Area Forecast Discussion
Issued by NWS Sterling, VA

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137 FXUS61 KLWX 221857 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 257 PM EDT Sun Oct 22 2017 .SYNOPSIS...
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High pressure will push eastward into the northwest Atlantic through Monday. Low pressure will move up the Appalachians Monday night into Tuesday. Upper troughing will remain over the region through the second half of the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Mostly clear this evening, but low clouds and areas of fog are expected to develop late tonight as low-level moisture increases in SSE flow. Warmer tonight due to more clouds and SSE winds.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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Low clouds mix out during the late morning, but mid-high clouds thicken through the day with showers developing over the Appalachians and spreading east Mon night as moisture deepens in response to strenghtening low-level jet. Models indicate a thin convective line will move quickly across the area with potential for gusty winds of 35-40 mph as strong 850mb low-level jet of 50-60 kts mixes down with the showers. The pre-frontal convective line should be east of the Chesapeake Bay by 18Z Tue but light showers will remain possible Tue-Tue night as actual cdfnt moves through and upper trough remains still to the west. Rainfall amounts of 0.5 to 0.75 inches are expected except higher over the Appalachians and Blue Ridge mountains. Given progressive nature of front and dry antecedent conditions, no flooding is anticipated.
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&& .LONG TERM /WEDNESDAY THROUGH SUNDAY/...
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The cold front will be well off the eastern seaboard 12Z Wednesday as weak high pressure builds. Moderate 850 hPa cold advection on Wednesday afternoon in NW flow should bring a fair amount of strato-cumulus cloudiness across the area which will hold down temperatures during the afternoon. 12Z/GFS shows 850 hPa temperatures falling to at or just below 0 degC overnight Wed into Thursday, but quickly start to rebound by 12Z Thursday. Along upslope mountain areas Wednesday night, there could be some a few showers, with temps fall aloft to below freezing...some wet snow could mix with rain after midnight. Thursday...high pressure builds across the southeast and into our region...and will be in control with dry weather and seasonal temperatures. Morning temperatures will probably be the coldest of the week...but nothing unseasonably cold. Friday...the center of the high will move off the southeast coast during the day...allowing a return of warmer readings to commence on Friday under clear skies. Weekend...Saturday...the center of high will be over New England...and temperatures will once again rebound to above seasonal normals. Dry weather is expected on Saturday. By Sunday, more uncertainty in the forecast associated with the timing of a cold front extending from a low centered over the northern Great Lakes, that is expected to cross the area. The new 12Z GFS is faster brining in the colder air in on Sunday compared to the slower 00Z/ECMWF ensemble guidance. Looks like this is the next best chance for widespread wetting precipitation.
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&& .AVIATION /19Z SUNDAY THROUGH FRIDAY/...
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Widespread low clouds (MVFR cigs) and areas of 2-4sm fog are expected to develop late tonight as low-level moisture advects in SSE flow. Dense fog is possible at KBWI and KMTN. Showers expected to overspread the area quickly late Mon afternoon and early evening as cdfnt approaches. Gusty winds will accompany these showers with gusts up to 40 mph possible. LLWS is also possible Mon night. Convective line will move east of the terminals by 15Z Tue with flying conditions improving. Expect VFR conditions through the extended. Possibly on Friday there could be locally IFR visibilities in early morning fog at IAD/MRB terminals.
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&& .MARINE...
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Winds are expected to strengthen some this evening but especially Mon afternoon and night. Have issued a gale warning for much of the Chesapeake Bay and lower Potomac for Mon night in anticipation of a convective line to move across the area when a 850-mb jet of 50+kt is expected to spread the area. Small craft winds possible Wednesday. As high pressure builds in on Thursday, expect winds/waves to remain below small craft advisory levels for the remainder of the week.
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&& .TIDES/COASTAL FLOODING...
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Water levels are expected to rise rapidly Mon night as strong srly low-level flow develops with minor coastal flooding expected at most of the sensitive locations.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 11 AM to 6 PM EDT Monday for ANZ531>534-537-539>541-543. Gale Warning from 6 PM Monday to 6 AM EDT Tuesday for ANZ531>534-537-539>541-543. Small Craft Advisory from 11 AM Monday to 6 AM EDT Tuesday for ANZ538-542. Small Craft Advisory from 6 PM Monday to 6 AM EDT Tuesday for ANZ530-535-536.
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&& $$ SYNOPSIS...LFR NEAR TERM...LFR SHORT TERM...LFR LONG TERM...SMZ AVIATION...LFR/SMZ MARINE...LFR/SMZ TIDES/COASTAL FLOODING...LFR

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