Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 050238 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 938 PM EST Sun Dec 4 2016 .SYNOPSIS... An upper-level disturbance will pass through the region tonight. High pressure will briefly return for Monday. Coastal low pressure will impact the area Tuesday before high pressure briefly returns again for Wednesday. A potent cold front will move through late in the week. && .NEAR TERM /THROUGH MONDAY/... Forecast is largely on track this evening outside of minor details. 00Z IAD sounding shows a shallow sub-freezing layer between approximately 2400 and 3600 ft. This layer can be seen with some of the high elevation mesonets (i.e. Wintergreen) below freezing, with some reports of freezing rain glaze at those elevations as well. Winter Weather Advisory covers these areas. As the column saturates, temperatures should gradually rise through the night, at least on the ridges. Doubt much if any snow is falling. However, some ice pellets are being reported all across the region on the leading edge of precipitation due to the evaporative cooling aloft. There shouldn`t be enough sleet to cause any issues. Low pressure centers to our north and south as well as a shortwave trough will continue to progress east tonight. Rain will continue at times. Cooling will be limited as dew points/wet bulb temps rise, so p-type issues should dwindle with time. Back edge of precip will progress east quickly late tonight, and most areas may be dry by daybreak except communities near the Chesapeake Bay. The system quickly heads east Monday morning and a weak ridge returns. This should allow clouds to break for some sun by afternoon. Highs will return to the upper 40s and lower 50s. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... Closed low over northern Mexico will eject eastward into Texas on Monday, then open up and head northeastward towards us Monday night and Tuesday. In response, surface low pressure will develop and head northeastward along the Atlantic seaboard. A primary low will lift north west of the Appalachians while a secondary will redevelop in the Carolinas. In between, a wedge of colder air is likely to hold firm east of the Allegheny Front. In short...this spells trouble...though the extent of it remains uncertain. Skies may still be mostly clear early Monday night, allowing for some radiational cooling. However, clouds are likely to build in by midnight and put a stop to that, perhaps even allowing temps to rebound a bit. That said, the further inland one is, the less of a bump one might expect. Warm advection precip will start moving northeast across the region late Monday night with precip likely to arrive during rush hour in the metro, perhaps near the end of the rush up towards Baltimore. Dry air again will be in place so even though much of the I-95 corridor may still be above freezing, some snow and sleet may initially make their way to the surface. No impacts are currently anticipated in this corridor, however, as the moistening of the already warm atmosphere should cause any sleet and snow to go over to rain with little accumulations. Further inland, especially across the Shenandoah Valley, cold air may be trapped for the duration. These areas may get significant icing if the colder guidance is to be believed. After also possibly starting as snow/sleet, freezing rain is likely to be the predominate precip type most of the morning, perhaps changing to rain as temps only very slowly warm up in the afternoon. Given its usual good handling of cold air damming, the NAM temps were favored. This suggests that some areas could stay freezing rain nearly all day on Tuesday. However, since temps will be marginal, and most of precip will fall during daylight, we are skeptical about a warning level ice event, so for now still will advertise potential advisory. Highs will be stuck in the 30s in the SHenandoah Valley, while struggling to reach 40 further east. As the system weakens and lifts north, expect rain to taper off Tuesday night, though if cold air damming is as persistent as some guidance suggests, there could still be a little freezing rain ongoing in the Shenandoah Valley. Temps will likely move little Tuesday night. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... High pressure will build in on the west side of the coastal low Wednesday and Wednesday night. Drier air will filter in from the northwest and temperatures will remain seasonably chilly. A few isolated rain showers cannot be ruled out near the Chesapeake Bay, while some snow showers could evolve over the Potomac Highlands. The next storm system will approach from the southwest Thursday and Thursday night. A chance of rain showers exist across the region each period. We can`t rule out some wintry precipitation as well, depending on the track of the storm system and whether or not there is enough cold air at the ground surface to support it. Uncertainty remains moderate to high. A chilly dome of high pressure is expected to build in from the west Friday through Saturday night. This high could bring the chilliest air so far this season. Some upslope snow showers could also occur in the Potomac Highlands. The high pressure will move offshore the East Coast Sunday ahead of the next approaching trough of low pressure Sunday night into Monday. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... Conditions will continue to deteriorate tonight as a weak storm system crosses the area. Lower cigs to the south will spread north, reaching IFR after midnight and likely staying in that category through morning. Periods of rain will continue until late tonight. Sleet should only be an issue with this initial onset, with temperatures expected to remain above freezing at the terminals. The rain may be clear of the terminals by the morning push. Conditions improve on Monday with VFR returning later in the morning. Going back down late Monday night and Tuesday with a slightly more wintry system, but still more plain rain than not along I-95 - however, significant icing is possible at MRB. Icing or not, IFR cigs and vis likely Tuesday and Tuesday night. MVFR conditions possible in any rain showers near MTN or BWI early Wednesday. VFR conditions elsewhere and rest of time. Winds northwest 5 to 10 knots Wednesday becoming light and variable Wednesday night. Gusty NW winds expected behind strong cold front later Thursday into Friday. && .MARINE... Winds are below SCA and should remain below through Monday night. Southerly winds are picking up to 15 kt tonight, and northwest winds may do the same Monday afternoon. Will have to keep an eye out for a potential marginal SCA. Coastal low pressure will develop to the south Monday night and it will pass by to the east later Tuesday into Tuesday night. As it passes, SCA winds may redevelop on the bay as the system passes. High pressure will briefly build toward the waters Wednesday, with a potent cold front expected to pass through Thursday. A Gale Warning may be needed for the waters Thursday night and Friday behind the cold front. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Winter Weather Advisory until 6 AM EST Monday for MDZ501. VA...Winter Weather Advisory until 6 AM EST Monday for VAZ503-504- 507-508. WV...Winter Weather Advisory until 6 AM EST Monday for WVZ501-503- 505-506. MARINE...None. && $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...RCM LONG TERM...KLW AVIATION...ADS/RCM/KLW MARINE...ADS/RCM/KLW

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