Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 251329 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 929 AM EDT Tue Apr 25 2017 .SYNOPSIS... Low pressure near the South Carolina North Carolina border will move slowly northeastward along the North Carolina coast through this evening, and finally off the Mid-Atlantic coast Wednesday. A cold front will approach the region late Thursday and remain nearby through the weekend. A stronger cold front will pass through the region early next week. && .NEAR TERM /THROUGH TONIGHT/... Bands of heavier rain will pivot northwestward across the Mid- Atlantic region in response to an 850mb warm front this morning. Additional rain of 0.25 to 0.5 is expected through the afternoon. NE-E winds, clouds and rain will keep the Mid- Atlantic region cool today with temperatures in the 50s to L60s by afternoon. Previous discussion... Low pressure will remain nearly stationary over North Carolina today before moving up the Mid-Atlantic Coast tonight. High pressure over southern Canada into New England continues to wedge south and west into the Mid-Atlantic. A northeast flow around these systems has caused cool conditions for this time of year. The onshore flow has also resulted in plenty of moisture. Warm and moist air around the low will continue to overrun the cooler air in place...resulting in more clouds and rain today. There has been lull in the rain early this morning...but a soaking rain is likely to fill from southeast to northwest between about 11z and 15z this morning. This is associated with the convergence from the 850mb warm front that will propagate into the area. Rain will continue through midday before tapering off in coverage later this afternoon into this evening as the dry slot rotating around the low builds into the area. Therefore...the steadiest rain is expected from about mid- morning through the middle afternoon hours. However...even when it is not raining that will be plenty of moisture underneath the subsidence inversion for low clouds and areas of drizzle. Leaned cooler than most guidance due to the cloud cover and rain expected today. The low become vertically stacked tonight as it moves up the Mid-Atlantic Coast. Therefore...it is expected to weaken. The weakening low along with the dry slot in place will continue to cause rain to diminish in coverage...but plenty of moisture underneath the inversion will result in more low clouds along with areas of light rain and drizzle. The best chance for measurable rainfall will be across northern and eastern portions of the CWA where wrap around moisture may build in as the low passes by just to our east. Areas of fog are possible overnight as well...especially in the valleys west of the Blue Ridge Mountains where there may be some breaks in the stratus deck toward morning. Fog may be locally dense. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... The low will continue to weaken and move away from the area Wednesday and upper-level ridging will build overhead. High pressure over the Atlantic Ocean will begin to take over the weather pattern...allowing for a southerly flow to develop. The southerly flow along with increasing sunshine and riding aloft will allow for noticeably warmer conditions compared to the previous couple days. Max temps will range from the upper 60s in northeastern Maryland where clouds may hang on a bit longer to the upper 70s/near 80 degrees across the central Shenandoah Valley where more sunshine is expected. A southerly flow will cause mild conditions Wednesday night as well. A cold front will approach from the west Thursday before moving into the area and weakening overhead Thursday night. A south to southwest flow will usher in unusually warm conditions for this time of year Thursday with max temps well into the 80s for most locations. A few showers and thunderstorms are possible ahead of the front...mainly late Thursday in the Potomac Highlands into Thursday night across the rest of the area. Coverage should remain isolated to widely scattered since forcing will be weak. Best chance for gusty thunderstorms will be across the Potomac Highlands perhaps into portions of the Shenandoah Valley Thursday afternoon and evening. More mild conditions are expected Thursday night. && .LONG TERM /FRIDAY THROUGH MONDAY/... Warm and humid airmass will reside over the Mid-Atlantic through much of the long term as our area remains in the warm sector...with temperatures remaining well above normal through the period. Heat and humidity will combine to foster moderate- to-strong instability Friday into the weekend...though focus for convective development expected to be rather nebulous...with terrain circulations likely main source for any diurnally-driven thunderstorm activity. Isolated strong storms are possible during the afternoons/evenings. Stalled front looks to remain far enough north initially to impact the area...though it could sag toward the area late in the weekend. It remains unclear if storms will remain over the higher terrain...or if they will be able push east toward the metro areas. Potential for more widespread thunderstorm activity (some of which could be strong) arrives Monday as strong cold front approaches the region. && .AVIATION /13Z TUESDAY THROUGH SATURDAY/... Cigs will continue to lower to IFR and even LIFR levels this morning. Rain will become more widespread by mid-morning and it will continue through most of the day before diminishing in coverage late this afternoon and evening. Low clouds will hang around. IFR conditions are likely...but there may be breaks of MVFR cigs at times late this morning and afternoon. Low pressure will pass by to the east tonight. Low clouds will hang around underneath the subsidence inversion. IFR conditions are likely. Cigs may improve at KCHO...but areas of fog may develop due to light winds and saturated soil. Low clouds and fog should give way to VFR conditions Wednesday...but the exact timing of that occurring is uncertain at this time. Most likely scenario is for MVFR conditions to develop in the morning and become VFR by the afternoon. Light winds and mainly clear skies may allow for areas of fog to develop overnight Wednesday into Thursday morning. A southerly flow will develop Thursday and an approaching cold front may trigger a shower/thunderstorm Thursday night. Coverage will be isolated to widely scattered. VFR expected to prevail Friday into the weekend...though, isolated to widely scattered thunderstorms are possible (especially during the afternoon/evening). && .MARINE... Low pressure over North Carolina this morning will weaken as it moves up the Mid-Atlantic Coast later this afternoon through tonight. A strong pressure gradient for today will cause gusty northeast winds. A Small Craft Advisory is in effect for the waters. A Gale Warning is in effect this morning for the Maryland Chesapeake Bay south of Drum Point and the lower Tidal Potomac River south of Cobb Island. The strongest winds will be between 11z and 15z across these areas with gusts up to 35 knots. Winds will gradually diminish later today and tonight as the pressure gradient weakens. Will continue with the Small Craft Advisory for the Bay and lower Tidal Potomac River tonight...but that may be able to be cancelled early based on the latest guidance. A southerly flow will develop Wednesday and the southerly flow will strengthen ahead of a cold front Thursday into Thursday night. A Small Craft Advisory may be needed for portions of the waters Thursday into Thursday night. Light winds expected Friday into the weekend...though, isolated to widely scattered thunderstorms possible (especially during the afternoon/evening). && .TIDES/COASTAL FLOODING... Easterly flow around low pressure to the south will continue to promote persistent onshore flow over the waters through today...which will keep tidal anamolies elevated. Minor tidal flooding is expected at Straits Point/St. Mary`s county and a coastal flood advisory has been extended for this afternoons high tide cycle. There is still some uncertainty in flood potential at other sensitive sites for the next high tide cycle...though it currently appears they will remain below minor flood stage. The flow should gradually turn northerly tonight into Wednesday as the low moves away from the area. Water levels should begin to decrease during this time...though minor coastal flooding will remain possible through tonights tide cycle at the most sensitive sites. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory until 5 PM EDT this afternoon for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory until 4 AM EDT Wednesday for ANZ530>533- 538>542. Small Craft Advisory until 4 AM EDT Wednesday for ANZ534-537- 543. Gale Warning until 11 AM EDT this morning for ANZ534-537-543. Small Craft Advisory until 6 PM EDT this evening for ANZ535- 536. && $$ SYNOPSIS...BJL NEAR TERM...BJL/HSK SHORT TERM...BJL LONG TERM...MSE AVIATION...BJL/MSE/HSK MARINE...BJL/MSE TIDES/COASTAL FLOODING...BJL

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