Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 291404 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1004 AM EDT Mon May 29 2017 .SYNOPSIS... Upper level low pressure will drift east across the north side of the Great Lakes this week. In the meantime, weak high pressure will prevail over the Mid Atlantic outside of weak cold frontal passages Tuesday night and Wednesday night. Low pressure approaches from the west this weekend. && .NEAR TERM /THROUGH TONIGHT/... Latest observations and satellite pictures indicate the edge of low clouds on the doorstep of FDK-IAD-EZF, with a pretty sharp improvement west of there. Therefore, am hopeful that we`ll be able to see a period of sun today, before any diurnal cu develop. It remain possible that the frontal convergence zone crossing the forecast area may spark a few showers and perhaps a thunderstorm across the eastern quarter of the area. Have preserved a period of isolated- scattered showers/thunderstorms along and east of I-95. Atmosphere remains stable due to the morning wedge, and doubt we`ll be able to recover from that. The front/trough axis becomes more nebulous tonight, as it will stall near the Bay. Onshore flow will be re-introduced, bring a return of lower clouds, especially the northeastern quadrant of the area. Fog and reduced visibility will be possible, whether due to low clouds (east) or radiational cooling of the moist airmass (west). Low pressure will develop along the frontal zone to our south, which could bring a shower threat to the far southeast, but otherwise dry. Lows will be in the upper 50s to mid 60s. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... Upper level low pressure will continue to spin over Ontario Tuesday and Wednesday with the jet stream located just to our northwest. Surface features will continue to be less defined though, as weak fronts (more pressure troughs than huge airmass differences) waver across the region. The first will push southeast on Tuesday afternoon. With low stratus to start the day, it is uncertain how quickly and how far east it will erode. West of the marine air, modest instability will develop. Deep layer shear increases in magnitude with northward extent. Thus there is some risk of strong/gusty thunderstorms developing ahead of the front. SPC has placed northern parts of the area in a Marginal Risk. This front will lose its definition as it slips southward Tuesday night, ending the chance for showers. A final and slightly more well- defined front will push south Wednesday afternoon and evening. While low level convergence isn`t great, some scattered showers and storms will still develop along the front. The front will align better with diurnal heating to our west, so expect a decreasing trend for any storms that enter the area. Once again though, deep layer shear will be strong, so any organized storms could have a gusty wind threat. A Marginal Risk is in place for the northwestern half of the area. Highs both days will be in the 70s and 80s with lows in the 50s and 60s. && .LONG TERM /THURSDAY THROUGH SUNDAY/... Canadian high pressure will dominate Thursday as a trough passes aloft. Warm air will begin returning northward on Friday, but by Saturday the next cold front is already dropping back south across the region as another trough rotates around the stalled upper low over southern Canada. That front looks to stall south of the region by Sunday as the upper low starts moving further east, with the front perhaps lifting back north as a warm front early next week. We will have a chance of showers and thunderstorms with the front dropping south across the area late Friday into Saturday. By Sunday, with the potential for the front to stall south of us, the weather is uncertain, but if high pressure builds in strongly enough from the north, it could be dry. Temperatures will mostly be cooler than normal, with the warmest day likely Friday. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... Low clouds have moved east of MRB/CHO already, and based on current progress should see return of VFR to IAD/DCA shortly. It`ll be a little bit longer for BWI/MTN as they`re more entrenched in the wedged marine air, but current trends suggest that VFR should return before noon. A few showers may redevelop around Baltimore this afternoon, but not enough coverage to put in the TAF. Onshore flow will redevelop tonight. Low MVFR or IFR likely in Baltimore area, and possible at the rest of the terminals except CHO. It`s uncertain how quickly these clouds will erode on Tuesday as well. Where they do, thunderstorms will be possible during the afternoon and evening as the next boundary drops southeast. It`s possible the low cloud pattern repeats Tuesday night as the front washes out. A more well-defined cold front will push through late Wednesday with scattered showers and thunderstorms possible. Mainly VFR later this week with high pressure returning. Thunderstorm chances are maximized late Friday into Saturday with a passing front. && .MARINE... Overall light flow is expected on the waters through Wednesday. Expect variable wind directions as weak boundaries waffle across the area, though onshore flow may persist more often than not until a more defined cold front passes through Wednesday night. With the frontal passage after sunset, am not thinking strong winds will be found in its wake. Also can`t completely rule out a few thunderstorms each day, mainly focused in the afternoon and evening, although it`s uncertain how many reach the waters at this time. Sub-SCA winds expected later this week. Thunderstorm chances increase late Friday with an approaching front. && .TIDES/COASTAL FLOODING... Water levels remain high and should continue above normal for the next few cycles due to the persistent onshore flow. Advisories remain for Anne Arundel and Alexandria/DC for this morning`s high tide cycle, and for St. Marys for this afternoon/evening`s high tide cycle. It is also probable that additional advisories will be needed for high tide cycles through Tuesday for the sensitive locations, and possibly elsewhere. After Tuesday, northerly flow may finally allow anomalies to start falling. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Coastal Flood Advisory until 2 PM EDT this afternoon for DCZ001. MD...Coastal Flood Advisory until 1 PM EDT this afternoon for MDZ014. Coastal Flood Advisory until 10 PM EDT this evening for MDZ017. VA...Coastal Flood Advisory until 2 PM EDT this afternoon for VAZ054. WV...None. MARINE...None. && $$ SYNOPSIS...RCM NEAR TERM...HTS/ADS SHORT TERM...ADS LONG TERM...RCM AVIATION...HTS/ADS/RCM MARINE...ADS/RCM TIDES/COASTAL FLOODING...RCM/MM is the U.S. government's official web portal to all federal, state and local government web resources and services.