Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 180206 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 906 PM EST Fri Nov 17 2017 .SYNOPSIS... High pressure will move offshore tonight through Saturday. A strong cold front will cross the area late Saturday night. High pressure will build to the south of the region late Sunday through Monday before moving offshore Tuesday. Another cold front will pass through Tuesday night into Wednesday and Canadian High pressure is likely to build overhead for Thanksgiving Day. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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High pressure is centered over eastern North Carolina, with winds nearly calm across much of the area. However, warm advection has already begun in advance of the approaching system to our west, and high level moisture is already moving overhead in the form of cirrus. So far the cirrus has done little to stop temperatures from dropping rapidly since sunset to around freezing. Just pushed a forecast update with an adjustment from current trends. The forecast is tricky though, since it`s possible temperatures level off as clouds thicken, or even rise in some spots as a light southerly wind develops late. Guidance has sped up leading edge of showers a touch quicker, reaching the Potomac Highlands around dawn. Initially this should be very light and falling from mid-level clouds. Wouldn`t be surprised if there were a few sleet pellets due to cold/dry low levels. There`s also some concern with how we`ve radiated if those temperatures will recover above freezing before precipitation arrives. At this time, don`t think there is an appreciable threat for any freezing rain though.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/...
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System developing in the central plains at present will bring some active weather to us this weekend, with the main focus being near and after cold frontal passage late Saturday night. To start Saturday, high pressure will be to the southeast, and a warm front will be approaching from the south as low pressure strengthens and heads northeast across the Corn Belt. As the warm front lifts north through the day, clouds will continue to thicken and lower, and some rain may break out across the area. Best chance is northwest of I-95 corridor, up in western MD and eastern WV, but guidance is shifting a bit further south as well. PoPs have been adjusted upward slightly across the northern half of the area, but may ultimately need to go higher along the US 50 corridor. That having been said, rain still looks likely to hold off until midday in the metro (if it makes it that far). Temps should rise above freezing everywhere by then, but there is a small chance a little ice could occur in a sheltered valley near Cumberland early Saturday. Otherwise, the low level southerly flow will be increasing through the day, and a late day or evening high looks more likely than not. In fact, temps could spike several degrees warmer than our forecast towards midnight, but given uncertainty, held temps down for now. One caveat: if steadier precipitation does develop across the north during the morning, an in-situ wedge/cold pool could develop, resulting in highs stuck in the 40s. Then this cold pool may not break, especially in the North Branch Potomac River area, until the cold frontal passage, as depicted by the NAM 3km nest (and supported by climatology). Warm front should lift north of the CWA late Saturday/early Saturday night, and the rain chances look to diminish for a time late Saturday afternoon and Saturday evening as it does so. Cold front then pushes east across the region late Saturday night into early Sunday morning, possibly accompanied by a low-topped squall line or squall segments. There is a lot of low level wind ahead of the front, but given the southerly flow and likely low level inversion, do not expect widespread wind advisory criteria ahead of the front - however, some ridges may touch wind advisory before the front passes Saturday night. Highs will also likely be reached predawn Sunday across most of the region, with temps possibly spiking into the 60s, but our forecast highs do not reflect this given they focus on the daylight hours Sunday when temps will be more or less steady, perhaps even falling. Winds with the actual front could touch wind advisory late Saturday night/early Sunday, especially if there is a band of showers with it. Behind the front, most areas will dry out as low pressure lifts northeast into southern Canada, but upslope flow will get going along the Allegheny Front, so snow showers are likely most of the day Sunday before diminishing Sunday evening. Advisory level accumulations are possible along the Allegheny Front. Further east, the main story will be the wind, which is likely to gust close to advisory criteria even in the low elevations of the metro, though odds are better in northern areas (Maryland) than further south (Virginia). Right now best gradient and wind aloft combined with mixing appears to maximize gusts during the morning Sunday, so this is when we peak them in the forecast, but its possible it peaks with the actual front overnight, or later in the day when mixing increases (but winds aloft and gradient diminish a bit). Given how far out in time it is, have held off on wind advisory for now, but it still looks like a decent bet especially near the Mason-Dixon line. By Sunday night, the low will be continuing to move away and the gradient, winds aloft and mixing will all decrease as heights start to rise. This should both cut off the snow in the upslope regions as well as allow gusts to fall off. Temps will be chilly Sunday night, but not as cold as we experienced a week ago.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/... High pressure will build into the region Monday and Monday night. Dry conditions expected with temperatures 5 to 10 degrees below average each period. The high pressure will move offshore Tuesday to allow for a strong cold front to plunge southeastward into the region Tuesday night. The front should be a dry front with the lack of moisture present. However, we can`t rule out a couple of upslope snow showers Tuesday night. High pressure will build into the region Wednesday through Friday. Temperatures will remain chilly for this time in November with dry conditions and some sunshine. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
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VFR through at least Saturday morning, though cigs will be lowering and vis may briefly be restricted in showers during the afternoon Saturday (earlier at MRB, where there`s also a chance of MVFR cigs by midday). There may also be some low level wind shear Saturday morning, particularly at MRB, but also possibly at the other terminals. As usual, NAM is much lower with cigs by Saturday evening, but have played conservatively at low end VFR for now. Brief IFR possible in showers with winds possibly gusting to 40 knots late Saturday night as cold front plows through. Going quickly back to VFR Sunday, but winds may continue to gust, with possibly 40 knot gusts at times Sunday, with best odds being at BWI/MTN. Winds diminish with continued VFR Sunday night. VFR conditions expected Monday through Tuesday night. Winds west around 10 knots gusts 15 knots Monday. Winds southwest 5 to 10 knots Monday night through Tuesday night.
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&& .MARINE...
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Winds diminishing at present as high pressure moves overhead. Most of night should be sub SCA, but southerly flow on back side of high will begin after midnight. Have pushed back Small Craft Advisory to 3 AM for the wider Bay waters, where channeling should be maximized. Winds above the surface look quite strong as Saturday wears on, but think this southerly flow over somewhat cooled waters should keep gales from happening Saturday and most of Saturday night, so just extended SCA through the night. We may need a gale late Saturday night with the cold frontal passage and accompanying gusty showers, and will likely need one in the wake of the front on Sunday for NW winds gusting perhaps as high as 40 knots. Winds diminish Sunday night as the front and storm moves further away and high pressure starts building in. Small craft advisories may still be needed Monday. Winds west 10 to 15 knots gusts up to 20 knots. No marine hazards Monday night through Tuesday night.
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&& .TIDES/COASTAL FLOODING...
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Anomalies remain around a third of a foot, but will likely rebound as the lingering anomalies to our south push back north with increasing southerly flow later tonight and Saturday. Minor flooding is anticipated at our sensitive sites, perhaps even some of the less sensitive ones, during the high tides later Saturday into early Sunday. After that, cold front will bring northwest winds back and should drop the anomalies enough to prevent minor flooding by midday Sunday at latest, probably earlier.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 6 AM Saturday to 6 AM EST Sunday for ANZ530-531-535>542. Small Craft Advisory from 3 AM Saturday to 6 AM EST Sunday for ANZ532>534-543.
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&& $$ SYNOPSIS...RCM NEAR TERM...ADS/RCM SHORT TERM...ADS/RCM LONG TERM...KLW AVIATION...ADS/RCM/KLW MARINE...ADS/RCM/KLW TIDES/COASTAL FLOODING...RCM

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