Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 110230 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 930 PM EST Sat Dec 10 2016 .SYNOPSIS... High pressure will drift east overhead rest of tonight before moving off the Mid-Atlantic coast Sunday morning. A warm front will move into and lift north Sunday followed by a cold front pushing east Monday. High pressure will briefly move over the area Tuesday into early Wednesday before the next reinforcing cold front arrives. && .NEAR TERM /THROUGH SUNDAY/... As of 9pm, a 1034mb surface high is centered over the area. Enough gradient for 10mph winds exists over Baltimore. Low pressure is spilling over the Front Range in Colorado with an associated warm front currently stretching east to the Midwest. A swath of snow along this front stretches WNW from central IN. This snow over the Midwest is a focus for precip tonight. The southern edge of this swath will enter the NWrn section of the CWA tonight and likely bring light snow accumulations north from a line from Grant County WV to Frederick County MD. Cannot not rule out some flurries south of this line, but it is notably dry tonight. It appears the most likely location for up to an inch will be near Frostburg/Cumberland. The most likely period for snow will be 2-10 AM. As the warm front lifts northward, the remainder of Sunday should be dry but relatively cloudy. The clouds will help hold temperatures down in the mid 30s to lower 40s. && .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/... The Sunday night-Monday morning time frame remains a tricky forecast due to uncertainty in surface temperatures and areal coverage of precipitation while temperatures are potentially below freezing. It is beginning to appear that moisture arriving from the west won`t arrive until late evening or later. By that time, a strong low level jet will be advecting much warmer air aloft, meaning only a brief chance of seeing snow or sleet. In addition, there will be some moisture advection off the Atlantic. The NAM spreads light precipitation much farther west into the Piedmont, which would pose a concern due to near-freezing temperatures. Have discounted this solution for now as it appears to be an outlier. The best chance for a period of freezing remains west of the Blue Ridge and perhaps near the PA border. Temperatures will likely slowly rise through the night since southerly flow will already be established...the question is just how quickly. The chance of freezing rain will linger longest across northwestern parts of the area. Rain will gradually continue eastward across the area on Monday. Since the rain will persist the longest east of the Blue Ridge, in-situ cold air damming may linger, as indicated in raw NAM/GFS temperature guidance. Trended high temperatures down into the 40s in this area. A weak cold front will clear away any lingering rain by sunset. Precipitation may end briefly as snow along the Allegheny Front. High pressure will build in Monday night, although clouds may be slow to clear. && .LONG TERM /TUESDAY THROUGH SATURDAY/... A powerful (180-knot) jet streak at 250 mb will intersect an Arctic front sagging through the area Wednesday. With fast westerly flow and not a ton of moisture, areas most likely to see precipitation would be the western ridges of the Allegheny Front as well as the southeastern half of the CWA (near and S&E of I-95), with perhaps a bit of a minimum in between. Temperature profiles aloft would support mostly or all snow, but surface temperatures during the early to middle afternoon may not allow much in the way of accumulation at the lower elevations at the onset. As both the background airmass and surface temperatures cool with sunset, any snow that is left falling may begin to stick. For now the trend in the guidance appears to be a bit quicker with a little less precipitation, but with such an energetic jet coupled with the time of day and rapidly cooling airmass, conditions will need to be closely monitored for wintry precipitation impacts. Another round of Arctic air tumbles into the region behind the front Thursday into Friday with model forecasts showing 850 mb temperatures dropping into the -16 to -20 C range. This would result in daytime highs struggling to get much above freezing across most of the area Thursday and Friday with overnight lows in the teens and 20s. This is about 15 degrees below normal, but not quite to record levels for this time of year. Most Arctic airmasses do not depart peacefully, and as such another storm system approaching from the south and west may bring a shot at wintry precipitation again by next weekend. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... VFR conditions expected through Sunday. Chance for light snow/flurries at MRB late tonight into Sunday morning, but should not result in significant impacts. Clouds Sunday morning with Cigs around 5kft across the DC metros. MVFR to IFR conditions will develop Sunday night as the next storm system develops. A period of freezing rain or sleet is most likely at MRB but uncertain. At this time, precipitation appears to be rain at the other terminals, but temperature forecasts will need to be monitored. LLWS may also be an issue late Sunday night. Rain and associated impacts continue for much of daytime Monday. Uncertain how quickly clouds will clear out Monday evening. Mainly VFR expected Tuesday into Wednesday as high pressure noses into the region with light northerly flow expected. && .MARINE... Winds continue to ease tonight as high pressure shifts overhead before drifting offshore Sunday morning. Southerly flow increases Sunday afternoon, and have gone ahead and issued a SCA for most of the MD Chesapeake Bay and lower Potomac. However, mixing becomes a little more uncertain Sunday night. SCA conditions will probably continue for portions of the waters Monday and Monday night as a cold front crosses. High pressure is expected to ridge over the waters Tuesday into Wednesday which would result in light winds below Small Craft Advisory levels. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 3 PM Sunday to 6 AM EST Monday for ANZ531>534-537-539>543. && $$ SYNOPSIS...BAJ/ADS NEAR TERM...BAJ/ADS SHORT TERM...BAJ/ADS LONG TERM...DFH AVIATION...BAJ/ADS/DFH MARINE...BAJ/ADS/DFH is the U.S. government's official web portal to all federal, state and local government web resources and services.