Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 132010 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 310 PM EST Wed Dec 13 2017 .SYNOPSIS... An Alberta Clipper will pass nearby tonight through early Thursday. Developing low pressure will pass off the east coast late Friday and high pressure will return for Saturday. Low pressure may impact the area early next week. && .NEAR TERM /THROUGH THURSDAY/... Weak ridging of surface high pressure is over the area at present, but is quickly moving east. Wind is slowly but surely diminishing as it starts to shift from northwest to south. The system which affected the area yesterday is now over the Canadian Martimes, while an Alberta Clipper is over northern Illinois heading southeastward. Tonight, this clipper will move by near or just north of the CWA, crossing eastward along or just north of the MD/PA state line. However, a warm front will advance northeast ahead of it, resulting in significant isentropic lift over our region. While surface air remains dry, the lift looks strong enough to generate some light snow across northern portions of the CWA, potentially reaching DC itself. Model guidance has shifted southward the southern extent of precip, so that while the operational NAM and GFS are still dry in DC, the operational global and regional Canadian, the HRRR, and EC all now bring measurable to DC itself. While the amounts will be light, generally less than 1 inch, and probably less than a half inch in DC if it occurs, with cold ground temps leading into it, the potential for icy/snow covered roads is elevated. Fortunately, nearly all guidance holds the precip back until after 7PM, so the bulk of the rush hour should stay dry, but as we head beyond 7PM the odds of encountering light snow increase significantly. The period of accumulation across the metro looks quite brief, accompanying the warm advection period this evening, and after midnight we should dry out by comparison. This can`t be said further northwest along the Allegheny Front, where upslope snow showers will continue through the night. Thus, the winter weather advisory for 2-4 inches remains in effect in this region. With the warm front crossing the region, temps will likely drop down to the high 20s across the metro early before staying steady or even slowly rising late tonight. Colder temps will dominate along the Allegheny Front. Cold front crosses the region early Thursday, and while a snow shower can`t be ruled out with it, the impact at this moment looks lower than potential impact this evening. The bulk of the day will be behind the front as the low pressure heads east into the Atlantic, with a gusty northwest wind but temps actually rebounding as a slightly warmer air mass dominates. Upslope snow will also end early. Highs will rise towards 40 in many areas. && .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... Weak high pressure crosses the region Thursday night into early Friday, with partial clearing and dry conditions with lighter winds. Trough passing aloft however will generate a surface low off the East Coast. While the chance of this low affecting our region remains small, with the trough crossing the region have kept slight chance to low chance pops for snow showers over the region. Highs will cool a bit with the passing trough, with 30s generally expected. System heads east of the region at night, with drying likely, except for the upslope areas, where accumulations are likely. Lows will be below freezing once again. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... By Saturday, the upper level pattern will begin to change, with upper level troughing pulling away to the northeast due to developing ridging downstream of a west coast trough. Surface high pressure will be building into the area, although temperatures may remain slightly below normal in continued northwest flow. The high will begin to move to the east on Sunday, which will allow a more southerly flow to develop with warmer temperatures. For the remainder of the long term, while there is moderate model agreement on the large scale pattern, there are considerable smaller scale differences. One shortwave trough will be crossing late Sunday, with deeper troughing digging toward the eastern United States in the Tuesday to Wednesday time frame. These two time frames represent the best opportunity for some precipitation, although with low confidence due to the spread in timing, locations and moisture sources. With a milder westerly flow aloft overall, temperatures should remain above normal (even above freezing at night), so any precipitation should be in the form of rain, except minimal chances for upslope snow. && .AVIATION /20Z WEDNESDAY THROUGH MONDAY/... VFR conditions at present have an increasing chance of deteriorating this evening as a brief period of snow is looking more and more likely at the terminals near and north of DC. Cigs and vis are likely to drop to at least MVFR at BWI/MTN/MRB, and quite possibly doing likewise at DCA/IAD. CHO is most likely to stay VFR. Conditions should improve after midnight and generallye expect VFR to dominate Thursday through Friday night, though the risk of snow showers will be elevated again on Friday as another weak system crosses the area. Winds will relax tonight but become reinvigorated from the northwest on Thursday. VFR with high pressure for Saturday and most of Sunday. A weak weather system could bring some showers late Sunday into Monday, but it`s uncertain at this time if sub-VFR conditions would result. && .MARINE... SCA remains in effect for all waters through rest of today, then drops off the more sheltered waters tonight as winds shift southerly and we get some channeling ahead of an approaching cold front. This front crosses the area early Thursday and SCA redevelops across all waters. We then drop back below SCA over the course of Thursday night, but another system crossing the area could bring SCA back on Friday. Small Craft Advisory conditions will remain possible through Saturday in west/northwest flow. Winds will become lighter Saturday night into Sunday with high pressure. Southerly flow on Sunday will become westerly on Monday as a weak system passes. At this time, it appears winds will remain below criteria through this period. && .TIDES/COASTAL FLOODING... Strong northwest winds have caused tidal anomalies to drop sharply. Anomalies are however already starting to rebound this afternoon through tonight as winds turn to the southwest. They should drop again on Thursday behind a cold front, but likely not as much as they dropped last night. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Winter Weather Advisory until 7 AM EST Thursday for MDZ501. VA...None. WV...Winter Weather Advisory until 7 AM EST Thursday for WVZ501-503. MARINE...Small Craft Advisory until midnight EST Thursday night for ANZ531>534-537-539>541-543. Small Craft Advisory until 6 PM EST this evening for ANZ535- 536-538-542. Small Craft Advisory from 8 AM to 6 PM EST Thursday for ANZ535- 536-538-542. Small Craft Advisory until 6 PM EST Thursday for ANZ530. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...ADS AVIATION...ADS/RCM MARINE...ADS/RCM TIDES/COASTAL FLOODING...RCM

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