Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 110843 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 343 AM EST Mon Dec 11 2017 .SYNOPSIS... Weak high pressure will slide offshore through tonight. An arctic cold front will pass through the area Tuesday and weak high pressure will build overhead Wednesday. Weak low pressure will pass through Thursday and another one will pass by to the east Friday. High pressure will return for Saturday. && .NEAR TERM /THROUGH TODAY/... Upper level shortwave is passing across the region early this morning with some clouds and perhaps a few scattered flurries. Behind it, we`re seeing a shift to westerly winds and some terrain- driven cumulus. A ridge of surface high pressure will build in over the region by this afternoon, which will lead to dry conditions and relatively light winds. Aside from the departing clouds associated with the shortwave and the cumulus over the terrain this morning, mostly sunny skies are expected through the first half of the day, before high clouds increase during the afternoon hours, out ahead of the next fast-approaching shortwave trough. Temperatures will nudge back up towards climatological normals, with highs in the 40s. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY NIGHT/... A clipper low will dive through the Great Lakes region later today, before tracking up the Saint Lawrence Valley tonight and Tuesday. Warm air advection will overspread the area tonight, followed by a cold frontal passage from west to east beginning along the Allegheny Front early Tuesday morning, and pushing eastward towards the Chesapeake Bay by Tuesday afternoon. A potent upper level shortwave trough will also move eastward into the region by Tuesday afternoon. While much of the forcing and therefore precipitation is expected to remain north of the region tonight into early Tuesday morning, a few areas of light precip are possible late tonight and early Tuesday morning, mainly north towards the PA border. With marginally cold temperature profiles, there is the potential for some very light mixed precip, but coverage and intensity will be limited. Precip chances then increase along the Allegheny Front along and behind the cold front as northwest flow begins. Lows tonight in the 30s. On Tuesday, there may be a few light showers along and ahead of the front, but as temperature profiles crash behind the front and forcing increases as the strong upper level trough moves towards the region, chances for convective snow (and possibly rain to start) showers increase. Along and near the Allegheny Front, with strong northwest flow and favorable Froude numbers and moisture profiles, upslope snow showers and squalls are expected. Some of these may spill east of the Allegheny Front with the deep mixing, strong northwesterly flow, and strength of the trough. Highs Tuesday will be met prior to frontal passage, generally in the 30s/40s, but near 50F across parts of central VA and southern MD. Upslope snow showers will continue into Tuesday night with continued northwest flow. Will currently show total snowfall accumulations in the 3-7" range along the Allegheny Front, and Winter Weather Advisories will likely be needed. In addition, a very cold air mass is expected to move overhead, with 850mb temps from -15C to -18C by Wednesday morning. This will allow for lows to bottom out in the teens to around 20F. That coupled with a strong wind field will likely lead to wind chills in the single digits for most locations and near -10F for the higher elevations. A brief window of calm weather will move overhead by Wednesday afternoon, but this will be short-lived, as another weak clipper system will approach the region Wednesday night, with additional chances for some light snow by Thursday morning. Highs Wednesday may not break freezing area-wide, with lows Wednesday night 15-25F. && .LONG TERM /THURSDAY THROUGH SUNDAY/... A weak clipper low will pass through the area Thursday. Guidance has diverged on the track with solutions ranging from this system tracking well to our north vs. to our south. The ECMWF has been consistent on keeping the low farther south with other guidance having it pass farther to the north. However...the 00Z GFS has trended a bit farther south with the track of the low. Should the low track to the north...dry conditions would most likely occur but should the low track to our south...then a period of light snow could affect the morning rush Thursday. Temperatures would be cold enough for light snow to stick as well. This will have to be closely monitored over the next couple days. Will carry a chance for snow during this time. High pressure will briefly build into the area late Thursday into Thursday night...but another northern stream system will dig through the Great Lakes Thursday night and then through our area Friday. A southern stream system will also pass through the southeastern CONUS and then offshore during this time. It is unlikely that these systems will phase in time for strong coastal low pressure to develop...but some of the southern stream moisture could get drawn into our area bringing the chance for rain or snow. High pressure will briefly build into the area for Saturday...bringing dry and seasonably chilly conditions. The high will move offshore Saturday night and cold front will approach Sunday. A southwest flow in between these systems will allow for milder conditions Sunday...but showers cannot be ruled out ahead of the cold front. && .AVIATION /07Z MONDAY THROUGH FRIDAY/... VFR is expected area-wide today and tonight with some high clouds and relatively light winds. A patchy area of light precipitation is possible towards Tuesday morning, but coverage/impact low. A cold front will then cross the region on Tuesday, which will lead to an increase in winds by Tuesday afternoon. Winds may gust near 30 knots out of the northwest by Tuesday afternoon. A few rain/snow showers are also possible, especially at MRB. Gusty northwest winds will continue Tuesday night into Wednesday morning, but conditions expected to be VFR. The next chance of sub-VFR conditions will then come late Wednesday night into Thursday morning with potential for light snow. A clipper low will pass through the area Thursday. A period of light snow is possible Thursday morning and light accumulations are possible. Confidence in this occurring is low at this time. SubVFR conditions are expected in any snow that does develop. Another area of low pressure will pass by to the southeast Friday. There may be enough moisture for a period of rain or snow. && .MARINE... SCA in effect until 7 AM this morning, with winds diminishing for the rest of the morning and afternoon. Tonight, winds will increase out of the south ahead of an approaching frontal system, and SCA is in effect for portions of the waters. SCA then goes into effect for all waters on Tuesday for southerly winds ahead of a cold front Tuesday morning, as well as northwest winds behind the front. Gale conditions are then possible late Tuesday through Wednesday morning. SCA conditions may continue into Wednesday night. A clipper low will pass through the area Thursday and another low will pass by to our southeast Friday. Winds may approach SCA levels during this time. && .TIDES/COASTAL FLOODING... Elevated water levels are occurring this morning due to a southwest flow. Anomalies should drop some today as the flow goes west to northwest...but anomalies will likely increase as a southerly flow picks up for tonight into Tuesday. At this time...it does not appear that there will be minor flooding but it will have to be monitored closely for sensitive areas. Strong northwest winds are expected later Tuesday and Tuesday night. Tidal blowout conditions are possible Tuesday night into Wednesday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from midnight tonight to 6 PM EST Tuesday for ANZ531-532-538>543. Small Craft Advisory until 7 AM EST this morning for ANZ531>534-536-537-540-541-543. Small Craft Advisory from 7 AM to 6 PM EST Tuesday for ANZ530- 535-536. Small Craft Advisory from 9 PM this evening to 6 PM EST Tuesday for ANZ533-534-537. && $$ SYNOPSIS...BJL NEAR TERM...MM SHORT TERM...MM LONG TERM...BJL AVIATION...BJL/MM MARINE...BJL/MM TIDES/COASTAL FLOODING...BJL

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