Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 191339 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 939 AM EDT Sat Aug 19 2017 .SYNOPSIS... Weak high pressure will build in today...but an upper-level disturbance and reinforcing cold front will pass through the area this evening. High pressure will build overhead late tonight through Sunday before moving offshore Monday. High pressure will remain offshore Tuesday and a stronger cold front will pass through Wednesday. && .NEAR TERM /THROUGH TONIGHT/...
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Clearing has occurred behind the cold front this morning. Mostly sunny skies will continue this morning and into the afternoon. Previous discussion... Drier air will continue to move into the region later this morning into this afternoon from the northwest behind the cold front and also behind the upper-level disturbance. More warm conditions are expected...but it will be noticeably less humid compared to Friday along with some sunshine. Max temps will be in the upper 80s to lower 90s across most locations. Another upper-level disturbance and its associated cold front will approach the area late this afternoon before passing through this evening. Most of the forcing from the upper-level disturbance will be to the north and forcing along the cold front will be weak. A few popup showers and perhaps even an isolated thunderstorm are possible across northern and central portions of the CWA late this afternoon through this evening. Coverage should remain isolated to scattered and much of the time will turn out dry. The weak cold front will move off to the east late tonight and high pressure will return...bringing dry conditions. Min temps will be range from the 50s in the Allegheny Highlands to near 70 in downtown Washington and Baltimore. Dewpoints in the 50s and 60s will make it feel more comfortable compared to recent nights.
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&& .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... High pressure will remain over the region Sunday...bringing dry conditions along with sunshine. More seasonably warm conditions are expected...but humidity will remain on the low side with dewpoints in the low to mid 60s for most areas. Surface high pressure will remain overhead Sunday night...but a return flow at the low-levels will allow for warm and moist air to overrun the relatively cooler surface air. This will bring more clouds. An isolated shower cannot be ruled out...but most areas should be dry. The surface high will move offshore Monday and a southerly flow will allow for more humid conditions to return. At the same time...there may be a couple upper-level disturbances that pass through the area in the zonal flow aloft. A few showers and thunderstorms are possible...especially during the later afternoon and evening hours due to more instability and forcing from a surface trough and upper-level disturbances. Still some uncertainty as to how much moisture makes its way back into the region...but instability may be on the high side which could cause some stronger or locally severe storms. Confidence is low at this time. As for clouds...there may be a bkn/ovc deck around 5 to 10kt feet from isentropic lift to start the morning. Do think the isentropic lift will dissipate some and this should allow for breaks of sunshine during the late morning into the afternoon hours. Then...more clouds are likely with any thunderstorms later in the day. Therefore...the most likely sky cover during the solar eclipse will be partly sunny. A southerly flow will continue Monday night...bringing very warm and humid conditions. && .LONG TERM /TUESDAY THROUGH FRIDAY/... High pressure will remain over the Atlantic while a cold front approaches from the north and west Tuesday. More very warm and humid conditions are expected ahead of the boundary. Showers and thunderstorms are also possible ahead of the boundary...especially Tuesday afternoon into Tuesday night. Some storms may become strong to severe due to strengthening shear profiles associated with the upper-level trough that will be digging from the Great Lakes into New England. The cold front will pass through Wednesday. A few more showers and thunderstorms are possible...but confidence is low due to a downsloping westerly flow that may develop behind a pressure trough and ahead of the cold front. High pressure will build toward the area Wednesday night through Friday while an upper- level trough digs south from the Great Lakes over our area. A northwest flow will usher in much cooler and less humid conditions compared to recent days. Highs in the 70s to lower 80s with lows in the 50s and 60s will make it feel more like mid to late September instead of August. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... VFR conditions are expected most of the time through Wednesday. Patchy fog cannot be ruled out early this morning. An isolated shower or thunderstorm is possible late this afternoon and evening across the northern terminals...but most areas will be dry. Scattered thunderstorms are possible later Monday into Monday night as a southerly flow allow for more atmospheric moisture to return. A cold front will approach Tuesday before passing through Wednesday. Showers and thunderstorms are possible ahead of the boundary. Some thunderstorms may become strong to locally severe during the early and middle portions of next week. Confidence remains low at this time. && .MARINE...
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A reinforcing cold front will pass through the waters this evening. A popup shower cannot be ruled out but most areas will be dry. High pressure will build overhead for Sunday before moving offshore Monday. A return southerly flow will develop Monday and persist through Tuesday night ahead of a cold front. Winds will turn to the northwest behind the cold front Wednesday. A Small Craft Advisory may be needed for Monday night and Tuesday and it will likely be needed Wednesday into Wednesday night behind the cold front.
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&& .TIDES/COASTAL FLOODING...
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An offshore flow will develop behind a cold front early this morning. This should allow for anomalies to decrease a bit today. Another reinforcing cold front will pass through tonight and an offshore flow will develop behind that boundary late tonight into Sunday. Therefore...minor flooding is not forecast at this time. However...it will be close for sensitive areas because the offshore flow will be weak and we are approaching a new moon phase which means astronomical norms will be higher during the preferred high tide. Therefore...we will have to keep a close eye on anomalies late tonight into early Sunday. A southerly flow will increase early next week and this may cause minor flooding near high tide for sensitive areas Monday night into Tuesday.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...None.
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&& $$ SYNOPSIS...BJL NEAR TERM...BJL/HSK SHORT TERM...BJL LONG TERM...BJL AVIATION...BJL MARINE...BJL TIDES/COASTAL FLOODING...HSK

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