Area Forecast Discussion
Issued by NWS Sterling, VA

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178 FXUS61 KLWX 160203 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1003 PM EDT Sun Apr 15 2018 .SYNOPSIS...
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A strong low pressure system 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.
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&& .NEAR TERM /THROUGH MONDAY/...
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A stalled front lies just to the south of our southernmost zones. A line of showers and embedded thunderstorms are moving east across central Virginia as of 930 pm. Individual stronger thunderstorms are not only moving toward the northeast at 35 mph but tend to be weakening as they move over a more stable environment from the earlier backdoor cold front. The main cold front is still aligned from north to south across central West Virginia. Any flood potential for our zones remains to be in the central Shenandoah Valley and parts of the central Potomac Highlands due to recent thunderstorms and heavy prolonged moderate rain. Rain amounts of 1 to 3 inches are expected. There is currently a flash flood watch in effect through the overnight for these zones. The main upper trough will take on a negative tilt overnight. The main surface cold front will move east. The strong synoptic forcing will likely result in the band of moderate to heavy rain to continue eastward. Nocturnal timing and possible boundary layer stability is and has decrease the threat for severe thunderstorms with exception to our extreme southwestern zones in the central Shenandoah Valley and the Virginia Piedmont. Strong gusty winds are expected over the Potomac River and Chesapeake Bay and in any stronger thunderstorms in the Virginia Piedmont through the remainder of the night. The extent of the severe threat should be east of the area by daybreak Monday (if not a few hours earlier). SPC will be monitoring trends for southeast Virginia over the next several hours as well. Our region will be most likely to be affected by multiple rounds or extended periods of moderate to heavy rainfall before the trough takes on a negative tilt. However, we don`t foresee any large concern for flooding through the remainder of the overnight as flash flood guidance is in the 2.50 inches to 3 inch range east of the Blue Ridge and into eastern West Virginia and central and northeast Maryland. Lows tonight are tricky given the warm front lifting back north just ahead of the main cold front. See additional hydro details in the special section below. Even though the front will move eastward early Monday morning, lingering showers behind the front, associated with the upper trough and wraparound moisture, will likely persist much of the day, especially in northern parts of the area. 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.
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&& .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 into Tuesday. A few inches of snow will be possible, though the chance is low enough occurs in any 12 hour period to warrant a Winter Weather Advisory. Elsewhere, a chilly breeze will bring temps down into the 30s Monday night 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 mention potential freeze in 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 have a risk of freeze in the HWO as well. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Forecast area will be near warm frontal boundary on Wednesday. Its passage will allow for temperatures to return closer to seasonal values. Most of the day should be dry, but the highlands may see a shower late in the day. This chance of rain will overspread the area Wednesday night and Thursday as low pressure will pass north of the area. This will place the forecast area on the warm side of the boundary. The reprieve will be short-lived, as a cold frontal passage should arrive sometime Thursday PM. A ridge of high pressure will overspread the area for Friday and Saturday. Aside from gusty winds Friday, tranquil conditions can be expected. && .AVIATION /01Z MONDAY THROUGH FRIDAY/...
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IFR cigs and visbis at CHO and MRB through the overnight with moderate to heavy rain moving across the terminals. Expect MVFR and IFR conditions farther east at the IAD, DCA, BWI and MTN terminals as a band of heavy rain moves east into Monday morning. Gusty winds expected with heavier showers and a few thunderstorms overnight. LLWS due to strong winds above the surface as the cold front approaches and passes. VFR conditions should mainly prevail around mid-morning Monday through Thursday.
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&& .MARINE...
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A strong low level jet could bring gusty south winds back to portions of the waters. 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. Winds will gradually relax as we head through Tuesday, but the SCA will likely need to be extended for at least portions of the waters. Anticipate backing winds Wednesday, 10 kt or less. Winds will become northwest and increase behind a cold front Thursday, possibly into SCA range.
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&& .HYDROLOGY...
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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. Flash flood guidance is lowest west of the Blue Ridge (approximately 2 inches in 3 hours), so have focused the Watch here. Urban areas could also experience ponding or isolated flooding, though confidence is lower in a true flash flood threat. Most rivers should be able to handle the rainfall given recent dry conditions. The latest RFC guidance suggests the Shephardstown and Martinsburg gauges will be closest to flood stage, but this is highly contingent on QPF. River flood watches or warnings may eventually be needed.
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&& .TIDES/COASTAL FLOODING...
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Water levels are beginning to rise on the lower Potomac. 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. ESTOFS is especially strong in its surge, and SNAP-Ex ensemble takes Annapolis to near moderate flood. The greatest threat is with the Monday morning cycle, but Straits Point needs to be watched for this evening. Guidance suggests that minor flooding events could last into Monday night, maybe Tuesday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...Flash Flood Watch until 5 AM EDT Monday for VAZ025>027-029-030- 503-504-507-508. WV...Flash Flood Watch until 5 AM EDT Monday for WVZ055-501-502-505- 506. MARINE...Gale Warning until 8 AM EDT Monday for ANZ530>543.
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&& $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...CJL AVIATION...KLW/CJL MARINE...KLW/CJL HYDROLOGY... TIDES/COASTAL FLOODING...

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