Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 181408 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1008 AM EDT Sun Mar 18 2018 .SYNOPSIS... High pressure will build overhead through tonight. Low pressure will pass through the Tennessee Valley on Monday, with another coastal low developing off the Mid- Atlantic 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/... High pressure will build overhead through tonight, bringing dry conditions. Plenty of sunshine is expected the remainder of the day to allow our high temperatures to rise nearly 10 degrees warmer than our high temperature on Saturday. High temperatures will be in the upper 50s across most locations with lower 60s in central Virginia. Seasonably chilly conditions are expected tonight. Clouds will increase toward morning well ahead of low pressure over the central CONUS. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... No big changes to the forecast regarding the low pressure system Monday night through Tuesday night. Models are still showing a transfer of energy to the East Coast Monday night into Tuesday after the low moves across the Tennessee Valley Monday and reaches the Appalachian Front. The newly-formed coastal low will move well offshore by Tuesday afternoon. Another coastal low will emerge off the Mid- Atlantic Coast Tuesday night as additional upper level energy moves in behind the departing first coastal low. For Monday through Tuesday: A confluence zone around upper- level low pressure over the Canadian Maritimes and ridging over the north-central CONUS will pump high pressure to our north over New England. This should cause the primary low to transfer its energy to the coastal low south of our latitude...putting our area on the cold side of the storm. is the middle of March so there will be issues with warmer air near the surface. Latest guidance is in good agreement that the Canadian airmass will remain to our north and you can see that in the surface theta-e profiles. This means that an easterly flow will allow for relatively warm conditions Monday with max temps well into the 40s and perhaps even 50s for some areas. Clouds will increase Monday and light precipitation may move into the area from warm advection well ahead of the approaching low. However...the p-type is expected to be rain with the relatively warm boundary layer in place. The low will pass by to the south Monday night and emerge as a coastal low by Tuesday morning near the North Carolina Coast. As the low is passing by to the south, colder air will get drawn in from the north from an isallobaric wind associated with rapidly falling pressures to the south. Precipitation will increase in coverage Monday evening and widespread precipitation is expected overnight Monday into Tuesday morning. The colder air will cause rain to mix with and change to snow across much of the area. The details of the forecast will be dependent on how quickly the rain can change to snow before the heaviest precipitation moves out Tuesday morning. The farther north and west you go the better the chance you have for this to occur, also higher elevations will change over much quicker without the warm boundary layer in place. The most significant snow accumulations are expected across northern Maryland and for locations near/west of the Blue Ridge and Catoctin Mountains. For the metro this point it appears more likely that the warm boundary layer will hold on for a while before changing over to snow...thus cutting down on accumulations. Uncertainty is high due to the many factors that will contribute to the forecast such as where the banding precipitation sets up, exactly how quickly the cold air can move in, and how strong is the low as it passes by to the south. Those uncertainties along with a fairly deep warm boundary layer precluded any headlines for this update, but that will have to be considered for later today especially over the higher elevations. The low will move well offshore later Tuesday morning and Tuesday afternoon. However...another shortwave in the southern stream of the jet will be fast on its heels. This will cause low pressure to develop to our south later Tuesday and another coastal low will likely develop Tuesday night. A lull in the precipitation is most likely later Tuesday in between these systems. Will continue with the chance for light snow (or rain/snow mixed) but little if any accumulation is expected Tuesday afternoon due to the high sun angle this time of year and the light precipitation rates. In fact...there is a chance that it turns out mainly dry. Precipitation will fill in as the coastal low develops Tuesday night. Guidance still diverges on how strong the low will be and consequently how much precipitation fills in. It does appears that the best chance for precipitation will be across the west and south...closer to the track of the low. Thermal profiles will be colder due to northerly winds and this will cause the p-type to be snow for most areas. Accumulating snow is possible. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... Behind the exiting low pressure system on Tuesday, another area of low pressure is progged to develop off the Carolina coast late Tuesday night, likely residing off the NC/VA coast by Wednesday morning. As this low strengthens, low pressure to our southwest over the Appalachians will weaken as energy transfers off the coast. This will result in precipitation across much of the area during at least the first half of Wednesday before waning in the afternoon/evening as the coastal low pulls away to the east northeast. Sufficient cold air at the surface and aloft looks to be in place early Wednesday on the heels of northerly breezes, favoring snow as the primary ptype during the morning hours. Boundary layer warming is likely to occur as the day progresses considering it is mid to late March, which would favor more of a rain/snow mix by the afternoon hours, at least for locations south and east of the District. Precipitation tapers off Wednesday night with an upslope component remaining over our western mountains as the trough axis aloft starts to pivot east of the area and off the coast. Breezy conditions expected as the low strengthens and moves off to our northeast, resulting in a tightened pressure gradient over the region as Canadian high pressure builds southeastward from the Great Lakes region. Dry conditions are forecast to win out Thursday and Friday with high pressure taking control, as well as moderating yet continued below normal temperatures. Highs will hold mostly in the 40s, approaching the 50 degree mark on Friday south and east of the metros. Lows will fall in to the 20s to near the freezing mark. Low pressure moving eastward from the central U.S. will near our region Saturday, but some timing differences remain between global guidance. There is agreement in our CWA remaining very much in the warm sector with strong WAA under southerly flow out ahead of the system, which favors precipitation of the liquid variety. Highs will range from the upper 40s to lower 50s. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/... High pressure will build over the terminals through tonight. VFR conditions are expected. Low pressure will pass through the Tennessee Valley Monday and it will emerge as a coastal low off the Mid-Atlantic Monday night into Tuesday. Precipitation will overspread the area later Monday into Monday night mainly as rain. Rain will mix with and possibly change to snow by Tuesday morning. Accumulating snow is likely across KMRB and possible across the terminals in the Washington and Baltimore metro areas. SubVFR conditions are likely during this time. There may be a lull in the precipitation later Tuesday but subVFR cigs are still possible. Snow may return Tuesday night but confidence is low at this time. MVFR/IFR conditions expected during the day on Wednesday as low pressure strengthens off the coast and precipitation continues across the terminals. Snow is the favorable ptype Wednesday morning, with a transition to rain/snow possible for DCA/CHO, while northern and western terminals could see less precipitation but remain colder, favoring light snow. Improving flying conditions expected Wednesday afternoon/evening as precipitation wanes and low pressure moves off to the east northeast. High pressure will build toward the region on Thursday with gusty north northwest winds and prevailing VFR conditions expected. && .MARINE... High pressure will build over the terminals through tonight. Low pressure will move through the Tennessee Valley Monday and it will transfer its energy to a coastal low Monday night into Tuesday. Winds will increase Monday night into Tuesday and a Small Craft Advisory will likely be needed. Gale force winds are possible across the middle portion of the Maryland Chesapeake Bay and the lower Tidal Potomac River Tuesday. Another coastal low may develop Tuesday night and SCA conditions are likely. SCA conditions likely Wednesday through Thursday as low pressure strengthens off the coast and high pressure builds southeastward toward the area, delivering a strong pressure gradient over the waters. Winds will favor a north northwest trajectory during this period, gusting upwards of 20 knot. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until noon EDT today for ANZ530>534- 537>541-543. && $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...BKF AVIATION...BKF/KLW MARINE...BKF/KLW is the U.S. government's official web portal to all federal, state and local government web resources and services.