Area Forecast Discussion
Issued by NWS Sterling, VA

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569 FXUS61 KLWX 151932 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 332 PM EDT Sun Apr 15 2018 .SYNOPSIS... A backdoor cold front will remain stalled across the area through this evening. Tonight, 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. && .NEAR TERM /THROUGH MONDAY/... The backdoor cold front has definitely slowed to a crawl, running from south of CBE, FRR, CJR, EZF, and NUI. As warm moist air continues to lift over this boundary, showers and drizzle have been developing. Convection is developing southward from SW Virginia, and only slowly pivoting eastward due to unidirectionalflow east of a deep trough. It`s possible this could eventually pose a severe weather threat to the far southern portion of the area later this evening. The upper trough will take on a negative tilt tonight. A cold front extending southward from low pressure in the Great Lakes will be moving eastward, with a triple point low developing where it intersects the backdoor front. Model consensus has this low tracking near or just northwest of I-95, which will cause the backdoor front to lift back northward, at least into southern Maryland, and perhaps as far north as the Baltimore- Washington metro. In addition to strong deep layer shear, high helicity will be found along this frontal zone. The strong synoptic forcing will likely result in the band of moderate to heavy rain to continue eastward, with a more organized QLCS in the warm sector. Nocturnal timing and possible boundary layer stability may mitigate the severe threat, but the strong wind fields can`t be ignored. Locally damaging winds and perhaps a brief tornado are the primary threats in the warm sector. The extent of the severe threat will need to be monitored through the evening, and 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. The other threat is flooding. Very strong forcing from the surface front and upper trough and high PW`s over 1.5" will likely lead to plenty of rainfall. Rainfall amounts generally look to be on the order of 1-3 inches, with highest amounts near and west of the Blue Ridge where orography and isentropic lift are maximized. This area is most likely to be affected by multiple rounds or extended periods of moderate to heavy rainfall before the trough takes on a negative tilt. Hi-res guidance suggests locally higher amounts are possible. Thus a Flash Flood Watch has been issued for this area where there is higher confidence in higher rain totals. This confidence trails off toward the Potomac River, with the HRRR-TL being most aggressive in probabilities of 2 inches of rain. Localized totals of 3 inches could occur across the metro, but the band of rain is generally expected to be more progressive. Flash flood guidance is also much higher outside of the urban cores. For now will continue to carry flood potential in the HWO. 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. && .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 /19Z SUNDAY THROUGH FRIDAY/... Backdoor front has stalled between Washington and CHO for now, although it`s not out of the question it makes a further push south. IFR to LIFR has locked in north of the front, and will likely remain in place most of the night. Vis restrictions have developed in the deeper marine airmass at BWI and MTN, but may not occur at other terminals until more significant showers and drizzle develops toward evening. As a cold front pushes east tonight, rain will become widespread and heavy at times, some thunder is possible, and gusty winds are also a threat tonight as the front barrels through. Multiple wind shifts are possible, along with LLWS due to strong winds above the surface. 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 should mainly prevail, with only a small chance of flight restrictions Wednesday night into Thursday. && .MARINE... Backdoor cold front has now pushed through the waters, with easterly SCA level winds being observed on all waters. Tonight, as the front lifts back north, 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. && .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. Flash flood guidance is lowest west of the Blue Ridge (approximately 2 inches in 3 hours), so have focused the Watch here now. The Blue Ridge could enhance rainfall as well, so will need to monitor counties just to the east. The 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. && .TIDES/COASTAL FLOODING... Water levels are near astronomical normals, though they 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. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...Flash Flood Watch through late tonight for VAZ025>027-029-030- 503-504-507-508. WV...Flash Flood Watch through late tonight for WVZ055-501-502-505- 506. MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ530>543. Gale Warning from 11 PM this evening to 8 AM EDT Monday for ANZ530>543. && $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...HTS AVIATION...ADS/HTS MARINE...ADS/HTS HYDROLOGY...ADS TIDES/COASTAL FLOODING...ADS is the U.S. government's official web portal to all federal, state and local government web resources and services.