Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 110213 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 913 PM EST Sun Dec 10 2017 .SYNOPSIS... Weak high pressure will slide by south of the area through Monday. A low pressure system will cross the lower Great Lakes on Tuesday, bringing a cold front through the region. High pressure will build overhead Wednesday with another low pressure area approaching from the west on Thursday. && .NEAR TERM /UNTIL 7 AM MONDAY MORNING/... A weak shortwave will pass just to our north tonight bringing an increase in clouds, but not much precipitation with it. The latest guidance even leaves the favored upslope areas dry, but the pattern would suggest that at least some flurries likely. Have lowered PoPs further in the chance/slight chance range, and eliminated any mention of snow accumulations. In term of temperatures, the advancing cloud shield, associated with this wave, will reduce the extent of radiational cooling. Thus, not much of a city-suburb spread. However, would expect some advective cooling by dawn. As a result, guidance temperatures not looking too bad. We`ll be below freezing areawide (in many cases, we`re almost there). Favored a multi- model mesoscale ensemble for low temps. && .SHORT TERM /7 AM MONDAY MORNING THROUGH TUESDAY NIGHT/... The shortwave will exit to our east Monday morning, with clearing skies in its wake. Weak high pressure will move in from the west, keeping winds light, and bringing mostly sunny skies. This ridge will quickly move through our area, turning winds out of the south by Monday afternoon. This will allow temperatures to moderate a bit, topping out in the low to mid 40s. High clouds will start to stream overhead during the afternoon hours ahead of the next approaching shortwave over the lower Great Lakes. A stronger shortwave (compared to tonight) will near our area late Monday night in to Tuesday morning. The associated surface low pressure will pass to our north, dragging a cold front through the area during the day on Tuesday. Dynamics appear somewhat favorable with strong mid-level vorticity advection and an upper jet nearby. Add to that low-level isentropic upglide looks possible. Model guidance continues to keep the bulk of the moisture north of the Mason Dixon line, with upslope snow looking very likely Tuesday morning. Temperature profiles hint at the possibility of light mixed wintry precipitation along the northern portions of our CWA, particularly near the MD/PA border. That being said, most of the moisture and lift will remain to the north, so do not expect much in the way of accumulation. The best chance for accumulating snowfall will be along and west of the Allegheny Front through early Tuesday afternoon, where advisory criteria snow looks possible. On the heels of this shortwave is another stronger mid-level trough which will help push a cold front through the region, bringing strong cold air advection in its wake and strong/gusty northwesterly winds. Wind direction will be favorable for continued upslope snow and snow squall potential. Temperatures at the surface and aloft do not seem too supportive of snow squalls making much impact east of the mountains. The combination of strong gusty winds and the cold air filtering in, wind chills may drop below zero over the western ridges of Allegheny Front. Tuesday night, temperatures will fall in to the teens across our western zones, lower 20s elsewhere, and the middle 20s in the metro areas. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... A highly amplified upper level flow pattern with east coast troughing will result in a wintry pattern through Friday. Wednesday will be blustery and cold, with strong winds possible as low pressure pulls away from Maine. High temperatures may fail to crack the freezing mark across a large part of the area. With a slightly faster exit to the upper trough, flurries are looking less likely on Wednesday. Weak surface ridging will relax the winds by Wednesday night, but lows will be in the teens and 20s. There`s some model spread in timing on the next clipper system which will arrive sometime between late Wednesday night and Thursday evening. While the best chance for snow will be along the upslope areas, it`s not out of the question some light snow could spread east of the mountains as well, particularly near the Pennsylvania border. The next, sharper shortwave trough will arrive on Friday, prolonging chances for upslope snow. Instability through the snow growth zone could allow snow showers to move east of the mountains as well, although surface temperatures will likely rise above freezing at lower elevations. The upper level pattern will break down to more progressive flow next weekend. Surface high pressure will arrive on Saturday and temperatures will rise closer to normal. There`s still a large amount of uncertainty regarding the next frontal system which may arrive late in the weekend. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... VFR conditions are expected at all the terminals through Monday as weak high pressure traverses the area. Clouds will be mostly mid deck, with sct-bkn stratocumulus around 040-050 by dawn. These clouds will dissipate after sunrise. A clipper system will pass to the north Monday night in to Tuesday, bringing the possibility of brief flight restrictions along and to the north of MRB/BWI/MTN. Behind a cold frontal passage on Tuesday, gusty northwesterly winds will overtake the terminals in to Tuesday afternoon and Tuesday night, gusting 25 to 30 knots. Overall VFR conditions are expected Wednesday through Friday. W/NW winds could gust over 30 kt on Wednesday. There are a couple disturbances which could bring snow showers Thursday and Friday, but impacts are uncertain at this time. && .MARINE... Winds greatly diminished near sunset. However, a 25-kt low- level (near 2000 ft) jet ahead of a shortwave traversing the waters at this time. Since water temperatures are warmer than air temps, momentum transfer has been favorable. Multiple reports of 20 kt gusts noted from meso sites. Consequently, reissued a Small Craft Advisory for most of the waters through daybreak. Left out the upper Tidal Potomac, Patuxent, Baltimore Harbor, north of Pooles Island, and Chester. Weak high pressure moving through on Monday will bring a return of light winds (less than 10 kt). Ahead of an approaching cold front Monday night and Tuesday, winds may reach SCA level, with SCA likely behind the front Tuesday afternoon and night with strong gusty northwesterly flow. Gale conditions look possible behind the front, and through Tuesday night. Strong winds will continue into Wednesday behind departing low pressure. Small Craft Advisory conditions are likely, and gales are possible early in the day. High pressure will bring lighter winds by late Wednesday night. Low pressure will cross near the area on Thursday, but winds may not increase appreciably at the surface until a second disturbance moves through on Friday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 7 AM EST Monday for ANZ531>534-536- 537-540-541-543. && $$ SYNOPSIS...BKF NEAR TERM...HTS SHORT TERM...BKF LONG TERM...ADS AVIATION...HTS/BKF/ADS MARINE...HTS/BKF/ADS

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