Area Forecast Discussion
Issued by NWS Sterling, VA

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982 FXUS61 KLWX 280123 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 923 PM EDT Tue Sep 27 2016 .SYNOPSIS...
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A deep upper level low will impact the area Wednesday afternoon through Thursday night...bringing the potential for heavy rain. The upper level low will slowly move away from the area Friday through the weekend.
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&& .NEAR TERM /THROUGH WEDNESDAY/...
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Main issue for tonight will be development of a stratus deck and/or possible fog as suggested by NSSL WRF-ARW. Low clouds already seen banked against the Blue Ridge mtns with low clouds and/or fog advecting northward to northern VA. Perhaps, a slight chance of showers southern areas. Will be adjusting PoPs downward for tonight. Overnight, will see northeasterly winds develop at the surface as high pressure builds over New England. At the same time will see south/southeasterly flow increase at 850 mb and southwesterly above 800 mb. This will help to usher in additional moisture to the region. Later tonight and towards morning will likely see the development of a low stratus deck across central Virginia, shifting northward towards the DC Metro. Could also be some patchy fog and a few showers as well. Lows tonight will range from the upper 40s in the far west where skies remain mostly clear to the low 60s south/east. A flash flood watch has been issued for potential heavy rainfall late Wed night through Thu night. As of right now, there is a strong consensus between ECMWF and GFS ensembles for widespread 2-5 inches mostly in a 12-hr period between 06Z-18Z Thu. Even with the very dry antecedent conditions, believe there is a flash flood threat given deep layered southerly flow suggesting repeated activity and potential for continued convective regeneration. Guidance also indicates negative Showalter indices and sufficient CAPE for t-storms late Wed night through Thu which could greatly enhance rainfall rates. .SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/... A strong cutoff upper-level low will drop south from the Great Lakes into the Ohio Valley Wednesday and stall through the end of the week. This will act to pump sub-tropical moisture into the region on strong south/southeast flow from Wednesday through Friday. At the same time, low level boundary that passed through today will gradually lift northward through the region, and this will provide for added forcing/focus for development of heavy showers through the period. Showers and possibly a few thunderstorms will develop Wednesday, with coverage increasing through the day. May see enough instability develop in southern Maryland and central Virginia that we could see a strong storm or two. SPC has placed a Marginal Risk for severe thunderstorms to the south. However, even with relatively dry antecedent conditions, main concern will be development of heavy rain. The timeframe for the greatest risk of flash flooding will occur Wednesday night through Thursday night. Will see low level boundary shift northward and as it does so, precipitable values surge to 1.75" across portions of the region, which is significantly above the 90th percentile of around 1.5" for the date. Warm cloud depths rise to well above 10k feet as well, indicative of efficient warm rain processes. Divergence aloft from the upper low sitting to the west will further enhance the prospect for the development of heavy rain. Current indications are for the greatest risks for flash flooding will occur across northern/central Maryland and back across into northern Virginia and the panhandle of West Virginia and western Maryland as upslope component likely develops on the strong south/southeast flow. Current forecast will show a relatively large area of 2-5", with locally higher amounts possible in any training convection that develops. WPC has placed portions of the region in a Moderate Risk for excessive rainfall. There may also be a secondary risk for a few stronger storms on Thursday as well with strong shear profiles and marginal CAPE. Showers then gradually taper off towards Friday. High temperatures through the period generally 70-75F, with lows in the 50s/60s.
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&& .LONG TERM /FRIDAY THROUGH TUESDAY/... Upper level low will be lingering to our west on Friday and start moving north towards the Great Lakes Saturday. Unsettled weather conditions are possible Friday into Saturday as moisture is advected from the Atlantic towards our area. As the system moves north, conditions should be clearing Saturday into Sunday from south/southeast to north of our CWA. High pressure builds in on Sunday into Tuesday. High temperatures will be in the upper 60s and 70s. && .AVIATION /01Z WEDNESDAY THROUGH SUNDAY/... Mainly VFR conditions are expected through this evening and the first half of tonight as high pressure builds in to the north. Some lingering MVFR ceilings at CHO should scatter out shortly. Overnight, low stratus deck expected to develop over central Virginia and move northward. May also see some patchy fog and even a shower. Greatest chance for MVFR/IFR conditions at CHO, with MVFR possibly into IAD/DCA by morning. An upper-level low will then impact the terminals Wednesday through Thursday night. A mix of flight categories expected Wednesday in areas of low clouds and developing showers. More widespread showers and possible thunderstorms are expected to impact the terminals Wednesday night through Thursday night with widespread sub-VFR conditions likely. Sub-VFR conditions possible Friday and maybe into Saturday as upper level disturbance lingers to our west on Friday and then moves away from us on Saturday. Rain showers possible Friday and Saturday due to this disturbance. Conditions becoming VFR sometime on Saturday and into Monday as high pressure builds into our area. && .MARINE... High pressure will briefly build overhead following the frontal passage this morning. Sub-SCA winds expected to continue through tonight and much of Wednesday. Low pressure will then impact the waters later Wednesday through Thursday night. Widespread showers and isolated thunderstorms are expected during this time which will cause locally lower visibilities. A small craft advisory may be needed for portions of the waters Wednesday night through Thursday night. Small craft advisory possible Friday with winds near the threshold. Winds will be diminishing later on Friday as they fall below 15 kt. && .TIDES/COASTAL FLOODING...
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No coastal flooding anticipated now until late Thu night or early Fri morning at the earliest.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch from late Wednesday night through Friday morning for DCZ001. MD...Flash Flood Watch from late Wednesday night through Friday morning for MDZ003>006-011-013-014-016>018-502>508. VA...Flash Flood Watch from late Wednesday night through Friday morning for VAZ027>031-038>040-050>057-501-502-505>507. WV...Flash Flood Watch from late Wednesday night through Friday morning for WVZ050>053-055-504. MARINE...None.
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&& $$ SYNOPSIS...MM NEAR TERM...LFR SHORT TERM...MM LONG TERM...IMR AVIATION...MM/IMR MARINE...MM/IMR TIDES/COASTAL FLOODING...LFR

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