Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 102015 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 315 PM EST Sat Dec 10 2016 .SYNOPSIS...
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High pressure will build overhead tonight, then move off the Mid- Atlantic coast Sunday morning. A warm front will 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.
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&& .NEAR TERM /THROUGH SUNDAY/...
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High pressure is centered over the Ohio and Tennessee Valleys this afternoon. The gusty-at-times winds will subside after sunset as the high builds eastward. While some flurries are still falling along and west of the Allegheny Front, these will begin to taper off. Temperatures will likely fall quickly after sunset. Widespread teens would likely be reached if not for an advancing mid-level cloud deck ahead of the next storm system. At this point, only have some spotty teens in the typical colder spots in the south where clouds may be thinner, but adjustments may be warranted. Lows should be closer to 30 in the urban areas regardless. While the aforementioned low pressure center will be over the central Plains tonight, a glancing shot of isentropic lift will cross the area as a warm front aloft develops. Some light snow will likely develop in response to this forcing, although the amount that reaches the ground in our area is questionable due to low level dry air. Model QPF is pretty consistent with keeping QPF very close to the PA border, but would not rule out some flurries farther south. 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.
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&& .SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
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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.
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&& .LONG TERM /TUESDAY THROUGH SATURDAY/...
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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.
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&& .AVIATION /20Z SATURDAY THROUGH THURSDAY/...
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VFR conditions expected through this evening. NW wind gusts up to 20 kt will subside with sunset. Light snow/flurries may fall at MRB late tonight into Sunday morning, but should not result in significant impacts. Cloudy but VFR on Sunday. 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.
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&& .MARINE...
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Northwest flow still gusting to SCA levels at times this afternoon, especially northern waters, but will be subsiding with sunset. Light winds with high pressure tonight into early Sunday. 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.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ530>533-535-538>541. Small Craft Advisory from 3 PM Sunday to 5 AM EST Monday for ANZ531>534-537-539>543.
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&& $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...DFH AVIATION...ADS/DFH MARINE...ADS/DFH

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