Area Forecast Discussion
Issued by NWS Sterling, VA

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608 FXUS61 KLWX 140813 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 413 AM EDT Sat Apr 14 2018 .SYNOPSIS... High pressure will remain across the western Atlantic today. A large area of low pressure over the Central Plains will move northeastward into the eastern Great Lakes by Sunday, dragging a powerful cold front across the Mid-Atlantic Sunday night and early Monday. High pressure will return for the middle portion of next week. && .NEAR TERM /THROUGH TONIGHT/... Today will not be a carbon-copy of Friday, but it won`t be much different either. High pressure will remain to our southeast over the western Atlantic today. A backdoor cold front will start slipping southward toward us, but won`t reach the PA/MD border until after sunset. Meanwhile, the storm to our west will continue approaching, but significant impacts are not yet to be had. There will be some increased cloud cover today, and humidity will be a bit higher than the last two days with more of a gulf air mass starting to infringe on the region. Combined with a bit less wind and reduced mixing, and today should end up slightly cooler than yesterday, but that still means low to mid 80s for most of the region. No precipitation is expected during the daylight hours, so all-in-all, a pretty nice June day... in April. Backdoor front drops southwest across the CWA tonight. Still some uncertainty about timing, with the very fast and aggressive NAM bringing it down into northern Virginia by early Sunday morning, while the GFS holds it much further north. The EC is a bit in between, with the front pushing southwest of Baltimore, but not yet reaching DC by early Sunday. Given the uncertainty, brought lows down mainly across northern and eastern portions of the forecast area, but maintained another warm night elsewhere. Most of the night now looks dry, but some light showers or patchy drizzle may develop late as the backdoor moves south and some leading energy ahead of the main trough to our west moves in. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Sunday will be a day of transition as the backdoor front continues sliding southeast while the strong system to the west begins to spread more significant rain and perhaps some isolated t-storms eastward into the region. Most of the heavy rain should hold off until after dark, but there are a few models such as the RGEM which generate some isolated heavy showers/t-storms as early as Sunday afternoon. With some modest instability especially south of the backdoor front, there can be a little thunder in the afternoon, but forcing doesn`t really take off til nightfall. Forecasting highs is extremely difficult given the uncertainty of just how far southwest the backdoor cold front pushes. The NAM sweeps the front down to Fredericksburg, Culpeper and Harrisonburg before having it turn around Sunday night and move back northeast as a warm front, while GFS doesn`t even have it reach DC. The EC is in between, with the front passing DC, but not reaching as far as Harrisonburg/Culpeper. Given increased confidence that the front will reach DC metro, have dropped highs across the metro and points north and east, while keeping it toasty down towards Charlottesville. The extremely powerful, negatively tilted trough and associated strong cold front will cross the region Sunday night. Timing has sped up a little, with latest guidance having the front clear most of the CWA by 8AM Monday morning. Did likewise with the forecast precip, but not quite as fast as some of the latest guidance would suggest given potential for a flip flop. There still looks like modest instability could be tapped for a QLCS, and with plenty of wind aloft, isolated wind damage can`t be ruled out should this occur. In addition, with a very strong moisture feed from the Gulf and Atlantic and plenty of forcing, heavy rain is a big concern. We could get 2-3 inches locally, maybe closer to 4 inches in the most favored upslope locations near the Allegheny Front. Fortunately, most of the CWA has been quite dry for the last few weeks, so the ground should have an above average ability to absorb the water, but localized flooding can`t be ruled out, especially closer to the Allegheny Front where more significant rains have fallen recently. Will continue mention of flooding potential in the HWO. Behind the cold front, showers may linger for a bit, but most areas will dry out by day`s end Monday. Otherwise, Monday should generally be a drying out and cooling down day, with highs dropping into the 60s most areas, with 50s further northwest of I-95, and 40s in the mountains where upslope rain showers will mix with snow. A minor accumulation can`t be ruled out Monday night along the Allegheny Front as the upslope continues and temps drop below freezing, but most locations will just have a breezy, chilly night Monday night. && .LONG TERM /TUESDAY THROUGH FRIDAY/... High pressure will continue to build into our region on Tuesday from the south as upper level energy moves east and away from us. Some snow showers may persist west of the Allegheny Front, but they should be tapering off on Tuesday morning. Dry conditions are expected elsewhere. Temperatures will be below normal on Tuesday with high temps reaching the 50s for most locations, lower at higher elevations. Gusty winds are expected Tuesday but should diminish later that day. Dry conditions are expected to continue Tuesday night and into Wednesday as high pressure centered to our south moves offshore. A southwesterly flow will settle in and warm air will advect into our area allowing high temperatures to reach the 60s and 70s. Guidance diverges on their solutions for the second half of the week with the track of a low pressure and/or its associated front that should impact us. Deterministic GFS keeps it centered near the Great Lakes, moving east and pushing a front over us late Thursday. Deterministic Euro has a weaker low pressure over the Great Lakes that eventually transfers its energy over VA/NC late Thursday and then continuing to slowly move offshore. The Euro solution would allow for a wetter end of the work-week than the GFS, but QPF amounts are not too impressive. Therefore precipitation chances are possible starting Wednesday night and maybe into Friday. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... VFR today and most of tonight. Sub VFR possible BWI/MTN later toinght as backdoor cold front drops south and low cigs develop. Otherwise, main concern today is continued gusty southwest winds, though gusts likely are a little lighter than yesterday, more like 20-25 knots. Increasingly sub-VFR weather as we head through Sunday and Sunday night as low cigs become more widespread as backdoor front slides southwest, then strong cold front to the west brings heavy rains and isolated thunderstorms across the region Sunday night. Could be some strong gusty winds with a thunderstorm, or with the cold front itself, but this will be mostly overnight. Conditions should return to VFR later Monday. VFR conditions are expected Tuesday and remaining into Wednesday with high pressure in control. Gusty winds, up to 20 knots possible on Tuesday. Sub-VFR conditions are possible sometime Wednesday night onto Thursday with another system affecting us. && .MARINE... Gusty southwest winds continue today with Small Craft Advisories for all waters by afternoon. Forecast tonight into Sunday is much trickier, as a backdoor front will drop south across the waters. Northeast winds may become quite gusty behind it, so have raised SCA for tonight and Sunday as this occurs. That said, confidence is not as high as preferred on this. Sunday night could require special marine warnings or a gale as a powerful cold front crosses the region, which could include isolated gusty thunderstorms. Winds drop back to SCA on Monday behind the front. Small craft advisory possible Tuesday as winds are expected to remain above the threshold. Winds should gradually decrease Tuesday night and remain below criteria into Wednesday. Another system will affect us on Tuesday and winds could increase above SCA criteria. && .FIRE WEATHER... Dewpoints are expected to increase through the day today, so while RH will remain low, it should not be as low as recent days. Widespread rainfall is expected to greatly reduce fire danger Sunday night. && .HYDROLOGY... A strong upper trough and attendant surface front will barrel through the area Sunday night. PWATs ahead of the front are progged to be around 1.5 inches, near record highs for the month of April. Unidirectional flow and the parent upper low cutting off suggests training of heavy rain is possible, though the flooding threat may be mitigated by fast rain/storm motions and antecedent dry conditions. WPC placed the western portions of the CWA in a slight risk for excessive rainfall, with marginal risk further east. Will assess need for any flood watches in future shifts. && .TIDES/COASTAL FLOODING... Anticipate water levels to increase slightly today due to southerly flow. The water levels will increase more markedly Sunday night in strong flow in advance of a cold front. Minor inundations would be possible for the Sunday night and Monday tide cycles. However, uncertainty also increases in the specifics. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ530-531- 539-542. Small Craft Advisory from 6 AM to 6 PM EDT Sunday for ANZ530- 531-538-539. Small Craft Advisory from 11 AM this morning to midnight EDT tonight for ANZ535-536-538. Small Craft Advisory from noon to 6 PM EDT Sunday for ANZ535- 536-542. Small Craft Advisory until 6 PM EDT Sunday for ANZ532>534-537- 540-541-543. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...IMR AVIATION...IMR/RCM MARINE...IMR/RCM FIRE WEATHER...RCM HYDROLOGY...RCM TIDES/COASTAL FLOODING...RCM

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