Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 182212 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 612 PM EDT WED MAY 18 2016 .SYNOPSIS...
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High pressure over the upper Great Lakes will slowly build over the region Friday. Low pressure will approach the area Friday night from the Tennessee Valley and track to our south Saturday. High pressure will build overhead early next week...but an upper- level low will remain close by just off the Mid-Atlantic coast.
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&& .NEAR TERM /THROUGH THURSDAY/...
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Upper-level low pressure over the western Great Lakes will remain nearly stationary this evening. Low pressure is reflecting at the lower levels of the atmosphere near the Appalachian Mountains and this will also remain nearly stationary during this time. A northeast flow will continue to usher in unseasonably cool conditions for this time of year. Plenty of clouds are also expected due to moisture being trapped underneath the subsidence inversion... thanks to the low-level easterly flow. Periods of rain will continue across the Allegheny and Potomac Highlands as well as portions of the Shenandoah Valley...eastern West Virginia and western Maryland this evening...closer to the upper- level low. However...the rest of the area will remain far enough away from this system to be dry...but a few sprinkles cannot be ruled out due to the moisture trapped underneath the subsidence inversion. The upper-level low will shift to the east tonight...and the 850mb low associated with this system will also shift east from the Appalachian Mountains into our area. However...this system will weaken as the upper-level support shifts into New England...outrunning this system. The eastward shift will cause light rain to move east through the Shenandoah Valley toward the Blue Ridge Mountains this evening and overnight. Light rain may even push toward the Washington and Baltimore Metropolitan areas into the Virginia Piedmont tonight...but it will have a tough time since the low will be weakening. Points farther east should remain mainly dry with just a sprinkle or two. Rainfall tonight will be light...under one tenth of an inch with the best chances near and west of the Blue Ridge Mountains...closer to the dissipating low. Min temps will range from the 40s west of the Blue Ridge Mountains to the lower and middle 50s near Washington and Baltimore. The upper-level low will remain over northern New England Thursday and this will carve out an upper-level trough over our region. Surface high pressure will build slowly build in from the north and west...allowing for drier air to work its way into the area. However...it will be slow to do so so clouds may hang around through midday before giving way to some sunshine. A couple showers cannot be ruled out...especially near the higher terrain but coverage will be isolated so most areas will be dry. Highs will be a bit warmer due to some afternoon sunshine...but still below climo. Highs will range from the 50s in the mountains to the upper 60s/near 70 around Washington and Baltimore.
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&& .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... High pressure will continue to build over the area for Thursday night into Friday...bringing dry conditions. Friday will likely be the best day of the week with some sunshine and max temps ranging from the 60s in the mountains to the lower and middle 70s for most other locations. High clouds will slowly increase throughout the day ahead of the next system. Low pressure will track into the Tennessee Valley Friday night and it will begin to transfer its energy to a coastal low near the Carolina coast. Warm and moist air ahead of this system will overrun the cooler air in place...causing clouds to quickly lower and thicken and rain to overspread the area from southwest to northeast. The rain may be locally heavy overnight due to copious amounts of moisture associated with this system along with significant forcing from an increasing low-level wind field and forcing from the upper-level system. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Low pressure will move off the Carolina coast and out to sea Saturday. Rain will slowly come to an end from west to east as the day progresses, but the upper-level trough driving the surface low will linger over the area through the weekend. There is still a lot of spread in the ensemble guidance regarding exact rainfall amounts. This is due to the convective nature of the forcing driving the system which is not easily resolved at this time range. The placement of heavier bands of precipitation and exact amounts will likely remain in question until about 48 hours out, but a general 1 to 2 inches looks likely given the synoptic setup. Of note, guidance has trended a bit slower with the whole progression (which again makes sense given the blocky nature of the pattern) which means much of Saturday may end up being a washout. Sunday will be the better of the two weekend days, but intermittent lighter rain and drizzle will remain possible as the upper-level low cuts off and lingers over the area. Showers may pop up during the afternoon hours Monday and possibly Tuesday as the upper low spirals slowly off to the east. A second area of surface low pressure may try to spin up off the east coast early next week but details are very fuzzy this far out especially given the erratic nature of the cutoff upper low that would be forcing it. High pressure should begin establish itself off the southeast coast during the middle to latter portions of next week leading to a general warming trend. && .AVIATION /22Z WEDNESDAY THROUGH MONDAY/...
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VFR conditions this evening for most of the terminals as some drier air near the surface moves in on a northeast flow. Low pressure will weaken as it moves toward the area tonight. This may bring some light rain...especially to KMRB and KCHO. Light rain may approach the other terminals as well. Cigs will lower a bit toward MVFR levels. Will keep cigs around 3kft for most of the terminals...but went with MVFR conditions for KCHO. Cigs will gradually lift Thursday as high pressure builds overhead. High pressure will build overhead Thursday night through Friday. Areas of fog are possible Thursday night into early Friday. Low pressure will approach the terminals Friday night. Rain will overspread the area along with lowering cigs/vsbys. Sub-VFR conditions are expected Saturday with low ceilings and visibilities in rain. Sub-VFR conditions will remain possible into Sunday with low clouds and drizzle or intermittent light rain lingering across the area. Winds will be easterly 10 to 15 knots Saturday...becoming northerly 10 to 15 knots Sunday.
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&& .MARINE...
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High pressure will gradually build toward the waters tonight through Friday. Winds should remain below SCA criteria during this time. Low pressure will approach the waters Friday night...bringing rain to the waters late. Winds will strengthen a bit due to a strengthening gradient. A small craft advisory may be needed for portions of the waters overnight Friday. A tightening pressure gradient between high pressure departing to the northeast and low pressure passing to the south will lead to easterly winds gusting well into Small Craft Advisory criteria on Saturday (20 to 30 knots). Winds will gradually diminish Sunday but likely gust in excess of 20 knots for at least part of the day as winds pivot to the north and channel down the Chesapeake Bay.
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&& .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ UPDATE...WOODY! PREVIOUS...BJL/DFH

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