Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 281825 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 225 PM EDT Wed Sep 28 2016 .SYNOPSIS... An upper level low will impact the area through Thursday night... bringing the potential for heavy rainfall to the Mid Atlantic. This will gradually move away from the area this weekend. && .NEAR TERM /THROUGH TONIGHT/... Complex weather situation is unfolding across the area...as strong upper-level low is dropping southward into the Ohio River Valley. At the surface, frontal boundary is across southern VA...remaining nearly stationary. Very challenging forecast over the next couple of days. It`s interesting to see how much the atmosphere moistened up in the 12 hours today between the 00Z and 12Z IAD sounding. Training storms could lead to some flooding issues later today. For hours the HRRR has been projecting a band of rain setting up from south to north this evening over the I-95 corridor. Given the atmosphere has been outputting heavy rainfall already in this air mass believe that if this line does indeed develop that 1-2"/ locally more could be possible overnight. Another factor with the sounding/VWP is the wind field - low level winds are from the east...turning abruptly to the southwest above 6000 feet. Turning in the atmosphere has led SPC to put the CWA east of the Blue Ridge at a 5% tornado risk. Limiting factor would be extensive cloud cover. If a tornado were to impact our CWA it would likely develop in SE VA and track north. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... Thursday is a day for all to be concerned about. BUFKIT soundings are showing a strong low level easterly flow. This is a prime heavy rain/flood producer. It is difficult to pick an exact spot where the heaviest rain could fall but two areas of conern appear to be 1) the area between DC and the PA border and 2) the eastern slopes of the Blue Ridge. Flash flood watch is in effect through Friday morning for the CWA with the exception of the Highlands. Depending upon rainfall rates and fact that ground has been dry this month we may see steadier rises on streams/creeks...and over time see main stem rivers rise. To summarize - high concern for possible flooding Thursay but it is too early to pinpoint where. Despite the weak instability on Thursday...strong wind field remains... so there could be a few stronger storms with wind/hail threats. The concern will definitely continue Thursday night as the upper cutoff low continues to spin to our west. East/southeast flow will keep the moisture streaming into the northern Mid Atlantic. Flooding threat will persist. Showers will also be possible through Friday. The thought process will continue to evolve with later forecasts. && .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 /18Z WEDNESDAY THROUGH MONDAY/... Upper-level low will impact the area today through Friday...bringing IFR/LIFR 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. VFR should generally prevail by Sunday. && .MARINE... SCA in effect for the lower half of the Chesapeake Bay and lower Tidal Potomac...and for all waters starting later this evening through 12z Friday...as gradient winds increase 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. This will be monitored to see if coastal flooding concerns arise sooner. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flash Flood Watch from 6 PM EDT this evening through Friday morning for DCZ001. MD...Flash Flood Watch from 6 PM EDT this evening through Friday morning for MDZ003>006-011-013-014-016>018-502>508. VA...Flash Flood Watch from 6 PM EDT this evening through Friday morning for VAZ025>031-036>040-050>057-501-502-505>508. WV...Flash Flood Watch from 6 PM EDT this evening through Friday morning for WVZ050>053-055-504. MARINE...Small Craft Advisory from 6 PM this evening to 8 AM EDT Friday for ANZ530>532-535-536-538>541. Small Craft Advisory until 8 AM EDT Friday for ANZ533-534-537- 542-543. && $$ PRODUCTS...WOODY!

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