... Low pressure will press off to our northeast through this evening as another wave of low pressure tracks along a frontal boundary draped across the Appalachians Sunday. A cold front will push through the area late Sunday night into Monday, with strong Canadian high pressure building into the Mid-Atlantic through Tuesday. Unsettled weather may return to the region the second half of next week.
NEAR TERM /THROUGH SUNDAY/... Evening update: Current thinking remains in line with the previous forecast. Two minor changes were made this evening. The first was to add Hampshire and Hardy Counties to the Flood Watch. Despite not having any remnant snow pack, flash flood guidance is very low over those locations, and it appears that rainfall totals later tonight into early tomorrow morning may approach the 6 hours flash flood guidance values in those areas. The second change made to the forecast was to lower high temperatures tomorrow. Much of the area to the east of the Blue Ridge likely won't make it out of the 40s tomorrow, with precipitation reinforcing the cold air wedge, and low clouds holding strong even if there is a lull in the precipitation to the south of I-70 tomorrow afternoon. Previous discussion follows...
Clouds increase again this evening and overnight as a warm front tries to nudge northward from the Tennessee Valley and warm advection aloft commences. Shortwave energy will also encroach from the southwest as precipitation increases after midnight over West Virginia and western Virginia, reaching the metro areas and north- northeast Maryland by daybreak Sunday.
A period of moderate to heavy rain is expected to develop the second half of the night and toward morning along the Allegheny Mountains and Potomac Highlands, where a Flood Watch goes into effect at 4am and continues through late Sunday night. Boundary layer winds will align parallel to the existing frontal boundary, coupled with a decent low level jet and sufficient upper forcing. Add in the assistance of orographic lifting along the Alleghenies, and confidence of moderate to heavy rainfall in the Flood Watch is increasing. Factoring in the snowpack that resides across this area, concern for flooding is elevated. Available observations in the Allegheny and Potomac Highlands indicate a snow water equivalent of the snow depth in the 2 to 3 inch range.
Further east, cloudy skies and periods of rain will be the story Sunday and Sunday night. The Flood Watch currently does not extend east of the mountains, but cannot rule out isolated instances of flooding in those flood prone areas as the ground remains pretty saturated, however rainfall amounts are expected to be lower than the Watch areas. The rain may taper off a bit late Sunday afternoon and evening, before regenerating once again Sunday night as the upper jet nears and additional shortwave energy crosses. Storm total rainfall into early Monday morning is forecast to range between 1.5 to 2.5 inches across Allegheny Mountains and adjacent Potomac Highlands, while 0.75 to 1.25 inches is more likely to the east and into the metro areas.
SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... The cold front associated with the system is expected to push through the region early Monday morning, allowing for a drier northwest wind to allow for precipitation to come to an end. As colder air behind the front filters in across the mountains, some snow may mix with the rain, but little to no accumulation is expected.
Strong Canadian high pressure will build toward the region from the Upper Midwest Monday and Monday night, with a surge of gusty northwest winds expected behind a reinforcing cold front late Monday. Highs Monday will top out above average for this time of year, but behind the front and building high, temperatures will drop sharply Monday night into the teens and 20s. Gusty northwest winds will deliver wind chills in the single digits and teens by Tuesday morning.
LONG TERM /TUESDAY THROUGH SATURDAY/... A fairly broad ridge of high pressure will be migrating from the Midwest towards the NE and Mid-Atlantic on Tuesday, bringing dry conditions to the area with gusty winds prior to high pressure being directly overhead and a tighter gradient. North-westerly flow along with the introduction of high pressure will keep temperatures below normal for this time of year on Tuesday along with wind chills potentially dipping down into the teens for some areas (especially the ridge tops).
For Tuesday night and into Wednesday, a spread of guidance for the potential for precipitation across the area. GFS continues to keep moisture to our south Tuesday into Wednesday while ECMWF is slower in trends and also lifts the low across the NC/VA border, bringing moisture into the area. Given the timing and previous conditions, cannot rule out the potential for wintry precip (mostly for the ridge tops) with any intruding moisture Tuesday night into Wednesday. High pressure returns early Thursday, bringing breezy conditions for the onset period before calming and cooling yet again. Another low pressure system will be introduced for the end of the workweek and most model guidance is agreeing on a more southern result, leaving the bulk of the moisture to our south. With this system being a number of days, will continue to monitor the progression of the model trends and adapt accordingly.