Area Forecast Discussion
Issued by NWS Sterling, VA
Issued by NWS Sterling, VA
222 FXUS61 KLWX 200805 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 405 AM EDT Fri Sep 20 2024 .SYNOPSIS... Low pressure will continue to drift off the Mid-Atlantic coast through tonight with high pressure building north of the region. A frontal system will move into the area Saturday afternoon before stalling through Sunday. A strong front and area of low pressure over the Great Lakes and Upper Midwest will approach the area during the early or middle part of next week. && .NEAR TERM /THROUGH TONIGHT/... Low pressure remains southeast of Cape Cod this morning with high pressure building into the area from the north. With mainly clear skies and light winds, fog is presenting the primary issue of the morning. Satellite and web cams suggest thicker fog is confined along the valleys, especially west of the Blue Ridge. Some dense fog has been observed at various airports at times, so will be monitoring for a potential Dense Fog Advisory should it become more widespread. After the fog dissipates, it will be a warm day with few to scattered clouds. Highs in the lower to mid 80s are about 5 to 8 degrees above normal. Recent guidance has backed off a bit on the shower chance this afternoon, mainly along the immediate Allegheny crest where there will be some weak convergence. Tonight remains dry. The best signal for fog is across southern parts of the forecast area. Some guidance is also indicating potential for low clouds to develop east of the Blue Ridge. Lows will be in the upper 50s to mid 60s. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... This is my seventh night shift, and I feel like this amplified upper level pattern has changed this weekend`s forecast a little bit each day. For Saturday, the low remains off the coast and high pressure remains to the north. However, now there appears to be a bit more defined low and frontal pattern that approaches from the Great Lakes combined with a shortwave in northwest flow aloft. The warm sector will feature highs in the lower to mid 80s and dew points in the lower to mid 60s. This will aid in developing some instability, so showers and thunderstorms should develop with the approaching forcing. Most models suggest decent coverage (40-60 percent) starting midday west of the Blue Ridge and then spreading east during the afternoon and evening. While instability will be somewhat modest, shear of 30-40 kt is forecast by some guidance. This could result in some organized storms that could produce hail and localized wind damage. Right now the greatest chance for severe weather appears to be west of I-95, where storms are most likely during peak heating (along with slightly higher instability). While storms should weaken some as they push east, there does appear to be enough forcing for showers and isolated storms to last well into the night. At some point, the frontal system overhead gets overtaken by the surging high pressure to the northeast...what we`ve been describing as a backdoor cold front. The conglomerate boundary now appears to stall across the area Sunday, with cloudy, cool, and drizzly conditions to its northeast, and more sun and temperatures closer to 80 to it southwest. The wedge of high pressure pushes further south Sunday night, so cloud cover may overspread most of the area. Some drizzle is possible, and a few showers may approach the Appalachians ahead of the next low pressure system in the Great Lakes. && .LONG TERM /MONDAY THROUGH THURSDAY/... High pressure will build over the region on Monday before sliding offshore on Tuesday. There is a chance for a few showers over the Appalachians on Monday as weak energy moves up and over a ridge of high pressure. Temperatures will remain cool. Once the high moves offshore Tuesday, a shortwave trough of low pressure will move across the eastern Great Lakes and help to lessen the strength of the high. This will enable or increase the chance for showers and a thunderstorm Tuesday into Wednesday, not just in the Appalachians but also east through the Shenandoah Valley and perhaps the metro areas. Temperatures each day will remain cooler than average. A more significant cold front could approach late Wednesday and bring an enhanced threat for showers or a thunderstorm into Thursday. Timing and intensity of this stronger front is uncertain. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... Fog will be the main issue for CHO and MRB through sunrise, becoming dense at times. Expect improving conditions by 13-14Z. IAD could have a brief period of BR, but not expecting restrictions at the other terminals. For the remainder of the day, expect VFR conditions and light NE winds becoming SE by tonight. There`s a little more uncertainty in fog development for tonight, but would most likely be at MRB at CHO again. However, some guidance indicates low clouds could form east of the Blue Ridge. Will see if refinement can be made in this for the 12Z TAFs. A front will approach Saturday with showers and thunderstorms possible during the afternoon and evening. This boundary will stall near the area through Sunday. Sub-VFR ceilings and occasional light rain or drizzle may continue at times Sunday and Sunday night, although confidence is low. VFR conditions Monday through Tuesday night. Winds predominantly southeast 5 to 10 knots gusts up to 15 knots each period. && .MARINE... Upper level low pressure will remain off the coast through the weekend as high pressure builds north of the region. Light northerly winds turn toward the east and southeast through tonight and Saturday. A frontal system will approach Saturday, with thunderstorms potentially reaching the waters during the late afternoon or evening. The front will stall near the area Sunday before high pressure surges from the northeast Sunday night and turns winds to the northeast. There is potential for marginal SCA conditions at times late Saturday through Sunday night, although in general computer guidance has backed off on stronger winds. No marine hazards expected Monday through Tuesday night. Winds southeast around 10 knots with higher gusts each afternoon. && .TIDES/COASTAL FLOODING... Anomalies are above one foot across much of the waters this morning, which has been resulting in minor flooding in several locations. Water levels are forecast to rise further into the weekend as offshore low pressure traps water in the Chesapeake. Combined with high astronomical tide levels, additional, and more widespread, coastal flooding can be expected. Advisories will likely need to be expanded to northern parts of the bay by tonight. A Coastal Flood Watch remains in effect for Anne Arundel County Saturday evening. However, there could be additional bouts of moderate flooding thereafter, and DC SW Waterfront may also near moderate flood stage. Unfortunately this looks like a long duration coastal flood event as an onshore wind component persists well into next week. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Coastal Flood Advisory until 2 AM EDT Sunday for DCZ001. MD...Coastal Flood Advisory until 6 PM EDT Saturday for MDZ014. Coastal Flood Watch from Saturday evening through late Saturday night for MDZ014. Coastal Flood Advisory until 8 AM EDT Sunday for MDZ016. Coastal Flood Advisory until 11 AM EDT Sunday for MDZ017-018. VA...Coastal Flood Advisory until 2 AM EDT Sunday for VAZ054. Coastal Flood Advisory until 8 AM EDT Sunday for VAZ057. WV...None. MARINE...None. && $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...KLW AVIATION...ADS/KLW MARINE...ADS/KLW TIDES/COASTAL FLOODING...ADS