Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 181557 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1057 AM EST Sat Nov 18 2017 .SYNOPSIS... High pressure will move offshore today. A strong cold front will cross the area late tonight into early Sunday. High pressure will build to the south of the region late Sunday through Monday before moving offshore Tuesday. Another cold front will pass through Tuesday night into Wednesday before stalling well to our south and east on Thanksgiving Day. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Surface high pressure continues sliding east off the coast. At the same time, a low pressure system organizing over IL will move east-northeastward towards the eastern Great Lakes this afternoon and through the Saint Lawrence Valley tonight, deepening to ~980 mb. The main weather feature that will affect the region for the rest of today is a warm frontal zone associated with the aforementioned low pressure system. Clouds and showers driven by the warm and moist air advection are crossing the area, mainly north of the immediate DC metro. Temperatures are warming quickly in some areas thanks to strong southerly flow and most areas have already reached the 40s, with some spots into the 50s. Strong southwest winds will continue through the day, and while mixing will be limited enough in the lowlands to keep gusts below 30 mph, across the higher elevations, gusts are expected to reach 45-50 mph by late in the day and a Wind Advisory has been issued for these locations after 4 PM, mainly above 2000 feet. Some isolated gusts to this magnitude may occur earlier across the highest elevations. Our current high temperature forecast only reflects values through 8PM due to the programming of NDFD. In reality, many spots may reach their midnight-midnight high after this point thanks to strong southerly flow continuing this evening ahead of the cold front.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/...
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The surface low will be near far northern NY and the Ontario/Quebec border by Sunday morning with the strong surface cold front crossing our region tonight. This will bring a period of rain showers as it moves across with gusty south/southwest winds continuing out ahead of the front, especially across the higher elevations where a the Wind Advisory is in effect. While instability will very limited, with strong wind field aloft, a shallow-topped convective fine line cannot be ruled out where gusts may be locally higher. Temperatures will likely continue slowly rising through frontal passage, with readings likely to be into the low 60s after midnight Sunday morning. Behind the front, low level temperature profiles will crash, with precipitation changing to snow along and west of the Allegheny Front and taking on an upslope characterization as northwest winds increase. Wind gusts will be increasing area-wide behind the front as well, and gusts to 50 mph are possible beginning early Sunday morning. Therefore the Wind Advisory may need to be expanded. Lows by morning drop into the 30s along the Allegheny Front to the 40s and low 50s elsewhere. The strongest winds will occur Sunday morning when the best combination of mixing and strong wind field aloft overlap. The winds will gradually lessen in the afternoon hours. Upslope snow showers will continue along and west of the Allegheny Front through the day with several inches of snow accumulation likely. Temperatures Sunday won`t rise much following the front so highs very similar to tonight`s lows. A potent upper level shortwave rounding through the base of the large scale trough will then cross the region Sunday evening. This will not only act to re-invigorate the upslope snow showers, but may also bring some scattered flurries and snow showers eastward towards central and northern MD, the eastern WV panhandle and northern Shenandoah Valley of VA. Snow showers and gusty winds will then gradually dwindle Sunday night with lows falling into the 20s and 30s. By Monday high pressure will be building into the region with much quieter weather expected. Highs in the 40s to around 50F, with lows Monday night in the upper 20s to mid 30s.
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&& .LONG TERM /TUESDAY THROUGH FRIDAY/... High pressure will move offshore Tuesday and a return southwest flow will usher in somewhat milder conditions. A cold front will pass through the region Tuesday night into Wednesday...possibly triggering some showers along with more clouds. High pressure is most likely to build into the area late Wednesday through the end of the week...bringing dry and chilly conditions. However...shortwave energy may interact with a stalled boundary over the southeastern CONUS...causing low pressure to develop. If this occurrs...it could have an impact on our weather Thanksgiving Day into Friday with more clouds and even possible rain. Confidence in this occurring is low at this time since a majority of the guidance keeps the low far enough to our south from impacting our area...but this will have to be monitored over the next few days. && .AVIATION /16Z SATURDAY THROUGH WEDNESDAY/...
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Primarily VFR expected through today, although with thickening and lowering clouds. A warm frontal zone is crossing the region with some rain showers likely at MRB, and possibly as far south and east as IAD/DCA/BWI/MTN. A period of MVFR is possible at MRB with these showers. An additional period of sub-VFR conditions is possible overnight tonigt as a cold front moves across the region with another period of rain showers. However, the main aviation weather concern will be increasing low level wind field today through Sunday. Winds are gusting to 20-25 knots or so at many locales already, and those where it is not will likely see increasing gusts through the remainder of today. After cold frontal passage late tonight, gusty northwest winds will develop with gusts 30-40 knots likely. Winds will gradually lessen late Sunday into Monday as high pressure builds in. VFR conditions are expected Tuesday. A cold front will pass through the terminals Tuesday night into Wednesday. A few showers and subVFR conditions are possible during this time.
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&& .MARINE...
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Small Craft Advisory is in effect for all waters through tonight with increasing low level wind field. Gusts may approach gale force later tonight associated with cold frontal passage, but will keep SCA for now. The strongest winds are expected to occur Sunday morning behind the cold front, and a Gale Warning has been issued for Sunday. Winds gradually lessen late Sunday and Sunday night, however SCA level winds still likely, and may continue into Monday. High pressure will move offshore Tuesday and a cold front will pass through Tuesday night into Wednesday. A Small Craft Advisory may be needed for portions of the waters during this time. High pressure is most likely to return for late in the week...but low pressure has the potential to develop over the southeastern CONUS and it could impact our weather. Confidence in this occurring is low at this time.
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&& .TIDES/COASTAL FLOODING...
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Water levels continue to increase as southerly wind increases. Strait`s point should hit minor this afternoon. Many sites may hit tonight but will evaluate trends through early this afternoon before issuing a headline. Current tide trends suggest a 2:1 blend of ETSS and more aggressive CBOFS would work best (the latter model has moderate at Annapolis tonight; believe this is overdone but not impossible so will monitor closely).
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Wind Advisory from 4 PM this afternoon to noon EST Sunday for MDZ501. Coastal Flood Advisory from 2 PM this afternoon to 9 PM EST this evening for MDZ014. Coastal Flood Advisory until 4 PM EST this afternoon for MDZ017. VA...Wind Advisory from 4 PM this afternoon to noon EST Sunday for VAZ025-026-503-504-507-508. WV...Wind Advisory from 4 PM this afternoon to noon EST Sunday for WVZ501-503-505-506. MARINE...Small Craft Advisory until 6 AM EST Sunday for ANZ530>543. Gale Warning from 6 AM to 6 PM EST Sunday for ANZ530>543.
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&& $$ SYNOPSIS...BJL/RCM NEAR TERM...MM/RCM SHORT TERM...MM/RCM LONG TERM...BJL AVIATION...BJL/MM/RCM MARINE...BJL/MM/RCM TIDES/COASTAL FLOODING...DHOF

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