colder-than-normal weather is possible on most days starting Friday into late January.
Beyond that, there is much greater uncertainty as to how the winter will play out.