Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 250737 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 337 AM EDT Sun Sep 25 2016 .SYNOPSIS... High pressure will build in for today before moving off the coast Monday. A cold front will pass through the area overnight Monday into Tuesday. An upper-level low will likely impact the area during the middle portion of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... High pressure located over eastern Ontario early this morning will build southeastward over the Northeastern US through today. Abundant low level cloud cover currently exists over southern and western portions of the region due to trapped low level moisture under the developing subsidence inversion in combination with light upslope flow into the terrain. Some very light rain/drizzle/patchy fog also possible this morning across these areas. Extent of cloud cover not expected to change a whole lot today, although some shrinking of the coverage is likely later today. Thus a mostly sunny day expected north/east and mostly cloudy south/west. Highs generally 70-75 degrees, although in locations that remain cloudy through the day, temperatures may be stuck in the 60s. && .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/... The high will continue to shift eastward into eastern New England tonight, promoting persistent light easterly flow. This will likely lead to the expansion of a low stratus deck of clouds again tonight, especially in western areas. Again some patchy fog/drizzle or an isolated shower possible west of the Blue Ridge. Lows mainly in the 50s. A frontal system will then approach on Monday and cross the region late Monday night into early Tuesday morning. Clouds will thicken through the day Monday across the entire region with chances for some showers increasing, especially west and late in the day. A thunderstorm or two also possible in the evening as the front approaches. The most widespread rain showers will overspread the area Monday night, out ahead of the front. Again a thunderstorm or two is possible as well. Highs Monday in the 70s, with lows Monday night a bit milder ahead of the front, in the 50s/60s. Front will push eastward Tuesday, slowing as it does so though as associated upper low closes off and shifts southward through the Great Lakes. Thus chances for rain showers continue into Tuesday, mainly in the morning, and east of the DC/Baltimore metro areas. Highs Tuesday will be similar to Sunday/Monday, generally 70-75F. Lows Tuesday night 40s/50s. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... A cutoff upper-level low over the Great Lakes early Wednesday will drop south and east toward our area for Thursday and Friday. The cutoff low may even hang around for Saturday as well. Exact details in the forecast for Wednesday through Saturday remain highly uncertain because guidance continues to diverge with exactly how far south the upper-level low will drop before eventually moving away to the north and east. Should the low drop far enough to the south...then coastal low pressure will develop along the Mid- Atlantic Coast and widespread rain will occur across our area along with the chance for thunderstorms. However...should the low stay farther north...then precipitation amounts will be light and coverage would remain scattered. Guidance over the last 24 hours has trended a bit farther south with the cutoff low...and the forecast will reflect that trend. However...the ensemble forecasts also show high uncertainty and the latest GEFS and EPS mean indicate that the upper-level low will remain just to our north. Therefore...the latest forecast will also contain an element of continuity. Basically...will allow for more clouds along with a chance for showers. && .AVIATION /07Z SUNDAY THROUGH THURSDAY/... VFR at all sites today except CHO, which will experience MVFR ceilings through about 15z or so, before lifting to VFR. Could also be a few periods of patchy drizzle during that time period. Winds generally northeast, less than 10 knots. Additional development of sub-VFR conditions (primarily ceilings) expected tonight, mainly CHO, although also possible at MRB. A few areas of patchy fog/drizzle also possible again. Sub-VFR conditions possible Monday/Monday night as clouds and chances for showers increase. Will see some improvement on Tuesday. Light south winds develop tonight and increase on Monday/Monday night ahead of frontal passage. Winds will turn light westerly on Tuesday. An upper-level low may impact the area Wednesday and Thursday. Details remain uncertain...but subvfr cigs and vsbys are possible along with showers. && .MARINE... Marginal SCA remains in effect across the northern Chesapeake Bay early this morning with expected surge of northerly surface winds. Some gusts to about 18-20 knots expected. Sub-SCA conditions then return later today and continue into tonight. Southerly winds will then be on the increase Monday and especially Monday night ahead of an approaching frontal system when additional Small Craft Advisories will likely be needed. Winds gradually lessen on Tuesday/Tuesday night. An upper-level low will drop south toward our area for Wednesday through Friday. Details are uncertain...but if the upper-level low drops far enough to the south. This will cause coastal low pressure to develop along the Mid-Atlantic Coast. A small craft advisory may be needed for the waters Wednesday through Friday. && .TIDES/COASTAL FLOODING... High pressure will build toward the coast today before moving offshore tonight through Monday. An onshore flow will persist today and this will cause tidal anomalies to increase. However...the onshore flow should be light. The onshore flow should be light so water levels should remain below minor flooding thresholds. Water levels will be close to minor flooding thresholds in sensitive areas during the high tide cycle tonight into early Monday...this will be the higher of the two astronomical norms. The onshore flow will increase and turn to the southeast for Monday through Monday night. Minor flooding is possible during this time...especially during the high tide cycle Monday night which will be the higher of the two astronomical norms. && .CLIMATE... Unusually warm weather has persisted over the area for quite some time. Autumn-like temperatures are finally making an appearance over the area, and DCA may fall below 60 degrees early Sunday morning for the first time since June 9th (total of 107 days through September 23rd). The record most consecutive days at or above 60 degrees in the Washington DC area is 112 set in 2012. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 10 AM EDT this morning for ANZ530>533-538>541. && $$ SYNOPSIS...BJL NEAR TERM...MM SHORT TERM...MM LONG TERM...BJL AVIATION...BJL/MM MARINE...BJL/MM TIDES/COASTAL FLOODING...BJL CLIMATE...DFH is the U.S. government's official web portal to all federal, state and local government web resources and services.