Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 150810 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 410 AM EDT Sun Apr 15 2018 .SYNOPSIS... A backdoor cold front is dropping south across the Mid-Atlantic this morning and will stall out across the area this afternoon. Tonight, a strong low pressure over the Midwest will move into the eastern Great Lakes, dragging a strong cold front eastward across the Mid-Atlantic by Monday morning. High pressure will gradually build back in for Tuesday and Wednesday before another low pressure system crosses the region Thursday. && .NEAR TERM /THROUGH MONDAY/... Backdoor cold front is currently sliding southwest across Maryland and is presently situated between Baltimore and Washington. It will continue to push southwest this morning, crossing Washington metro by midday. Guidance continues to waver as to just how far the front gets, with the aggressive NAM sending it all the way to Charlottesville and Harrisonburg before turning back north overnight tonight, so while temps are mild now, they will be cooling off and our highs have likely already occurred. Some low clouds and even patchy drizzle are possible behind the backdoor front, but other than the dramatic drop in temps, it isn`t quite the main story. The other strong cold front, associated with low pressure to our west and a potent negatively tilting trough aloft, will be approaching the region today, and increased divergence ahead of the front plus whatever instability can be mustered south of the backdoor front will cause showers to start breaking out, most likely no later than midday. With the CAPE available especially south of the backdoor front plus increasing shear, thunder will be possible, and with the rapid rise in PW`s, some heavy rainers could be had, but the main forcing will not arrive until tonight, so think the risk of excessive rain or severe weather this afternoon is not that great. That said, something isolated in both regards could still occur. Tonight, as the powerful cold front crosses the region, the backdoor front will reverse and come back north as a warm front. It likely won`t make it back into the northern Shenandoah Valley, but the I-95 corridor will likely end up warming again just before the main cold front passes. This combined with the very strong forcing from the surface front and upper trough and the high PW`s over 1.5" will likely lead to plenty of rain and embedded thunder. Potential exists for a QCLS feature as well as perhaps an isolated tornado, with that risk generally confined to regions that end up getting back into the warm sector. However, the risk will extend right to the warm front itself as it will be a source of shear. Rainfall amounts generally look to be on the order of 1-3 inches, with highest amounts near and wets of the Blue Ridge where orography and isentropic lift are maximized, but localized totals of 3 inches could occur across the metro. For now will continue to carry flood potential in the HWO, but will wait one more cycle before issuing any flood watches. Lows tonight are tricky given the warm front lifting back north just ahead of the main cold front... places that see temps falling most of today could see them rising much of tonight, the reverse of the usual diurnal pattern. Latest guidance suggests the main hazards will clear the region by 8AM if not sooner, but lingering showers behind the front, associated with the upper trough and wraparound moisture, will likely persist much of the day. These should be much lighter, however. With temperatures cooling rapidly, especially aloft, rain will mix with and change to snow at the higher elevations, and minor accumulations may occur. Temps will probably be steady or falling much of the day behind the cold front with the continuing clouds and showers. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... Upslope flow behind the surface low and deep trough aloft will combine to promote continued snow showers along the Allegheny Front, with an inch or two possible by Tuesday morning. Elsewhere, a chilly breeze will bring temps down into the 30s across much of the region, though freezing temperatures are questionable given the winds may stay up. However, some areas in the southern Shenandoah Valley could decouple, so will add mention of potential freeze to the HWO. With the trough lifting out a bit and winds relaxing behind the low as it pulls further away Tuesday, clouds and upslope snow will diminish, but temps will remain quite chilly for mid April, with highs in the 40s and 50s. With a better shot of decoupling under lighter winds on Tuesday night, will also add in risk of freeze to HWO. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... High pressure centered off of the southeast coast remains in control for the early part of the day on Wednesday keeping dry conditions over our area. Warm air advection due to southwesterly flow will allow the temperatures to become more near normal, with highs reaching the 60s and 70s. Clouds will be increasing as a low pressure system moves into the southern Great Lakes on Wednesday night, and eventually pushes a cold front over us sometime on Thursday. Timing and exact track of this low and the fropa is still somewhat divergent among guidance, but they do agree that moisture will be limited. Therefore rain showers expected Wednesday night and Thursday with total mounts less than a tenth of an inch. Gusty northwesterly flow behind this front will allow for some snow showers west of the Allegheny Front and dry everywhere else Thursday night. A high pressure system will build over us from the northwest on Friday and remain in control into Saturday with northwesterly flow keeping below normal temperatures, reaching the 50s and 60s. both days. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... Backdoor front sliding southwest across the region will play havoc with aviation this morning, as low cigs, potentially IFR, will spread southwest towards MTN, then BWI, and then potentially MRB/IAD/DCA. For now will keep them low end MVFR but slightly lower is very possible. Vis restrictions not likely until significant rainfall develops tonight with the cold front sliding eastward toward us. Rain can be heavy, isolated thunder is possible, and gusty winds are also a threat tonight as the front barrells through, aside for the low cigs and reduced vis. Conditions improve a bit on Monday behind the front, but showers will linger, so MVFR will remain possible. Should improve to VFR Monday night and remain so thru Tuesday night as high pressure gradually builds back in. VFR conditions are expected on Wednesday with high pressure in control. Sub-VFR conditions are possible sometime Wednesday night onto Thursday with another front moving across. VFR conditions are expected to return sometime Thursday night into Friday as high pressure builds into the region. Gusty northwesterly winds are expected on Friday, with some gusts reaching up to 20 knots. && .MARINE... Winds have reintensified to SCA for parts of the bay, both south and north of a backdoor cold front, so have reinitiated SCA for portions of the waters. These spread over all waters by afternoon as the front lifts back north and strong low level jet brings gusty south winds back. Have maintained gale over waters for tonight with potential for 35+knot gusts in heavy rain and embedded t-storms as cold front crosses the region. SCA will likely be needed Monday behind the front but held off issuing for now. Winds will gradually relax as we head through Tuesday. Winds should remain below small craft advisory criteria Wednesday and Thursday. A front will move across us on Thursday and winds could increase above SCA criteria behind it sometime Thursday night or Friday, and may require an advisory. && .HYDROLOGY... Strong negatively tilted upper trough and surface cold front expected to cross region Sunday night, with widespread heavy rain and embedded t-storms. PW`s rise to around 1.5", quite high for April, thanks to a very strong fetch from the Gulf and Atlantic. With the forcing and potential training, rainfall could locally exceed 3 inches in a few hours, so flooding is a concern. However, fast storm motion and the recent dry weather may mitigate the threat, so have continued with just HWO mention without an actual watch. This allows the day shift a final assessment for this threat. && .TIDES/COASTAL FLOODING... Water levels will be near astronomical normals on Sunday. Onshore flow followed by a strong southerly push Sunday night could lead to an influx of water causing minor flooding on late Sunday night or Monday. Guidance suggests that minor flooding events could last into Monday night, maybe Tuesday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ531- 532-538>540. Gale Warning from 11 PM this evening to 8 AM EDT Monday for ANZ530>543. Small Craft Advisory from noon today to 11 PM EDT this evening for ANZ533-535>537-541-542. Small Craft Advisory until 11 PM EDT this evening for ANZ530- 534-543. Small Craft Advisory until 6 AM EDT early this morning for ANZ533-537-541. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...IMR AVIATION...IMR/RCM MARINE...IMR/RCM HYDROLOGY...RCM TIDES/COASTAL FLOODING...IMR is the U.S. government's official web portal to all federal, state and local government web resources and services.