Area Forecast Discussion
Issued by NWS Sterling, VA

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520 FXUS61 KLWX 280840 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 440 AM EDT Wed Sep 28 2016 .SYNOPSIS... An upper level low will impact the area Wednesday through Thursday night...bringing the potential for heavy rainfall across portions of the area. The upper level low will gradually move away from the area Friday through the weekend. && .NEAR TERM /THROUGH TONIGHT/... Complex weather situation is beginning to unfold across the strong upper-level low is starting to drop southward into the Ohio River Valley. At the surface, frontal boundary is across central VA...where it will remain through much of today. Overrunning showers have already been developing this morning across the VA Piedmont and are now pushing into parts of northern VA. Expect this trend to continue through the morning and into the afternoon...with a northward shift in precipitation chances through the day. While instability remains rather anemic, strengthening wind field could lead to some stronger storms across southern portions of the area...with wind/hail the primary threats in the strongest storms. SPC has much of central VA and the VA Piedmont in a marginal risk for SVR...which seems very reasonable considering the setup. Training storms could lead to some flooding issues...but dry antecedent conditions should prevent most hydro issues at least for today. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... Heaviest rain expected later tonight into Friday morning as strong upslope flow (up to 50 kts out of SE) and high precipitable water transport set up over the area (axis of heaviest rainfall expected over our area). Spatiotemporal characteristics of the Flash Flood watch in good repetitive bouts of moderate-to-heavy rain could eventually saturate the dry soils and lead to runoff concerns. Storm total QPF still greater than 3 inches over a large area...with the potential for higher totals in localized areas. Despite the weak instability on Thursday...strong wind field could also be a few stronger storms with wind/hail threats. General consensus is for the bulk of the heaviest rainfall to begin to wind down during the day Friday as low pressure system begins to weaken as it becomes vertically stacked. && .LONG TERM /SATURDAY THROUGH TUESDAY/... Upper low will remain across the Ohio Valley this weekend. Hence, we will need to keep a chance of showers in the forecast for at least Saturday. However, that chance will be less than the end of the week as the low will begin to lift toward the Great Lakes and eventually the St. Lawrence Valley. That should permit a drying trend, making Sunday the better of the two weekend days. High temperatures will be near normal for the start of October. Lows, on the other hand, will be about 5 to 10 degrees above normal, due to the humidity from the influx of moisture. High pressure should be back for early next week. && .AVIATION /09Z WEDNESDAY THROUGH SUNDAY/... Upper-level low will impact the area today through Friday...bringing periods of IFR (and perhaps lower) to all the terminals. Heaviest rain expected late tonight into Friday. Isolated thunderstorms are also possible this afternoon and Thursday afternoon. Could see a gradual return to VFR later Friday afternoon. Gradual improvement through the weekend, with windows for flight restrictions becoming more scarce. VFR should generally prevail by Sunday. && .MARINE... Gradient winds should remain below SCA criteria through much of the day today...though SCA gusts are possible in showers/thunderstorms. Small craft advisory was issued starting later this evening through 12z gradient winds strengthen with strengthening system. Gradient should begin to relax by later Friday...with SCA winds less likely. Upper low will slowly lift and fill this weekend. No flags anticipated at this time. && .TIDES/COASTAL FLOODING... Current forecast keeps tidal sites below flood stage at least through most of Thursday. Believe guidance is not capturing strength of easterly flow this will need to be monitored to see if coastal flooding concerns arise sooner. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch from late tonight through Friday morning for DCZ001. MD...Flash Flood Watch from late tonight through Friday morning for MDZ003>006-011-013-014-016>018-502>508. VA...Flash Flood Watch from late tonight through Friday morning for VAZ027>031-038>040-050>057-501-502-505>507. WV...Flash Flood Watch from late tonight through Friday morning for WVZ050>053-055-504. MARINE...Small Craft Advisory from 6 PM this evening to 8 AM EDT Friday for ANZ530>543.
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&& $$ SYNOPSIS...MSE NEAR TERM...MSE SHORT TERM...MSE LONG TERM...HTS AVIATION...MSE/HTS MARINE...MSE/HTS TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.