Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 230125 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 925 PM EDT Sun Oct 22 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Like last night, blow-off cirrus overspreading the forecast area this evening. Unlike last night though, anticipate the shield to get thicker overnight, with a strong possibility of low clouds working inland as well across the Baltimore metro area toward morning...and maybe reaching metro DC too. These clouds in turn will keep temperatures higher than previous nights. Also at play will be a continuing south wind, which actually is quite strong (close to 20 kt) just off the deck per 00z LWX RAOB. RAP/NAM model sounding both suggest that this low level jet will remain most of the night. So, skies should be mostly cloudy, and radiational cooling looks unlikely. Therefore, believe that low temperatures likely won`t dip lower than the 50s. && .SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/... 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 strengthening 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. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... 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. && .AVIATION /01Z MONDAY THROUGH FRIDAY/... VFR conditions expected through late this evening. A southerly flow will allow for increasing moisture to get trapped underneath the nocturnal inversion late tonight into Monday morning. Did allow for a period of IFR cigs and MVFR vsbys during this time. There is still some guidance that keeps VFR conditions in place but as of now there appears to be a better chance for low clouds and patchy fog given the fact that most of the guidance is forecasting this and dewpoints are climbing due to the light southerly flow. Any low clouds will burn off by mid to late morning and it will turn out breezy Monday afternoon with gusts around 15 to 20 knots from the south and southeast. Showers are expected Monday night ahead of a cold front. Some showers may contain gusty winds...especially overnight along the frontal boundary as it passes through. Gusts around 30 to 40 mph are possible. LLWS is expected with southerly winds around 45 knots are most likely around 2000 feet. However...should more winds mix down to the surface it may be more turbulence instead of LLWS. Confidence in that is low. Conditions will improve behind the front early Tuesday morning. Expect VFR conditions through the extended. Possibly on Friday there could be locally IFR visibilities in early morning fog at IAD/MRB terminals. && .MARINE... A southerly low level jet resides just above the waters tonight. If these winds were to mix down, then 20 kt gusts would be realized. However, with water temperatures in the upper 60s, think that an inversion will hold. Have included gusts in the mid teens across most of the waters. Winds are expected to strengthen 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. && .TIDES/COASTAL FLOODING... 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. && .LWX WATCHES/WARNINGS/ADVISORIES... 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. && $$ SYNOPSIS...LFR NEAR TERM...HTS SHORT TERM...LFR LONG TERM...SMZ AVIATION...BJL/SMZ MARINE...HTS/LFR/SMZ TIDES/COASTAL FLOODING...LFR is the U.S. government's official web portal to all federal, state and local government web resources and services.