Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 200202 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 902 PM EST Mon Feb 19 2018 .SYNOPSIS...
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A warm front will slowly cross the region by midday Tuesday. Record warmth is possible behind this system for Tuesday and Wednesday. A cold front pass through the area Thursday before stalling out nearby during the end of the week.
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&& .NEAR TERM /THROUGH TUESDAY/...
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Latest surface analysis places cool high pressure over the western Atlantic south of Nova Scotia. A warm front snakes its way inland near Virginia Beach, dives south into northeastern Georgia, then wraps back north along the Appalachians through Tennessee, western Virginia, West Virginia and southwestern Pennsylvania, then turns west towards a low pressure near Davenport, IA. The "wedge" pattern is still very much evident east of the Blue Ridge and hasn`t changed much since this afternoon. The lowest visibilities due to fog and drizzle are now confined over north central Maryland, and this is where ensembled hi-res guidance depicts them to likely stay overnight. Fog may spread farther south/west, but a light southerly flow may result in just low clouds. While patches of dense fog are certainly possible, am thinking persistence of visibilities 1/2 to 1 mile will be most common. The ongoing warm advection/isentropic lift will promote continued low clouds, mist and drizzle at times, especially closer to the Chesapeake Bay. West of the Blue Ridge/Catoctins, skies will be clearer, but some patchy radiation fog could develop late tonight. In these areas, temperatures will actually fall tonight, but east of the mountains, nearly steady temperatures are expected. While odds of measurable precipitation are low, the strong warm advection combined with the approaching warm front may spark off a few showers closer to the Chesapeake Bay in southern Maryland later tonight into early Tuesday. Tuesday, the warm front essentially dissolves to our south and reforms over Pennsylvania as the cool air is expected to mix out across our region. Southerly flow of 20+ kt below the subsidence inversion should be able to accomplish this, but it may take well into the midday hours before clouds fully break. Assuming this happens, temperatures should reach the low 70s in most areas. Did nudge temps down a little bit given uncertainties about timing of this mix out, and it must be noted that some guidance wants to keep the mix out slow enough to limit our warming notably, but at this time feel reasonably confident that we will still see 70 across most of our region, albeit perhaps not til late in the day. Such temperatures are more normal for early May than late February.
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&& .SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/... Southerly flow beneath an exceptionally strong ridge of high pressure aloft will continue advecting warm moist air across the region Tuesday night. We should have mixed out by this point, but any place that can go light and variable could get patchy fog, and fog may be an issue near the bay as well as the warm air crosses the cool waters. Otherwise, lows should be quite mild, with 50s common. Cold front starts to encroach on the region later Wednesday and Wednesday night, but most showers should hold off until Thursday, especially east of the Blue Ridge and Catoctins. That should allow Wednesday to be the warmest day of the stretch and likely one of the warmest days on record during the month of February. See climate section below for some details. With the front not reaching us until Thursday, Wednesday night should remain fairly mild, with 40s and 50s remaining common. Regarding the showers later Wednesday into Wednesday night, there are hints of instability that might allow for some thunder, but at this time did not insert any into the forecast. && .LONG TERM /THURSDAY THROUGH MONDAY/... A frontal boundary will be laying along the western periphery of our CWA on Thursday, with rain likely as low pressure rides along the boundary and into our region. This frontal boundary may sink south of our region and stall Thursday night, and with high pressure settling in to our north. Guidance does drop temperatures back to near or slightly above normal through Friday. Another area of low pressure moves quickly northeastward toward the Great Lakes and across northern Pennsylvania on Friday, which will result in increased rain chances along the northern portion of our area. This weekend looks unsettled as waves of low pressure continue to streak northeastward toward the Great Lakes region while riding along the aforementioned frontal boundary. Guidance wants to bring that front back northward late in the week, and it appears to stall at least in the vicinity of our region. Where this front resides will obviously control temps/precip. GFS wants to keep our region in the warm sector and bring back 60s to perhaps 70s for daytime highs. It also keeps the axis of heaviest precipitation just north of the Mason Dixon Line. The ECMWF holds the boundary a bit closer to our region, so temperatures are not as warm, but still residing in the upper 50s to near 60. It is also the much wetter solution with the proximity of the boundary close by. Moral of the story, this weekends weather looks unsettled with above normal temperatures, and we`ll have to wait and see where this boundary decides to set up shop. A cold front will finally sweep through the area Sunday night, ushering in drier and cooler conditions. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/...
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IFR/LIFR cigs have been persistent across the metro areas into this evening, though obs are beginning to confirm select hi-res models that show the IFR cigs lifting north of DCA/IAD for at least several hours. Am not certain of the timing of any transition of categories, as IFR cigs could redevelop later tonight. CHO is solidly in the MVFR environment, but is wafting in and out of ceiling. MRB will be west of this and should stay VFR most of the time, but may see some patchy fog later tonight. Low level wind shear is also a concern late tonight and early Tuesday as a strong low level jet develops across the region, especially east of the mountains. Have conditions improving between 15-18Z across the area, although there is certainly low confidence in exact timing. Afterward, southerly wind gusts to 20 kt will develop. Will need to watch for patchy fog or low clouds redeveloping east of the mountains Tuesday night/Wednesday morning, but otherwise expect VFR to continue through Wednesday night. Showers may encroach on MRB late Wednesday/Wednesday night, but significant vis/cig reductions are not expected there or elsewhere until Thursday. A cold front will sink through our region Thursday and Thursday night, increasing the likelihood of sub VFR VIS/CIGs as rain chances also increase. This boundary will reside at least in the vicinity of our region through weeks end, and as waves of low pressure continue to ride northeastward along it, the pattern will be unsettled with rain chances each day and night. This will result in periods of MVFR/IFR conditions, but to the degree of flight restrictions will be highly dependent on where this boundary stalls. Winds will be light throughout this period at 10 knots or less, favoring a southerly trajectory.
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&& .MARINE...
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Light southerly flow will continue through Tuesday morning, with dense fog and drizzle being the main concern to mariners. After that, warm front should bring improved visibility overall, though will still need to watch for lingering patchy fog/haze. Areas near shore have strong potential for SCA gusts around 20 knots by Tuesday afternoon, so have issued SCA. The onset of the SCA may be too early, but didn`t feel confident to make adjustments at this time. With warm air temperatures expected over considerably cooler waters, the strongest gusts will likely be observed along the shorelines, with lower gusts over the wider and more open parts of the bay and Potomac. The same situation will occur Wednesday. A cold front will near the waters on Thursday and Thursday night, with the best chance of SCA conditions during the day on Thursday with gusts of 15 to 20 knots possible. The frontal boundary will stall and then lift northward as we finish the work week and head in to the weekend. Winds during this period will be light, with SCA headlines looking unlikely at this time.
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&& .CLIMATE... Some all-time February highest minimum temp records could be in jeopardy, but it depends on how quickly a cold front moves through Wednesday night. If the Wednesday calendar day low does not drop below 60 degrees, it would be the first time not dropping below 60 at DCA in February since 1891, and only the sixth time on record in the entire meteorological winter season. Even a low 56 or higher at DCA would be the warmest low in the month of February since 1976. The all-time February highest minimums are: 61 at DCA/Washington (2/17/1891) 58 at BWI/Baltimore (2/17/1891) 55 at IAD/Dulles (2/17/1976) - records only go back to 1960 at IAD It almost goes without saying that daily records are in jeopardy. A table of those records follows: Record warm daily maximum temperatures Tue 2/20 Wed 2/21 DCA 76 (1930) 75 (1953) BWI 76 (1930) 74 (1930) IAD 70 (1971) 70 (1997) Record warm daily minimum temperatures Tue 2/20 Wed 2/21 DCA 59 (1939) 51 (1954) BWI 57 (1939) 49 (1981) IAD 46 (1981) 45 (1981) && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 10 AM to 8 PM EST Tuesday for ANZ530>543. && $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...RCM LONG TERM...BKF AVIATION...ADS/BKF/RCM MARINE...ADS/BKF/RCM CLIMATE...LWX

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