Area Forecast Discussion
Issued by NWS Sterling, VA

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943 FXUS61 KLWX 210726 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 326 AM EDT Thu Jul 21 2016 .SYNOPSIS... High pressure centered over the Mid Atlantic this morning will drift offshore today. A system will push north of the region on Friday, followed by a front moving into the area on Saturday. High pressure will be in control of the area`s weather Sunday before another front approaches early next week. && .NEAR TERM /THROUGH TODAY/... High pressure will be in control today as it slides from a position overhead this morning to offshore of the Atlantic coastline by later this afternoon. This will provide for mostly sunny skies, aside from some fair weather afternoon cumulus. Can`t rule out a stray shower over the higher terrain but will go with a dry forecast for now. As high moves eastward, this will allow for a light southerly flow to return. Will be looking at highs in the upper 80s to around 90F today. Dew points however will still be on the "drier" side, with readings from the upper 50s to mid 60s. Therefore heat indices will remain in the 90-95F range. && .SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/... Calm and tranquil night expected tonight with mostly clear skies and mild temperatures. May be some patchy fog around again. Lows generally in the 60s to around 70F, locally 75F in the urban centers and along the Chesapeake. The weather then heats up Friday and into the weekend. Sprawling upper ridge will remain centered over the Central Plains with the Mid-Atlantic/Northeast overspread with northwest flow aloft. This pattern is typically conducive for thunderstorm complexes to develop along the northern periphery of the ridge and track east- southeastward in the northwest flow aloft. However the location and track of these systems is uncertain this far in advance. Model guidance hinting at one moving through the Great Lakes and towards the region Friday morning, however, current indications are that it will weaken and not impact the region on Friday. In addition there will be a frontal system pushing through the Northeastern US on Friday. This will be the trigger for additional scattered shower/thunderstorm development Friday afternoon. Most of the activity should remain north of our area towards the better forcing. However as the front sags southward, any ongoing convection may work its way into the region late Friday/Friday night. Otherwise, Friday`s temperatures will be hot with highs in the low to mid 90s. With dew points in the mid/upper 60s, heat index values will likely approach 100F. The frontal boundary, which at this point will be more of a moisture/dew point boundary, will push southward on Saturday and cross the region. This will become the focus for the possibility of some additional isolated-scattered shower/thunderstorm activity. Temperatures a few degrees hotter again with highs mid to upper 90s. Dew points will be sensitive to the location of the potential boundary, with mid 60s on the northern side to low 70s on the southern side. This will ultimately drive heat index values, but 100-105F looking most likely right now and a Heat Advisory may be needed. Any thunderstorm activity should wane Saturday night. Temperatures will remain warm with lows in the 70s. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... Signs continue to support excessive heat continuing through Sunday into Monday. Strong 500 mb ridge will dominate most of the continental USA, with the westerlies trapped across Canada. Temperatures at 850 mb solidly at 20C, perhaps up to 22C. Questions remain whether we will hit 100F (temperature). But with dewpoints at (or above) 70F, heat headlines appear quite likely. Although there will be plenty of instability, initiation will be challenged due to warmth and suppression aloft. Thunder chances will be low, but non- zero. Low pressure traversing the border will pass north of the Great Lakes Monday. This will have two effects: heights will lower, thereby offering a more favorable environment for thunderstorms to develop; and an attendant cold front will help provide a focusing mechanism for these storms. Consequently, PoPs will be higher Monday night. The pattern for Tuesday and Wednesday still looks hot and summery, with the impact of the cold front limited. && .AVIATION /07Z THURSDAY THROUGH MONDAY/... Predominantly VFR expected through at least Saturday night. May be some patchy fog early this morning and then again tonight, but not currently expected to be widespread enough to warrant mention in the TAF. Could also be some isolated to scattered showers/thunderstorms around both Friday and Saturday. Light and variable winds this morning turn light southerly this afternoon, southwest Friday, and then westerly Saturday. Primarily VFR Sunday. May have some early morning fog, with a low chance at late day thunderstorms. Thunderstorm chances improve Monday, but coverage still appears to be scattered. && .MARINE... Sub-SCA conditions expected today, tonight, and Friday. Light and variable winds this morning turn southerly later today and tonight, but gusts should remain below 18 knots. Winds turn southwest Friday, and a SCA may be needed for Friday night as southwest winds increase. Winds then turn west on Saturday. High pressure remains in control Sunday, with southerly channeling possible overnight leading to elevated winds near SCA criteria. Gradient winds will increase Monday ahead of cold front. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...MM NEAR TERM...MM SHORT TERM...MM LONG TERM...HTS AVIATION...MM/HTS MARINE...MM/HTS

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