Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 181400 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1000 AM EDT Tue Jul 18 2017 .SYNOPSIS... An upper level trough will affect the weather today. High pressure will dominate Wednesday and Thursday. A cold front will approach from the north at the end of the week, but will most likely stall before reaching the area. && .NEAR TERM /THROUGH TONIGHT/...
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Upper level trough that moved overhead yesterday will remain in place for today, providing additional chances for scattered afternoon showers/thunderstorms. Not much change in the air mass from yesterday, so nature of development likely to be similar to yesterday, developing off of subtle low level boundaries and terrain circulations. 12z IAD sounding and SPC mesoanalysis does depict mid levels a bit warmer with mid level lapse rates not quite as steep as yesterday, reporting 6.5 C/km this morning. That being said, RNK sounding this morning still showing in excess of 7 C/km. This combined with surface temperatures near or slightly above 90F and dew points near 70F should allow for development of near 2000 J/kg of MLCAPE again by this afternoon. Thus another pulse thunderstorm day with potential for isolated severe exists, with perhaps the highest chance in our southwest towards the Blue Ridge/Shenandoah Valley where steeper lapse rates exist. SPC has placed this area in a Marginal Risk today. In addition, once again, there is very little shear/steering flow, so heavy rain and isolated flooding is again a possibility. As instability wanes tonight, it should become completely rain free by midnight or soon thereafter as the atmosphere re-stabilizes, since the synoptic forcing which could keep storms going through the night will remain weak. Lows will be warm again with 70s common.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... The trough which has been affecting us will slide eastward a bit and weaken somewhat on Wednesday, which should allow several things to change a bit. The first is that with reduced instability thanks to warming aloft, we should see a noticeable decline in thunderstorm coverage, though isolated storms are still possible. The second is that with more sun and warmer temps aloft, surface temps will also likely increase a bit. The third is that with a bit more of a westerly flow at the surface in the wake of the system, dew points may in fact decline a little (though not much). Heat indices will approach advisory criteria in parts of the area, so will maintain mention in HWO for now, but right now expecting most places to fall short. On Thursday a weak ridge crosses the area, resulting in another slight increase on high temps, though with a stronger westerly flow progged in advance of a cold front dropping down from Canada, dew points may see a more noticeable decline, so we once again expect heat indices to approach, but perhaps not exceed, criteria. Will leave mention in HWO for time being though. Storm coverage, if any, again looks isolated, and by this time will likely be mainly terrain forced if any form at all. By Thursday night, the approaching front may start sending a few storms into the northwestern portion of the CWA, but otherwise, effects from the front should be limited. With the southwest flow keeping warm air moving in with diminished radiational decoupling, it should be among the warmest nights of the year, with temps staying well into the 70s, and perhaps 80s in the cities. && .LONG TERM /FRIDAY THROUGH MONDAY/...
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A particularly hot stretch is expected for days 4-7 across the Mid- Atlantic. Deep high pressure remains centered over the Mid- South states through this time. 850mb temps around 20C are expected over the Mid-Atlantic through this time. Just like last week, the only impediment to a notable heat wave is organized thunderstorm/MCS activity (and associated clouds) that will round the high. The high center positioned near Memphis allows streamlines from the southern Great Lakes to arc to the central Mid-Atlantic. This is of note as last week activity was generally regulated to the northern Mid- Atlantic (with the central Atlantic receiving quite a bit of debris clouds from Midwest activity the night before). Scattered terrain driven convective thunderstorms looks possible each day Friday through the weekend with a slight height decrease through this time per guidance. Max temps look higher this week compared to last as the heat is advected from the west instead of the south. This should limit dewpoints somewhat and allow for a higher ambient temperature. Risk for 105F heat index (heat advisory minimum criterion east of the Blue Ridge) is possible Thursday through Monday. Will blend ER Superblend with MEX guidance for max temps Friday- Monday.
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&& .AVIATION /14Z TUESDAY THROUGH SATURDAY/... VFR overall through the next few days. We still have the continued caveats of patchy fog over the next few mornings and scattered thunderstorms this afternoon, though coverage should drop to isolated on Wednesday and Thursday. Winds mostly 10 knots or less. Organized thunderstorms are possible at times Friday through Monday. Otherwise VFR with light westerly flow from high pressure centered over the Mid-South states. && .MARINE... Mostly sub-SCA over the next few days, with main concern being gusty winds from any thunderstorm which develops near or over the waters during the afternoon and evening hours. Highest risk of this is today and Wednesday, with a lower chance Thursday. Generally light Wly flow Friday to Monday with high pressure centered to our west. Organized thunderstorms are possible through this time. && .TIDES/COASTAL FLOODING... South flow will maintain elevated water levels through Wednesday night. Water levels are expected to remain just below minor coastal flood thresholds during the afternoon/evening preferred high tide. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...RCM NEAR TERM...MM/RCM SHORT TERM...RCM LONG TERM...BAJ AVIATION...BAJ/MM/RCM MARINE...BAJ/MM/RCM TIDES/COASTAL FLOODING...BAJ is the U.S. government's official web portal to all federal, state and local government web resources and services.