Area Forecast Discussion
Issued by NWS Sterling, VA

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577 FXUS61 KLWX 171955 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 255 PM EST Fri Nov 17 2017 .SYNOPSIS...
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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.
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&& .NEAR TERM /THROUGH TONIGHT/...
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High pressure is currently almost directly over the region, and winds have lessened considerably. 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 high cirrus. Tonight, high pressure will shift southeast into the Atlantic near Cape Hatteras, with winds now going light and variable becoming southerly by dawn. We may radiate quickly early this evening, but increasing flow late tonight mean we likely won`t get as cold as we could if it otherwise stayed near calm all night. The increasing high clouds will also likely limit radiational cooling. Thus, have not gone quite as cold as the coldest guidance, but still expect many areas near freezing.
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&& .SHORT TERM /SATURDAY 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, so future shifts may need to raise POPS all the way to DC from what we currently have. That having been said, rain still looks likely to hold off until morning across western zones, and until afternoon 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 increasing 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. 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.possible Behind the front, most areas will dry out as low pressure lifts northeast into southern Canada, but upslope flow will get going along the Alleghany Front, so snow showers are likely most of the day Sunday before diminishing Sunday evening. Advisory level accumulations are possible along the Alleghany 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/...
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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.
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&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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VFR through Saturday, though cigs will be lowering and vis may briefly be restricted in showers during the afternoon Saturday. There may also be some low level wind shear Saturday morning, particularly at MRB, but also possibly at the other terminals. 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, so SCA will be dropped shortly. Most of night should be sub SCA, but southerly flow on back side of high will begin after midnight, so have hoisted SCA for mid-bay zones after midnight, then all zones during the day Saturday. Winds above the surface look quite strong, 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 low, 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>533-535>542. Small Craft Advisory until 6 PM EST this evening for ANZ533- 534-537-541>543. Small Craft Advisory from midnight tonight to 6 AM EST Sunday for ANZ534-543.
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&& $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...KLW AVIATION...RCM/KLW MARINE...RCM/KLW TIDES/COASTAL FLOODING...RCM

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