Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 290559 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 159 AM EDT Thu Sep 29 2016 .SYNOPSIS... An upper level low will settle over Kentucky Thursday into Friday before drifting across the Great Lakes this weekend. A persistent onshore flow and rain event will occur across the Mid-Atlantic through Friday. The low looks to track east across the northern Mid-Atlantic early next week with a cold front eventually crossing the area as the low moves offshore. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Latest convective allowing models continue to have the south flow bring increasing coverage of showers and thunderstorms across the area through midnight with ongoing activity overnight. Continued local heavy rain with ongoing flash flood threats. The activity over southern VA has been slow moving and repeating in places. Flooding threat is greatest if this occurs over sensitive locations like urban areas. Looks like severe thunderstorm watch can expire on time. Will coordinate on that now. Low level flow increases and shifts easterly Thursday with swaths of heavy rain persisting across the forecast area. && .SHORT TERM /6 AM THIS MORNING 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 concern 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 localized flooding Thursday - widespread rain with embedded excessive. Despite the weak instability on Thursday a strong wind field remains, so there could be a few stronger storms with wind/hail threats with a Marginal risk west of the Blue Ridge. 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 expected to continue through Friday night. Intensity looks to decrease Friday evening, but this will need to be followed in future forecasts. As of now the flash flood watch until 8am Friday seems sufficient. && .LONG TERM /SATURDAY THROUGH THURSDAY/... 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 /06Z THURSDAY 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 overnight and Thursday afternoon. Expect low ceilings through Friday night with persists onshore flow. Gradual improvement through the weekend. VFR should generally prevail by Sunday. && .MARINE...
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Updated the SCA to Gale Warning for the main channel of the Chesapeake Bay and eastern inlets until 14z Thursday...as ongoing observations indicate gusts near Gales. SCA in effect for the lower half of the Chesapeake Bay and lower Tidal Potomac...and for all waters through 12z Friday...as gradient winds increase with strengthening system. Gales possible Thursday as a southeast wind increases. 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.
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&& .TIDES/COASTAL FLOODING... Persistent onshore flow with gusts 20 to 25 knots will bring a steady increase to positive tidal departures across the waters through Friday with at least southern areas (Annapolis and south) of the bay and the entire tidal Potomac likely to see at least minor coastal flooding by Thursday afternoon. Went ahead with a coastal flood advisory for overnight at Straits Pt in St Marys County and Thursday early afternoon which should be more widespread. Will continue to monitor trends both up the MD part of the Bay and Tidal Potomac. May need to issue coastal flood watches for high tides as early as Thursday night. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch through Friday morning for DCZ001. Coastal Flood Advisory from 6 AM this morning to midnight EDT tonight for DCZ001. MD...Flash Flood Watch through Friday morning for MDZ003>006-011- 013-014-016>018-501>508. Coastal Flood Advisory until 8 PM EDT this evening for MDZ014- 016-018. Coastal Flood Advisory until 6 PM EDT this evening for MDZ017. VA...Flash Flood Watch through Friday morning for VAZ025>031- 036>040-050>057-501-502-505>508. Coastal Flood Advisory until 8 PM EDT this evening for VAZ057. Coastal Flood Advisory from 6 AM this morning to midnight EDT tonight for VAZ054. WV...Flash Flood Watch through Friday morning for WVZ050>053-055- 501>504. MARINE...Small Craft Advisory from 10 AM this morning to 8 AM EDT Friday for ANZ530>534-539>541-543. Gale Warning until 10 AM EDT this morning for ANZ530>534- 539>541-543. Small Craft Advisory until 8 AM EDT Friday for ANZ535>538-542.
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&& $$ Update...Melliott Previous...Woody!/Jackson

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