Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 080820 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 320 AM EST Fri Dec 8 2017 .SYNOPSIS... Weak surface ridging weakens today. A low pressure system will impact the area tonight through Saturday. High pressure will return Sunday into Monday. && .NEAR TERM /THROUGH TODAY/... A ridge of high pressure at the surface will weaken through the day today as southern stream low pressure develops off the southeast U.S. coast and northern stream low pressure approaches from the Great Lakes. An overcast day is anticipated as abundant mid and high level clouds pour in ahead of a buckling upper level trough. Temperatures should be about 10 degrees cooler than yesterday as a result of the clouds and low December sun angle: upper 30s to around 40 east of the mountains and upper 20s to lower 30s over the higher terrain. && .SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/... The first widespread accumulating snowfall of the season seems inevitable late tonight into Saturday for much of the area. This is a significant change from the most likely scenario from 24 hours ago. What changed is 1) the southern stream surface low ejecting off the southeast U.S. coast appears a little slower and stronger, allowing the northern stream upper trough to catch up and interact/partially phase with it as it passes; and 2) the approaching northern stream trough takes on a neutral (as opposed to positive) tilt, placing the area more squarely in the favorable right entrance region of an amplifying upper jet (quite strong, 200-knot max at 250 mb Friday night over northern New England). Therefore, what was a lower (but non- zero) probability solution this time yesterday now appears to be the most likely scenario. Right entrance region jet dynamics coupled with mid-level PVA and frontogenesis would suggest some banded snow is possible. Snowfall events driven by strong jet dynamics tend to overperform, so have upped QPF/snowfall amounts, with 2-4" for the I-95 corridor, and measurable back to the I-81 corridor. This is consistent with the latest GFS/Euro trends as well as ensembles which are coming into better agreement. But this is still subject to change. A sharp cutoff on the northwest edge of the precipitation is possible, with locally higher amounts likely in any banded precipitation (most likely east of the Blue Ridge). Boundary layer temperatures may be a couple degrees above freezing through much of the event, but strong dynamics likely overcome this. Also, the sun angle is at about its lowest and least impactful point of the year, so even though most snow will fall during the daytime hours Saturday, it should still accumulate fairly well. If this scenario remains consistent for another model cycle, then winter weather headlines will be warranted. As the surface low pulls away, snow tapers off west to east Saturday afternoon and evening, though some flurries or a couple snow showers could spill over the mountains ahead of the northern stream trough. A decent round of upslope snow showers appears likely along and west of the Allegheny Front Saturday night, but subsidence behind the slightly stronger low offshore may result in less appreciable snow showers east of the mountains than previously thought. Of note, cold air pouring in on strong northwest flow behind the low will likely cause any residual moisture from snow that fell to re-freeze Saturday night. Dry, cold and breezy conditions are expected Sunday as an Arctic airmass plummets into the region. && .LONG TERM /MONDAY THROUGH THURSDAY/... The 500 mb trough axis pulls out Monday only to be reinforced Tuesday-Wednesday. Such a pattern would likely favor upslope snow showers; however, depending upon placement and timing of forcing, flurries or even scattered snow showers would be possible elsewhere too. These will be the considerations going forward. Will have highest Tuesday PoPs (50%) for the Potomac Highlands, and maintain 20-30 percent chance elsewhere. Will also need to monitor for warm advection aloft Monday night, which may trigger a mixed precipitation situation at onset. Temperatures will be at or below normal through the extended forecast, with the coldest period Tuesday night into Wednesday in the wake of the trough axis. && .AVIATION /07Z FRIDAY THROUGH TUESDAY/... VFR through much of the day, then IFR increasingly likely for I-95 corridor terminals by daybreak Saturday through early Saturday afternoon due to snow. Most likely accumulations range from around an inch near the Blue Ridge to 2-4 inches in the I-95 corridor. Locally higher amounts/briefly lower restrictions are possible in banded snow (mainly a factor east of the Blue Ridge). Accumulating snow is possible as far west at MRB. VFR should return by Saturday evening. Winds will be light out of the N/NE AOB 10 kts through Saturday, then increase markedly out of the NW Saturday night into Sunday 10-20 kts w/ gust to 30 kts. Flight restrictions will once again be possible by Tuesday due to scattered snow showers...affecting cigs and perhaps vsbys. && .MARINE... Winds will be relatively light through early Saturday as the region is sandwiched between low pressure approaching from the northwest and another of low pressure off to the southeast. As these lows interact then move away Saturday night, gusty northwest winds take hold and SCA will likely be needed, with gales possible into Sunday before winds diminish. The next impulse will come on Tuesday. The system will be potent enough to anticipate an increase in winds to SCA thresholds. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...HTS AVIATION...HTS/DHOF MARINE...HTS/DHOF

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