Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 231356 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 956 AM EDT Mon Apr 23 2018 .SYNOPSIS... High pressure will move offshore today. A low pressure system will bring unsettled weather to the region tonight through the middle of next week. A brief ridge ridge of high pressure will build in later in the week, ahead of the next low pressure system toward the weekend. && .NEAR TERM /THROUGH TONIGHT/... High pressure will move off of the East Coast today as a cut- off low pressure system moves through the southern United States and towards the Mid-Atlantic states. High clouds of varying thickness will stream overhead through the day today, with a general increase/thickening trend, especially later this afternoon. Rain will eventually spread northeastward and into the southern Shenandoah Valley, Blue Ridge, and Allegheny Highlands late this afternoon, and especially this evening. High temperatures will reach the upper 60s to near 70F for most locations, except mid 60s where clouds/rain chance increase first. Overnight tonight rain probabilities increase from southwest to northeast as moist/warm air advection continues and closed low gradually approaches. Rain should be falling by sunrise from the DC metro area south/west. Further northeastward, locations may remain dry through the night. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... As the cut-off low and a couple of pieces of upper level energy move into the mid-Atlantic region Tuesday, additional rounds of light to moderate rain will develop and overspread most of the region. This rain should gradually taper to showers Tuesday night and end from southwest to northeast. However, the ending time may be delayed as the next upper level disturbance will be quick on the heels of the Monday night and Tuesday low pressure system. Temperatures will be about 5 to 10 degrees cooler Tuesday due to cloud cover and rainfall and about 5 to 10 degrees milder Tuesday night due to the same two factors. Low pressure will be moving northeast away from the area Wednesday morning, leaving a chance for some residual showers in its wake. Another quick moving shortwave will move through Wednesday evening into Thursday morning. Some showers will be possible with this system as well, especially Wednesday night. Temperatures Wednesday will be below average, thanks to the cloud cover and precipitation expected over the area. && .LONG TERM /THURSDAY THROUGH SUNDAY/... The area of low pressure will depart off to the northeast on Thursday leading to dry conditions and slightly below normal temperatures. The forecast becomes much more uncertain for Friday. Several different shortwave disturbances will interact as they move across the CONUS between now and Friday. The high number of these complex interactions lend themselves to high uncertainty in the forecast for Friday through Sunday. The GFS forms a surface low in the Southeast Thursday Night out ahead of a compact shortwave, and then proceeds to track that low up the East Coast on Friday giving us a round of rain. On the other hand, the Euro keeps the shortwave and attendant area of low pressure suppressed to the south and weaker in nature. Roughly half of the members in the 00z GEFS give us over a tenth of an inch of rain, and only a few members of the 12z EPS gave us over a tenth of an inch with this system. So, to summarize...there`s at least a chance for rainfall on Friday, but it`s an uncertain forecast to say the least. Another upper-level trough and associated weakening cold front will approach the area from the west this weekend. Most model guidance keeps us dry with the passage of this front, but some light precipitation can`t be ruled out. Temperatures this weekend are expected to remain slightly below average. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... VFR conditions expected through this evening with winds southeast around 10 knots or so. MVFR conditions then become possible at CHO overnight tonight as light rain approaches from the southwest. Chances for MVFR conditions then increase at all terminals Tuesday and especially Tuesday night due to low ceilings and light to moderate rain. A period of IFR is also possible Tuesday night into early Wednesday morning. Clouds and showers will persist for much of the day on Wednesday with potential for sub-VFR conditions continuing. VFR conditions are likely on Thursday. Conditions could potentially drop to sub-VFR on Friday if a coastal low were to develop, but the forecast is highly uncertain at this point. Conditions will likely return to VFR for the weekend. && .MARINE... Small craft advisory goes into effect for all waters this afternoon as southerly flow increases ahead of an approaching low pressure system. The advisory continues tonight through Tuesday with gusty south/southeast winds 20-25 knots. An extension through Tuesday night may be needed. As the area of low pressure exits Wednesday morning, some gusty winds are possible across the marine areas, with SCA criteria winds possible during that time. No marine hazards are expected Thursday through Sunday at this time. && .HYDROLOGY... While the area saw significant rain a week ago, its been relatively dry since then. That said, 1-2 inches of rain appears likely across the region tonight through Wednesday, which could lead to some minor hydrology issues and cause river and stream rises, potentially nearing flood stages. Will be closely monitoring this for potential need of any flood watches over the next day or so. && .TIDES/COASTAL FLOODING... As low pressure approaches the waters from the southeastern U.S. late Monday into Tuesday, a persistent onshore flow will develop. This will yield increasing tidal anomalies, and the possibility of coastal flooding toward the middle part of the week. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 1 PM this afternoon to 6 PM EDT Tuesday for ANZ530>543. && $$ SYNOPSIS...KLW NEAR TERM...MM/KLW SHORT TERM...KLW LONG TERM...KJP AVIATION...MM/KLW/KJP MARINE...MM/KLW/KJP HYDROLOGY...MM/KLW TIDES/COASTAL FLOODING...MM/KLW is the U.S. government's official web portal to all federal, state and local government web resources and services.