Area Forecast Discussion
Issued by NWS Sterling, VA

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KLWX 151835 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 235 PM EDT Thu Mar 15 2018 .SYNOPSIS... Low pressure will linger over the Canadian Maritimes through Saturday. A weak cold front will cross the region by this evening and surface high pressure will build toward the region Friday. Another weak system will pass through the area Saturday before high pressure returns for Sunday. A stronger storm system will likely impact the area during the middle portion of next week. && .NEAR TERM /THROUGH FRIDAY/... Deep surface low remains over Atlantic Canada. A much weaker low is crossing Pennsylvania, with a weak cold front pushing southeastward to its southwest across western Maryland and West Virginia at present. This front will cross the rest of the CWA by sunset. Weak warm advection/isentropic lift has produce a little light precip over Maryland, but this should be no more than flurries or brief showers, and it should depart with the cold front over the next few hours. Gusty southwest winds will shift northwest by sunset as well. Tonight, northwest flow continues behind the front. Upslope flow will produce some snow showers along the Allegheny Front, but at this point think its sub-advisory so no headlines for snowfall - generally less than 3 inches are expected. A pretty strong shortwave/vort max does cross the region overnight, and this could be enough forcing to bring a stray snow shower or flurry east of the mountains, but odds are low, so have not yet included it in the forecast. Will let evening shift take another look. Lows tonight will be below freezing once again. Friday is more of the same, though with weakening upslope flow as we head through the day, the snow showers along the Allegheny Front should diminish. It will stay breezy and cool, however, with highs pushed back down into the 40s across most of the region behind today`s cold front. && .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/... High pressure will build back in briefly on Friday night, allowing winds to drop off overnight. Clear skies should result in another chilly night with most places dropping into the 20s again. A weak system passing through the region on Saturday is of some concern. With temps likely below freezing region-wide at dawn Saturday, the speed of precip`s progression into the area will likely be the key to whether we have any icing issues. In addition, soundings at the leading edge of precip, as well as towards the northern side of the system near PA, could result in a bit more snowfall. For now, given significant disagreement among various models regarding timing and placement of precip, not to mention some differences in temp profiles, have gone fairly general, with chance of mixing at the start and likely rain later Saturday. Best chance of ice and snow is west of the Blue Ridge, and this potential remains in the HWO. Temps are alos uncertain Saturday, with potential for precip and wedging holding temps low, but right now most guidance brings temps up towards the 40s. Will need to watch this closely, however. Guidance generally clears all precip from the region early Saturday evening, which will turn out partly cloudy and cold again. Some may linger in the far south (central VA) close to the front this system is traveling along, so have pops linger there. && .LONG TERM /SUNDAY THROUGH THURSDAY/... The large scale pattern will continue to be dictated by two stalled upper level lows: one over the Canadian Maritimes and one over the northeastern Pacific/western North America. The latter of the two upper lows will eject energy eastward across the United States, while the Canadian Maritimes low blocks energy from ejecting harmlessly into the Atlantic. The resultant troughing over the eastern U.S. will lead to a continuation of below normal temperatures over the Mid-Atlantic through much of next week. High pressure will build from the Ohio Valley eastward across the Mid-Atlantic Sunday into Monday. Current guidance suggests the next piece of energy/upper shortwave will cross the central Plains and mid-Mississippi Valley Sunday night into Monday, moving eastward through the Tennessee/Ohio Valley by Monday night, then exit the east coast Tuesday. As is to be expected at this time range, uncertainties exist in overall finer scale system evolution, but there is a high likelihood of the development of another coastal low pressure system along the eastern seaboard during this time. At least some wintry precipitation is possible, but by mid-March things would have to come together just right to result in a more significant winter storm east of the mountains. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... VFR through Friday night. Main issue through Friday is gusty NW winds. There may be a stray flurry or even brief snow shower with the passing front or behind it with the passing shortwave, but odds of an impact at a terminal are quite low. Winds will relax a bit overnight but should increase again after sunrise on Friday. They should finally drop off for a while later Friday night as high pressure briefly builds in. Saturday could bring issues as a weak wave of low pressure brings precip across the region. Most likely, this will just be rain at the terminals, but some wintry precip is also possible, especially at the onset. Cigs and vis could be reduced below VFR as well, especially if there is some front-end snow (most likely at MRB) or if rain persists for a while (most likely at CHO). Improvement back to VFR looks likely Saturday night. Mainly VFR Sun-Mon as high pressure moves from Ohio to over the Mid- Atlantic then eastward into the Atlantic Ocean. Winds Sun will be northerly around 10 kts, then go light and variable Sun night as high pressure moves overhead. Light easterly flow is anticipated Monday as the high moves offshore, but depending on finer scale details of the evolution of the pattern, winds could be more northeasterly. && .MARINE... Continued northwest flow around the low lingering to the northeast will keep Small Craft Advisory conditions going on the waters on and off through Friday evening. There could be a few intervals of low gale, most likely this evening behind a passing weak cold front, but confidence is low and expect any gale gusts to be sporadic. High pressure then builds briefly overhead late Friday night, allowing the gradient to weaken and in turn, the winds, so it should drop below SCA late Friday night. A weak wave of low pressure then crosses the region later Saturday, but winds should remain below SCA. There will likely be some rain with this wave, however. Generally light winds (N Sun, E Mon) are expected as high pressure moves from the Ohio Valley eastward over the waters and then out to sea. && .FIRE WEATHER... Low pressure will remain over the Canadian Maritimes through Friday. High pressure will approach from the north and west during this time. A tight gradient between these systems will bring gusty winds to the area, especially from the late morning through the early evening hours today and Friday. A downsloping west to northwest flow will cause low relative humidity. For the remainder of today, lower fuel moistures are expected along with gusty winds and low relative humidity. Temps will be in the 40s and 50s, a bit warmer compared to Wednesday. There will be an enhanced threat for the spread of wildfires, especially across northern and central Virginia, the eastern Panhandle of West Virginia and portions of central and southern Maryland where relative humidity will be lowest (20 to 30 percent). Meteorological conditions (wind and rh) support Red Flag Warnings, but fuel moistures are still above 8 percent. Will continue with a Special Weather Statement instead of a Red Flag Warning for now given the relatively higher fuel moistures. For Friday...more gusty winds are expected. Somewhat chillier conditions are expected as well with high temps in the 40s but fuels should be noticeably drier. The relative humidity will drop to between 15 and 25 percent across central Virginia and 25 to 35 percent across northern Virginia, eastern West Virginia and Maryland. The strongest winds will be displaced from the lowest RH values (strongest winds over Maryland, northern Virginia and eastern West Virginia). An enhanced threat for the spread of wildfires is possible Friday given the fact that fuels will be dry and RH values will be low. After review, believe that the Red Flag conditions may occur locally but with fuel moistures still marginally above criteria and the wind/RH colocation missing, have opted to not issue any headlines for fire weather yet. An SPS may be again needed for Friday, but with one currently in effect, have decided to let evening or mid shift issue a new one after the current one expires at 6 PM. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...Wind Advisory until 8 PM EDT this evening for VAZ025-026-503- 504-507-508. WV...Wind Advisory until 8 PM EDT this evening for WVZ505-506. MARINE...Small Craft Advisory until midnight EDT Friday night for ANZ530>534-537>541-543. Small Craft Advisory until midnight EDT tonight for ANZ535-536- 542. Small Craft Advisory from 9 AM Friday to midnight EDT Friday night for ANZ535-536-542. && $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...DHOF AVIATION...RCM/DHOF MARINE...RCM/DHOF FIRE WEATHER...BJL/RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.