This system is proving to be quite the forecasting challenge. As noted in my previous email/forecast, the most significant factor that will determine how much snow the DC metro and neighboring counties accumulate is the rain/snow line. While the rain/snow line related to snow storms impacting our region is often a factor of horizontal cold fronts, this situation is particularly unique in that the rain/snow line with be more so determined vertically through a boundary of colder air aloft and warmer air near the surface. Ground and air temperatures near the surface will be above freezing at the onset of precipitation tonight, and while there will be plenty of colder air aloft to support precipitation in the form of snow, there will be enough “warmer” air near the surface to keep precipitation as rain or a rain/snow mix at times tonight through Wednesday afternoon, before an almost certain full transition to all snow by Wednesday evening at the latest. I’ll outline general major and minor impact scenarios FOR DC AND NEIGHBORING COUNTIES (this is important to note since there could be significant variations in accumulations even within the District) and then provide my forecast. Of the potential outcomes, there is high confidence tomorrow morning’s commute will consist of moderate to significant traffic delays with hazardous road conditions, while tomorrow evening’s commute will be dangerous. Unless there is a drastic change in forecast, schools in DC and all counties mentioned below will be closed tomorrow.
Major Impact Scenario:
Precipitation begins in the form of rain Tuesday night, but becomes heavy enough before midnight to support sufficiently cold enough temperatures at the surface for a transition to all snow. Snow will be heavy at times, likely seeing a strong band move through the region late in the afternoon/early evening as the core of the low pressure system pulls out East/Northeast. The system is slowed in its northward progression by a blocking high sitting over the NE U.S./Canada, allowing for a lasting period of snow that wraps around the backside of the low pressure system and our region experiences additional minor accumulations through very early Thursday morning.
DC: 8-10” (highest totals to the west, lower totals to the east)
Arlington, VA: 6-10” (highest totals to the west, lower totals to the east)
Fairfax, VA: 10-14” in the far western portion of the county, 8-10” central, 6-8” eastern portion
Montgomery, MD: generally 8-10” for most of the county with a possibility of 10-14” in the far West
Prince Georges, MD: wide ranging depending on location within county. 8-10” in the extreme NW, 6-8” for the NW half of the county, 4-6” in the eastern half of the county with 2-4” in the extreme SE.
Minor Impact Scenario:
Precipitation begins in the form of rain Tuesday night. Heavier amounts of precipitation necessary to lower surface temperatures enough to support a transition to all snow stays to the West of DC and neighboring counties to the South and East, resulting in a mostly rain and rain/snow mix to persist through much of the day Wednesday before a transition to all snow as the system exits our region Wednesday evening. The system takes a more easterly path into the Atlantic and the impact of the ridge of high pressure to the North is negligible, resulting in precipitation ending Wednesday evening. Late period of snow from the backside of the system allows for some accumulation before the system and any associated precipitation exit completely.
DC: 1-3” (highest totals to the West, lower totals to the East)
Arlington, VA: 1-3” (highest totals to the West, lower totals to the East)
Fairfax, VA: 2-4” in the western portion of the county, 1-2” in the eastern portion
Montgomery, MD: generally 2-4” for most of the county with a possibility of 4-6” in the far West
Prince George, MD: wide ranging depending on location within county. 2-4” in the extreme NW, at best 1-2” the NW half of the county but likely an almost all rain event for the entire county with an inch or less of accumulation
Models were trending most of the day yesterday for a rain/snow line remaining to the East of DC for the majority of the event, then regressed West of DC late last night only to suggest once more a rain/snow line to the East of DC this morning. Given the vertical complexity of the rain/snow line in this instance, I don’t think the issue will be completely resolved prior to the precipitation entering our area, but will actually throw a few unexpected transitions of precipitation type tonight through Wednesday afternoon before a full transition to snow Wednesday afternoon/evening. It is a certainty precipitation will start as rain for everyone, then I believe there will be a transition to accumulating snow for most during the overnight hours before becoming a rain/snow mix during the morning into possibly the early afternoon (leaning more toward earlier transition rather than later) before a full transition back to all snow for most of us. The system exits to the East/NE, bringing intermediate bands of light to potentially moderate snow that persist through midnight and perhaps linger into the early morning hours Thursday. As highlighted by the wide range of potential snow totals over the relatively small area mentioned above in the major/minor scenarios, the timing of a transition to all snow will vary, but will generally occur in from west to east at a roughly 45 degree orientation (from SW to NE).
Western half of DC: 6-9″
Eastern half of DC: 4-7″
Arlington, VA: 4-8″ (highest totals to the west, lower totals to the east)
Fairfax, VA: 8-12″ in the western portion of the county, 5-8” central, 3-7” in the eastern portion
Montgomery, MD: 8-12″ far western portion of the county, 6-9” central, 5-8” in the eastern portion
Prince George, MD: wide ranging depending on location within county. 6-8” extreme NW, 4-8″ western half, 3-6” eastern half, 2-4” extreme SE.
School Closure Outlook: High confidence for widespread closures Wednesday.
Federal Government Outlook: Does it really matter??? I kid, only medium confidence in closures due to likelihood of eastern portion of DC accumulating lesser amounts than rest of forecast region.