Area Forecast Discussion
Issued by NWS Sterling, VA

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887 FXUS61 KLWX 040228 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 928 PM EST Sat Dec 3 2016 .SYNOPSIS... High pressure will gradually move into the region through Sunday. An upper-level disturbance will pass through the area Sunday night and high pressure will briefly return for Monday. Coastal low pressure will impact the area Tuesday before high pressure briefly returns for Wednesday. A potent cold front will move through late next week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Surface high pressure is located over the Great Lakes and Ohio Valley region this evening, with an upper level ridge axis in roughly the same area. However, the gradient remains with low pressure over maritime Canada, and winds have been slow to diminish across northern parts of the area. Likewise, stratocumulus has been persisting in this area longer than some models indicated. Meanwhile, fairly dense cirrus has been overspreading the area from a storm system along the Gulf coast. While the winds will diminish with time as the high pushes east overnight, the same can`t be said about the cloud cover. A thicker mid-level deck will be approaching by daybreak. However, hourly forecast temperature traces are verifying well, leading to little change in the min temperature forecast of upper 20s to mid 30s. Some variance could occur due to local differences in clouds/winds though.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/... Two systems will move across the region during the short term. Both of them have the potential to bring precipitation other than rain across parts of the area, primarily north and west of I-95 but possibly reaching the metro. Antecedent conditions suggest the Tuesday event is a bit more threatening. On Sunday, high pressure will slide east across the region as a shortwave trough approaches from the west. Warm advection will bring plenty of clouds through the day, with temps likely to struggle to rise - highs should be mostly in the 40s. Warm advection precip is likely to start breaking out late in the day in SW VA and S WV, moving across our region on Sunday night. Low level temps look warm on most guidance, but the mid level dry air is more suggestive of snow or sleet possibly reaching the surface, especially at the onset, thanks to evaporative cooling. Given warm ground and a afternoon in the 40s with no chance for radiational cooling, think impact to travel should be minimal with accumulations hard to come by even in those locales that have a period of all frozen precip, which is most likely to be NW of I-95 and closer to I-81. Higher terrain would be strongly favored for any accums given the rapid cooling just above the surface, though even in those spots it looks like accums would be perhaps an inch or so. Lows Sunday night will be in the 30s, but look mostly above freezing. This first system slides east on Monday and after some morning clouds and perhaps a little lingering drizzle or light rain near the bay, it should try to clear out during the afternoon with temps trying to get back into the 50s. This is a short respite however. In the evening, there is a window for radiational cooling which could bring temps down to freezing or below before clouds thicken again. The next system, which is currently sitting in northern Mexico, will approach the region later Monday night with warm advection/isentropic lift precip breaking out late at night and early in the morning from SW to NE. With sub-freezing temps possible, have highlighted this time period for potential impacts, particularly near and NW of I-81. Still lots of uncertainty, but guidance has trended a bit colder today, so the chance exists for accumulating snow and/or ice primarily NW of I-95, with some mix possibly reaching I-95 itself. Lows Monday night will be near freezing especially NW, with highs struggling to reach the 40s on Tuesday. Given this is a wedge situation, it would not be shocking if many spots along I-81 got stuck in the 30s on Tuesday. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... High pressure will build in on the west side of the coastal low Tuesday night through Wednesday night. Drier air will filter in from the northwest and temperatures will remain cool. Any precipitation will be rain showers near the Chesapeake Bay and snow showers at higher elevations of 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 it repeats the previous storm`s process of transitioning its energy to the East Coast. There is moderate to high uncertainty on the exact outcome at this time. A chilly dome of high pressure is expected to build in from the west Friday through Saturday. This high could bring the chilliest air so far this Fall. Some upslope snow showers could also occur in the Potomac Highlands. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/...
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VFR thru Sunday afternoon with mainly mid and high clouds, although stratocumulus has been slower to clear across northern terminals. Those clouds should gradually clear as high pressure builds. Likewise, winds will diminish late tonight into Sunday. Sub VFR cig/vis (with decent indications of IFR cigs) expected Sunday night as rain and perhaps a bit of sleet and snow moves through the region. Returning to VFR Monday then back to sub VFR by Tuesday as another wave of precip, possibly including freezing rain, moves across the area with the next system. Improving back to VFR Wednesday and Thursday.
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&& .MARINE...
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NW winds have been slow to diminish this evening, so have made some adjustments to the SCA...namely extending until 3 AM for the Potomac as winds aloft don`t diminish until that time. High pressure will be overhead by daybreak Sunday, with further lessening of the winds. Sub SCA winds expected Sunday thru Monday night as a weak system brings some rain and perhaps a little sleet Sunday night, with Sunday and Monday daylight hours being dry. Another system moving up the coast Tuesday may kick winds up to SCA levels, along with some rain and perhaps a little sleet or even snow, then dropping back to sub SCA later on Wednesday.
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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 3 AM EST Sunday for ANZ531-532-535- 536-538>540-542. Small Craft Advisory until 6 AM EST Sunday for ANZ533-534-537- 541-543.
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&& $$ SYNOPSIS...RCM NEAR TERM...ADS SHORT TERM...RCM LONG TERM...KLW AVIATION...ADS/RCM/KLW MARINE...ADS/RCM/KLW

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