Area Forecast Discussion
Issued by NWS Sterling, VA

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807 FXUS61 KLWX 201936 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 236 PM EST Sat Jan 20 2018 .SYNOPSIS... High pressure will remain near the southeastern US coastline through the rest of the weekend. A cold front will approach the region Monday before passing through Monday night into Tuesday. High pressure will then return for Wednesday through Friday of next week. && .NEAR TERM /THROUGH TONIGHT/... High pressure will remain over the southeastern United States through the rest of the day today leading to dry conditions and light southwest surface flow. West/northwest winds aloft have lead to downsloping flow, providing for mild conditions and mostly sunny skies aside from some high cirrus passing overhead. Temperatures have reached well into the 50s and low 60s across the region. A warm front, and in reality perhaps more of a moisture boundary, will lift northeastward and towards the region tonight and especially towards Sunday morning. While skies will start mostly clear tonight with just some cirrus, clouds will begin to increase overnight, especially across western areas as moisture increases and westerly flow persists. There may even be some light upslope rain or drizzle. Otherwise it will be a dry night for the remainder of the area. Lows will range through the 30s. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As noted by low level theta-e advection, moisture will continue to increase across the region on Sunday. This is expected to lead to a deck of strato-cumulus across the area during the day. Some light upslope rain or drizzle remains possible along and west of the Allegheny Front. Under the clouds, temperatures, while still mild, will likely be 5 to as much as 10 degrees cooler than today, with highs from 50-60 degrees, coolest across northeast MD and warmest in central VA. The warm front/moisture boundary will pull further north Sunday night, leading to continuing warming and moistening low levels. Some patchy light rain, drizzle, or fog cannot be ruled out. Lows should be within a few degrees of 40F. By Monday, with increasing southerly flow and surging warmth in the low and mid levels, the boundary should be well north of the area, placing the region fully in the warm sector by Monday afternoon. Thus, even with potential for morning low clouds and increasing afternoon high clouds as a cold front begins to approach the region, highs are likely to reach well into the 50s and low 60s again. The cold front will push towards the region Monday evening and begin to cross the area Monday night. This will lead to widespread rain, and with the potential for some marginal instability, there is the possibility of a convective fine line with some gusty winds. Rain totals will be generally from a half inch to an inch. && .LONG TERM /TUESDAY THROUGH SATURDAY/... Cold front will finish crossing the area Tue morning with a weak triple point low riding near or over the region. The details of how that low evolves will determine if enough instability can be brought far enough north over us to bring about gusty thunderstorms/showers. Strong winds off the deck will provide the winds if there is any convection robust enough to tap it. At the moment, instability looks very weak. Upslope snow showers will follow in the mountains. A secondary cold front trails and passes through Wed morning, ensuring temps don`t get out of the 40s on Wed...seasonable, and again with upslope mountain snow showers. After that, high pressure gradually passes over and east of the region late week with warming temps again for next weekend. && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... VFR expected through the rest of today with mostly sunny skies and some high cirrus. Winds will be generally southwest 5-15 knots. Late tonight and into Sunday, clouds will be on the increase with a strato-cumulus deck expected to develop, especially on Sunday. However, current indications are that ceilings will remain VFR with cloud bases generally around FL050. Some guidance still trying to hint at potential for MVFR at a few sites, but probability too low for inclusion. Winds become light and locally variable tonight and southerly around 5 knots on Sunday. Strato-cumulus deck likely to persist Sunday night into Monday morning, along with the possibility for some patchy fog, so potential for some flight restrictions exists. Southerly flow will increase by Monday afternoon and Monday night as a cold frontal system approaches, and gusty southerly winds and low level wind shear become concerns. Sub- VFR conditions also likely Monday night in increasing rain chances. Gusty showers and lowered vsbys and cigs may continue into Tue morning with the cold front. LLWS also possible. After that, VFR expected. && .MARINE... High pressure located south of the region is providing for southwesterly flow across the waters this afternoon. While winds are primarily less than 15 knots, some gusts to 20 knots have been observed across the northern Chesapeake Bay on the near- shore waters, and these may continue through late afternoon. A Marine Weather Statement was issued to provide more detail. Sub-SCA winds will continue tonight and Sunday. As low level winds increase by Monday, SCA probability increases, especially Monday night ahead of a cold frontal system. Gusty showers are possible Tue morning with a cold front. Small craft advisories are possible Tue and Wed as colder air moves back over the waters. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...MM NEAR TERM...MM SHORT TERM...MM LONG TERM...CS AVIATION...MM/CS MARINE...MM/CS

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