Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 111819 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 219 PM EDT Wed Oct 11 2017 .SYNOPSIS... A frontal zone will remain nearly stationary across central Virginia into the Potomac Highlands into this evening. An area of low pressure will track along the front and push it well to our south late tonight into Thursday. High pressure will build north and east of the area later Friday through Saturday before moving offshore Sunday. A stronger cold front will pass through the area early next week. && .NEAR TERM /THROUGH TONIGHT/... A nearly stationary boundary remains over the Potomac Highlands into central Virginia this afternoon. The boundary separates cooler marine air to the north and east vs. warm and humid air to the south and west. The boundary will remain nearly stationary over the area during this time while shortwave energy passes by to the north. This will cause the low-level boundary between 925-850mb to move north and east as a warm front. Thus...warm and moist air will be overrunning the surface cooler air in place and when combined with added lift from the low-level boundary...this will lead to areas of rain and drizzle for locations north and east of the surface stationary front. Popup showers are expected along and to the south and west of the front where more unstable air persists. Latest mesoanalysis shows around 250-500 J/KG of MLCAPE across our western and southwestern areas. An isolated thunderstorm is possible across these areas into this evening. Max temps will range from the 60s across northern the lower and middle 70s near Washington and the upper 70s and lower 80s across portions of central Virginia. More shortwave energy will pass by to the north this evening before moving off to our east late tonight. More rain and drizzle is expected through this evening. In fact...PWATS remain high and there may be a slight uptick in forcing as a surface low along the boundary approaches our area. Also...convection upstream may overrun the boundary enhancing chances for rain. Therefore...moderate to locally heavy rain is possible this evening. Still think chances for flooding are low due to the recent dry conditions we`ve had. The boundary will move south and west as a cold front overnight as the shortwave energy moves off to the east. The onshore flow will cause low clouds along with areas of rain and drizzle/fog. Min temps will be in the 50s and 60s. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... High pressure will along the New England coast will extend southwest into our area Thursday through Friday before building overhead Friday night. An onshore flow is expected during this time while warm and moist air overruns the marine air in place. Therefore...plenty of clouds are expected along with areas of rain and drizzle/fog. Cooler conditions will persist during this time with max temps in the 60s and min temps in the 50s/60s. Fog may be locally dense...especially along the ridge tops. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Large area of high pressure will drift eastward into the northwestern Atlantic over the weekend. A front will drop in from the northwest early next week but weaken as it does so. Some scattered showers are possible with this front, but significant rain is not expected due to an overall lack of moisture. Long term guidance has been reasonably consistent with timing the frontal passage late Monday. High pressure and more fall-like weather returns behind the front for the middle of next week. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... A stationary boundary will remain to the south and west of all the terminals except KCHO through this evening. An easterly flow to the north and east of the boundary will cause low clouds along with areas of rain/drizzle. MVFR to IFR conditions are most likely. Timing of IFR conditions still remain uncertain as of now. In fact...most guidance shows MVFR conditions across the eastern terminals for most of the time into this evening. Higher cigs upstream to the east suggest this is possible...therefore the forecast shows MVFR conditions for BWI...DCA and MTN. However...confidence is low and IFR conditions cannot be ruled out. Timing of rain is uncertain...but it appears that the most widespread rain will be this evening between 00z and 04z as shortwave energy passes through. For KCHO...low clouds have broken and high MVFR or VFR conditions are most likely through late this afternoon. Cigs will lower as the boundary shifts south and west tonight. A popup thunderstorm cannot be ruled out across KCHO as well. The boundary will drop south and west as a cold front late tonight. An onshore flow behind the boundary will cause more low clouds and some fog along with areas of rain and drizzle. Mainly IFR conditions are expected late tonight through Friday night. Sub-VFR possible in low clouds/fog Sat and Sun AM. Otherwise, mainly VFR this weekend. Light flow. && .MARINE... An onshore flow will strengthen later this evening as a stationary boundary drops south as a cold front and high pressure builds toward the area from the north and east. A Small Craft Advisory is in effect for the Bay and lower Tidal Potomac River tonight. A pressure surge is expected Thursday and a Small Craft Advisory is in effect for all of the waters. The strongest winds are expected Thursday across the middle portion of the Bay and lower Tidal Potomac River with gusts around 25 to 30 knots likely. The gradient will subside a bit for Thursday night and Friday...but a Small Craft Advisory continues for portions of the waters overnight Thursday and it may need to be extended into Friday evening as well. Generally light winds expected over the waters this weekend as high pressure drifts eastward. && .TIDES/COASTAL FLOODING... E/SE flow will keep water levels elevated with a persistent threat for minor flooding into the weekend. && .EQUIPMENT... The NWS 88D Sterling radar (KLWX) will be down during the day from 7 AM through 5 PM today through Saturday this week. This is for nationally scheduled maintenance on the radome. Our surrounding 88D radars as well as FAA terminal radars will continue to scan our skies during this downtime. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory until 9 PM EDT this evening for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory from 10 PM this evening to 6 AM EDT Friday for ANZ531>534-537-539>541-543. Small Craft Advisory from 6 AM Thursday to midnight EDT Thursday night for ANZ535-536-538-542. Small Craft Advisory from 10 PM this evening to midnight EDT Thursday night for ANZ530. && $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...DFH AVIATION...BJL/DFH MARINE...BJL/DFH TIDES/COASTAL FLOODING...DFH EQUIPMENT...BJL is the U.S. government's official web portal to all federal, state and local government web resources and services.