Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 160101 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 901 PM EDT Thu Mar 15 2018 .SYNOPSIS... Low pressure will linger over the Canadian Maritimes through Saturday. A weak cold front will continue southeast of the area tonight 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. Winds have shifted to the northwest after a weak cold front pushed through the area. Those winds quickly subsided with sunset and skies cleared, even along the western slopes. Expect clouds to increase (more so on the western slopes) and winds to tick up a bit later tonight as the next upper level disturbance crosses the area. This disturbance will combine with upslope flow to produce some snow showers along the Allegheny Front, but at this point think it`s no headlines for snowfall. The vort max is fairly strong, so it could be enough forcing to bring a stray snow shower or flurry east of the mountains along the Mason-Dixon line, but odds are low. 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 also 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. Have added this threat to the HWO based on 12Z probabilistic guidance, though exact details are subject to change. && .AVIATION /01Z FRIDAY THROUGH TUESDAY/... VFR through Friday night. Winds have relaxed with sunset, but may increase later tonight as an upper disturbance passes. The more notable increase will occur after sunrise on Friday when gusts of 20-30 kt will become possible. 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 pressure lingering to the northeast will keep Small Craft Advisory conditions going on the waters on and off through Friday evening. Winds actually subsided below 10 kt in many areas with sunset, but will limit changes to the headlines since an upper level disturbance will likely increase mixing and thus wind gusts later tonight. Widespread gusts of 20-30 kt will occur after sunrise Friday. The probability of any gale force gusts appears to be low. 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 Friday. A downsloping west to northwest flow will cause low relative humidity. 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 collocation missing, have opted to not issue any headlines for fire weather yet. An SPS may be again needed for Friday, but will let mid shift make final determination on its placement. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. 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...ADS/RCM SHORT TERM...RCM LONG TERM...DHOF AVIATION...ADS/RCM/DHOF MARINE...ADS/RCM/DHOF FIRE WEATHER...LWX is the U.S. government's official web portal to all federal, state and local government web resources and services.