Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 210904 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 404 AM EST Sun Jan 21 2018 .SYNOPSIS...
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High pressure will shift east off the coast of the Southeast today. A cold front will approach the region Monday before passing through Monday night into Tuesday. High pressure will then return for Wednesday through Saturday before another system approaches next Sunday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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High pressure shifting east into the Atlantic today is allowing a stronger southwest flow to develop, which is also aided by the developing storm system in the Plains. This flow is resulting in warm advection/isentropic lift over the region aloft, which has started to bring clouds into the region overnight. This has helped keep temps higher than the last few nights. This general pattern should continue today. A few showers may even make their way into the Allegheny Front today, but east of there, dry conditions are expected to prevail. Clouds may dominate much of the day with the warm advection aloft, and this will likely keep temps a little cooler than yesterday, but still expecting a mild day. Highs generally 50s, with 40s near the bay and in NE MD, and around 60 in central VA.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/...
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Warm advection/isentropic lift will continue tonight and Monday as the high continue slipping eastward and the storm in the Plains strengthens and moves towards the Great Lakes. However, moisture and forcing will remain limited, so not expecting much more than some clouds. A few showers or a little drizzle can`t be ruled out along the Allegheny Front tonight, shifting towards the Mason-Dixon line Monday morning, but otherwise expect dry weather. Temps will be held up tonight with the clouds, and should warm considerably on Monday thanks to the strong warm advection and some limited sun reaching the surface. Lows tonight should stay in the 40s overall, with highs Monday reaching the 50s and 60s. Strong cold front approaches from the west Monday night and crosses the region Tuesday morning. Still some timing differences among the models, but overall expect showers to move across the region starting late evening Monday and exiting the area Tuesday afternoon. Southerly low level jet combined with storng forcing aloft and weak instablity could be enough for a low-topped squall line, which could mix down some stronger gusts, and given the weak instability and strong forcing, have included slight chance thunder late Monday night and Tuesday morning. Rainfall totals look relatively light, mainly less than an inch, since the system should be moving along fairly well, but isolated totals could exceed an inch. Temperatures Monday night will stay very mild, with 40s and 50s common, and may spike back into the 60s on Tuesday across most of the area either just ahead of, or just behind, the cold front. Behind the front later Tuesday and Tuesday night, cold advection on a gusty NW wind will get underway. This isn`t an arctic air mass, but we do expect snow showers to get going across the higher terrain. However, given the marginally cold air mass, partly frozen lakes, and blocked flow, have kept pops in the chance range for now. Further east, Tuesday night will be dry and colder, but actually staying above normal for late January, with lows mainly in the 20s to low 30s. Will need to watch winds, especially in western Virginia, with a very strong 850 max coming through - we may need an elevation Wind Advisory here later Tuesday into Tuesday night.
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/...
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A secondary vort max looks to pivot around upper troughing that will be situated over the northeastern CONUS on Wednesday. East of the mountains, moisture is not that impressive or deep, upper jet positioning is relatively unfavorable and the shortwave is opening up (as opposed to deepening) as it crosses. But there could still be a (non-accumulating) flurry or two as the shortwave moves overhead Wednesday afternoon (GFS) or evening (EC). The Great Lakes are largely frozen, so the moisture source along the upslope areas likely won`t be tremendous either, but it would likely still be enough for some snow showers Wednesday afternoon/evening, gradually tapering off through Thursday morning as drier air moves in and the upper trough departs. High pressure will then dominate the weather pattern later Thursday into Saturday bringing dry weather. The high looks like it will move off to the east as opposed to retreating to the north, which would result in a warming southwesterly return flow Friday into Saturday. Another rather large/deep upper trough moves toward the area Saturday night into Sunday which would likely result in inclement (wet) weather for the second half of next weekend, if timing holds. For what it`s worth, even when it was beyond day 7 the guidance has been in decent agreement already showing this rather robust system passing to our west, bringing a soaking rain to the region.
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&& .AVIATION /09Z SUNDAY THROUGH THURSDAY/...
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Low confidence forecast in VFR prevailing through today as a weak backdoor front slips south and warm advection/isentropic lift develops over the low level cool/dry air. Cigs could get a little lower than forecast, so MVFR not out of the question, but given dry low levels, think cigs should stay just barely VFR. Tonight, increasing moisture even at low levels will likely lead to cigs lowering to MVFR. There is a chance we reach IFR late tonight with continued warm/moist advection from the south, and some drizzle is not impossible, but the atmosphere may not moisten enough to allow it, so confidence on this is very low. Any sub VFR cigs will likely go back to VFR for the balance of Monday, with lowest confidence in this being at MRB/MTN where the warm air may have more difficulty scouring out the low level cool air, but even here, moisture looks limited in low levels. Winds will increase from the south as a cold front approaches. Sub-VFR, quite possibly IFR at times, later Monday night and Tuesday morning as cold front crosses the region. May even get a few thunderstorms to develop, with low level wind shear also likely to be an issue. Outside chance that one of those storms taps into the shear and brings it to the surface, which could cause an isolate 40-50 knot gust, but odds on this are low. Going back VFR Tuesday afternoon with gusty NW wind and staying VFR Tuesday night. Mainly VFR expected Wed-Thu. NW flow 10-20 kts and gusty Wed, becoming light/variable late Wed night into Thu as high pressure moves overhead.
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&& .MARINE...
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Sub-SCA winds expected through tonight. Southerly flow increases just above the surface on Monday, with winds likely to reach SCA criteria over land. However, with very mild temps expected (60s), expecting mixing over the water to be much poorer, so have kept gusts below SCA criteria for now. Its possible there is a little more mixing than currently expected, especially right along southerly shores, so we may eventually need an SCA for Monday for some waters, but there is little confidence in this. Winds continue to increase just above the surface Monday night as cold front approaches, so expect enough of that to mix down to result in SCA gusts Monday night. Cold front pushes through Tuesday with SCA likely. Could be close to gale at some points Tuesday and Tuesday night, mainly in potential squalls Tuesday ahead of the cold front, then with cold advection behind the front Tuesday night, but confidence in this is also low at this time. SCA gusts/possible gales Wednesday in the wake of a departing cold front. Winds become light Thursday as high pressure builds over the waters.
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&& .TIDES/COASTAL FLOODING...
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Increasing southerly flow ahead of an approaching cold front will cause water levels to increase Monday into Tuesday. Minor coastal flooding is possible, especially during the midday high tide cycles Tuesday if the cold front isn`t through by then.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...DHOF AVIATION...RCM/DHOF MARINE...RCM/DHOF TIDES/COASTAL FLOODING...DHOF

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