Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 171907 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 307 PM EDT Sat Mar 17 2018 .SYNOPSIS... Low pressure will pass our region to the south this afternoon and evening. High pressure will return Sunday and Sunday night. A second, yet stronger, low pressure system will track across the Tennessee Valley Monday. It will transfer energy to the East Coast to allow for a developing coastal low to spawn that will ride up the coast Tuesday. A third low pressure system will follow the coastal low and pass our region to the south Wednesday. High pressure will return for late next week. && .NEAR TERM /THROUGH TONIGHT/... Weak energy is bringing some widely scattered rain and snow showers across Maryland, eastern West Virginia and northeast Virginia. Little to no snow accumulation expected across these areas. For the most part, trace amounts is about all we anticipate with this activity. This will be mostly across central Maryland and the higher elevations of eastern West Virginia. Surface observations reveal a weak boundary across the Central Shenandoah Valley and just south of the Virginia Piedmont. Temperatures on the north side of the boundary are in the middle 30s to near 40. Temperatures on the south side of the boundary are in the lower to middle 50s. It is along this boundary that a weak low pressure system will ride from northwest to southeast and bring a couple of rain showers to parts of the central Appalachians, central Shenandoah Valley and lower Virginia Piedmont late this afternoon into this evening. Rain amounts will average a few hundredths of an inch and not everyone will encounter one of these showers. The remainder of the night tonight, drier air will filter in from the northwest as high pressure builds into the region. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... High pressure will build overhead Sunday through Sunday night. Dry and seasonable conditions are expected. The high will shift to the East Coast later Sunday night. An area of low pressure is expected to track across the Tennessee Valley Monday. As it reaches the Appalachian Front Monday night, it will transfer energy to the East Coast in order to form a coastal low. This newly-formed coastal low is expected to intensify as it moves northeast and parallel to the mid-Atlantic Coast Monday night and Tuesday. Precipitation could begin as early as Monday afternoon in the form of a mix of rain and snow. There should be rain since the high pressure is along the coast and there will be an east to southeast wind bringing modified March air onshore ahead of the storm. There should also be snow since the high pressure that will be at the coast will have already surged a swath of chilly air along the eastern slopes of the mountains down into the Carolinas about 12 hours before the precipitation arrives. Gradually and steadily this wintry mix of precipitation will overspread the region from southwest to northeast late Monday through Monday night. Also, with cooling and saturation of the atmosphere from top down as the coastal low develops at the coast, precipitation should transition to all snow for a period of time before either changing back over to a rain and snow mix or remaining all snow and tapering off late Tuesday. That period of time is the big question in terms of precipitation types, precipitation duration, and precipitation amounts. Model guidance is agreeing more and more each run that we are anticipating a winter storm to some degree of magnitude. To what degree and affect will it have on our region. The latest forecast track takes a blend of model guidance which continues to keep the best chance for snow north and west of the Metropolitan areas. Confidence still remains low on the track, plus it is low on location of developing coastal low, and how much wintry precipitation is expected through Monday night that will linger into Tuesday. && .LONG TERM /TUESDAY THROUGH SATURDAY/... A low pressure system will be moving off of the NC/VA coast early on Tuesday with a high pressure to our north centered over Ontario. This system is expected to bring precipitation over our area through the day on Tuesday as it intensifies offshore, with precipitation transitioning from rain or rain and snow mix to snow later on. The system will then move northeastward into the west Atlantic Tuesday night into Wednesday with precipitation lingering over our region into possibly late on Wednesday. Latest guidance still differs on the solutions for its track, which impacts the amounts and p-types that we could experience. A high pressure system will then build over our region Wednesday night into Friday night before a frontal boundary could affect us next weekend. && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... For another hour or two, a brief period of light rain or snow cannot be ruled out at the MRB, BWI, and MTN terminals. Brief MVFR cannot be ruled out with this activity, but for the most part VFR conditions will evolve elsewhere. High pressure will keep VFR conditions in play through Sunday night. Low pressure will track through the Tennessee Valley before transferring its energy to a coastal low off the Mid-Atlantic Coast Monday night. Rain will likely overspread the terminals later Monday into Monday night. Rain could mix with or change to snow Monday night as colder air works its way into the area. SubVFR cigs/vsbys are likely later Monday through Monday night. Sub-VFR conditions expected Tuesday into Wednesday with coastal storm possibly moving near by during this time. Breezy conditions on Tuesday and Wednesday, some gusts up to 20 kts possible. VFR conditions expected on Thursday with high pressure building in. && .MARINE... Small craft advisory remains in effect through 6pm for the upper Chesapeake Bay. High pressure will build over the waters tonight into Sunday. The pressure surge may cause a period of SCA wind gusts late tonight into early Sunday across middle portions of the Bay but confidence was too low for an SCA at this time. Low pressure will track through the Tennessee Valley before transferring its energy to a coastal low off the Mid-Atlantic Coast Monday night. A Small Craft Advisory will likely be needed for the waters Monday night. Wind gusts will be above small craft criteria Tuesday into Wednesday as low pressure system moves near our region... therefore a small craft advisory is likely these days. Winds should decrease Thursday below criteria. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ530- 531-538-539. && $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...IMR AVIATION...IMR/KLW MARINE...IMR/KLW is the U.S. government's official web portal to all federal, state and local government web resources and services.