Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 201858 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 258 PM EDT Mon Aug 20 2018 .SYNOPSIS... High pressure will build across the northeastern United States through the rest of today while a front remains stalled to the south. Low pressure will pass northwest of the area Tuesday, lifting a warm front northward. A cold front will then push eastward on Wednesday, with high pressure building in from the west for the end of the week. && .NEAR TERM /THROUGH TONIGHT/... A stalled frontal boundary is positioned well to the south of the region this afternoon while high pressure builds into the northeastern United States. Northeasterly flow and moisture in the low levels have led to widespread stratus across much of the region into this afternoon. Breaks have begun to develop and this trend will continue through the rest of the day, but in general mostly cloudy skies are expected. Some instability has also developed across portions of eastern WV, central VA, and into southern MD, and with that, isolated to scattered showers and a rumble or two or thunder. The greatest coverage has been across southern MD and the middle Chesapeake Bay. Shower/storm coverage will trend downward at first this evening, but as the warm front lifts northward and forcing increases aloft from the south/west, additional showers and a possible rumble of thunder become possible late tonight and towards Tuesday morning. Low clouds are expected once again, as well as some patchy fog. Lows tonight in the 60s/70s. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... The warm front will gradually lift northward during the day Tuesday. Low clouds and a few showers/drizzle will likely start the morning for much of the region prior to warm frontal passage. By the mid-afternoon hours, the warm front is expected to have made some progress northeastward, although the extent of which is still in question. At the same time, an upper trough and cold front will be approaching from the west, with passage during the evening and overnight hours. MLCAPE values on the warm side of the boundary are progged to reach into the 1000-1500 J/KG range, with shear values of 30-40 knots atop it. This will provide for an environment where strong to locally severe thunderstorms are possible, and the Storm Prediction Center has continued to highlight portions of the region in a Slight Risk for severe thunderstorms, although the extent of the event is still uncertain given instability questions and frontal timing. With deep moisture, noted by forecast precipitable water values in excess of 2", heavy rain and flooding is also a concern, although storm motion won`t be all that slow. Highs Tuesday will be in the low to mid 80s, with lows in the 60s and 70s. A secondary front will cross the region on Wednesday, with additional chances for scattered showers and isolated thunderstorms. Highs Wednesday will be in the 80s. Much cooler and drier air will push in behind the front for Wednesday night, with lows in the 50s and 60s. && .LONG TERM /THURSDAY THROUGH SUNDAY/... Thursday into Friday, an upper level ridge will build over the region from the west. Surface high pressure will also move into our area starting Thursday from the midwest. Skies will be mostly clear with light west to northwesterly winds. Daytime temperatures will be mild and below average in the mid to upper 70s on Thursday and the upper 70s to mid 80s on Friday. Saturday through Sunday, a weak upper level shortwave trough will approach the region from the west and move through over the weekend. A weak frontal boundary associated with the shortwave will attempt to move through late Saturday into Sunday. The 00Z Euro and the 12Z GFS both agree that this front will be mostly dry leading to little to no precipitation occurring over the weekend. The best chance for precipitation will be near the mountains. Day time temperatures for the weekend will hover in the upper 70s to mid 80s. Winds will be light out of the south. && .AVIATION /18Z MONDAY THROUGH SATURDAY/... A mix of VFR/MVFR currently exists across the region this afternoon, and gradual improvement to VFR is likely at most sites by the late afternoon and evening hours. Ceilings will again be an issue tonight into Tuesday morning with widespread MVFR/IFR returning. Patchy fog and a few showers are also possible. A warm front will lift northward Tuesday, followed by a cold front Tuesday night. In between, ceilings will likely lift, but showers and thunderstorms will become likely, some of which could be strong with gusty winds. A secondary front will cross on Wednesday and may produce some showers as it crosses. On Thursday and Friday, winds will be light out of the west to northwest with mostly clear skies. VFR conditions expected. && .MARINE... Generally sub-SCA winds are expected through tonight, with easterly flow up to around 15 knots. Scattered showers and a few thunderstorms may produce brief gusty winds over the waters this afternoon and evening. A warm front will lift north Tuesday and flow will turn southerly. Wind speeds through the day may be marginal for SCA, but the strongest winds are likely Tuesday evening and Tuesday night, as an initial front passes across. A Small Craft Advisory has been issued for this period. Showers and thunderstorms will also become increasingly likely during the afternoon and evening and could contain gusty winds. A cold front will cross the area early Wednesday, and winds don`t look particularly strong behind it. However, a secondary cold front will pass late in the day with potential for a few showers and storms. In its wake, SCA conditions will be possible. On Thursday and Friday, winds will be light out of the northwest with no marine weather hazards expected. && .TIDES/COASTAL FLOODING... Onshore flow will continue through Tuesday morning, then turn southerly as a warm front lifts north Tuesday afternoon. Minor coastal flooding is possible tonight through Tuesday night, especially at the climatologically favored sites. A Coastal Flood Advisory has been issued for St. Mary`s county for tonight`s high tide cycle and additional advisories may be necessary. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Coastal Flood Advisory from 7 PM this evening to 2 AM EDT Tuesday for MDZ017. VA...None. WV...None. MARINE...Small Craft Advisory from 6 PM Tuesday to 6 AM EDT Wednesday for ANZ532>534-537-540>543. && $$ SYNOPSIS...MM NEAR TERM...MM SHORT TERM...MM LONG TERM...JMG AVIATION...MM/JMG MARINE...MM/JMG TIDES/COASTAL FLOODING...MM is the U.S. government's official web portal to all federal, state and local government web resources and services.