Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 101058 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 558 AM EST Sun Dec 10 2017 .SYNOPSIS... A gyre of upper level low pressure over Hudson Bay Canada will send multiple clipper-like systems through the area over the next several days. && .NEAR TERM /THROUGH TONIGHT/... Small update early this morning as of 6 AM to boost sky cover a bit this morning a mention a few flurries as energetic upper level shortwave has been slow to cross the region. Any flurries will be of minimal consequence with no accumulation expected. Mainly sunny and dry conditions are still anticipated this afternoon. Previous discussion... Weak and transient surface ridging will slide across the Mid- Atlantic today. Upper troughing aloft remains, but we will be in the subsidence region behind departing low pressure which gave us the first accumulating snowfall of the 2017-18 season. This will result in a partly to mostly sunny day, but breezy as a reinforcing shot of cold air advects into the region. A small but sharp 500 mb shortwave trough crosses the region tonight. Believe this will bring an increase in sky cover and a few scattered snow showers along the western ridges of the Allegheny Front, but of little consequence. Winds should diminish in the evening but it will be cold overnight with lows generally in the 20s. Light winds and cloud cover would argue against radiational cooling, but I could see a few favored spots in the central VA piedmont briefly dropping a few degrees below what is currently forecast. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... Another transient surface ridge will traverse the area Monday, the axis of which should be east of the area by midday along for light southerly flow and slightly milder temperatures (a few degrees milder than Sunday). A mostly sunny day seems likely in the wake of the departing Sunday night shortwave. The big wheel of low pressure keeps turning into Tuesday, sending another spoke (shortwave/clipper) towards the area. I think it`s quite possible there is a little more light precipitation than model guidance is explicitly printing out at the moment due to a subtle coupled upper jet, strong mid-level vorticity advection and low-level isentropic upglide. Temperature profiles will still be quite cool, so precipitation type should be wintry, though at the moment cold air appears deep enough to result in snow or sleet as opposed to freezing rain. The highest chances are along and west of the Allegheny Front and near the Mason-Dixon line Monday night into Tuesday. As the trough pivots through, another one fast on its heels catches up and phases with it, resulting in cyclogenesis to our northeast. The tightening pressure gradient behind this low as well as strong cold air advection in the wake of the upper trough will result in strong and gusty northwest winds. Trajectories are favorable for upslope snow showers and squalls and substantial accumulations are possible. The strong winds and very cold air (-12 C or colder at 850 mb) may also result in wind chills well below zero over the ridges of the Allegheny Front as well. Both Winter Weather and Wind Chill Advisories could be required for these areas. Despite the very strong cold air advection, model wind fields suggest winds should stay mostly below Wind Advisory criteria, but it is quite possible gusts over 45 mph may be observed for a time Tuesday into Tuesday night, even at lower elevations. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... Windy and cold Wednesday as an area of high pressure builds in behind a departing storm system. Winds should diminish Wednesday night but temperatures will not modify. They will stay cold. As winds back around from the northwest to the southwest Thursday, temperatures will not be as chilly. There is a chance for snow showers across much of the region Thursday, particularly in the western zones and along the Mason-Dixon region, as the next storm system pivots across the lower Great Lakes region. This chance will linger into Thursday night and Friday as well due to the upper level cold air support behind the surface storm. A modifying area of high pressure is expected to build eastward behind the low pressure system Friday night through Saturday night, bringing milder temperatures and temperate southwesterly breeze. By Sunday, the next threat for precipitation will come with a cold front sagging southward across the region. High temperatures, next weekend, could reach the 50s. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/... Conditions have improved to VFR for most terminals as of early Sunday morning, and VFR should persist through Monday. Flight restrictions are possible Monday night into Tuesday as a frontal system approaches bringing the potential for lower ceilings or brief visibility restrictions. A wintry mix is possible, most likely north of a line from MRB to MTN Monday night. Winds will become a bit gusty shortly after daybreak out of the northwest (10-20 G 25+ kts). Expect winds to abate with the loss of daytime mixing and a relaxing pressure gradient by nightfall. Light southerly flow AOB 10 kts is expected Monday into Monday night, then gusty NW flow returns Tuesday. VFR conditions Wednesday through Thursday night. Any brief snow showers could reduce conditions briefly. Winds northwest 15 knots gusting 20 to 25 knots Wednesday, diminishing Wednesday night, before becoming southwest at 5 to 10 knots Thursday and Thursday night. && .MARINE... 18-22+ kt gusts were being observed over the waters as of early Sunday morning. As a cold airmass surges toward the region and mixing increases over relatively warmer waters after daybreak, anticipate solid 20-30 knot gusts over the waters. The gradient relaxes late this afternoon so gusts should diminish in concert with the setting sun. Winds are then expected to remain light through Monday. Southerly channeling is possible ahead of a frontal system Monday night which may require a Small Craft Advisory. Strong northwest flow behind said frontal system will likely warrant the issuance of a Gale Warning Tuesday into Tuesday night. Small craft advisories likely Wednesday and gales possible. Otherwise, no marine hazards expected Wednesday night through Thursday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...Winter Weather Advisory until 9 AM EST this morning for VAZ503. WV...Winter Weather Advisory until 9 AM EST this morning for WVZ501- 505. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ530>543. && $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...KLW AVIATION...KLW/DHOF MARINE...KLW/DHOF

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