Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 171817 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 217 PM EDT Tue Apr 17 2018 .SYNOPSIS...
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An area of high pressure will cross the southeastern United States tonight, then move offshore Wednesday. An area of low pressure will move west to east across the Great Lakes Wednesday night into Thursday. High pressure will build in from the Ohio Valley Friday into the weekend.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Visible satellite shows an awesome depiction of widespread wave clouds this afternoon. As the upper low/trapped mid level moisture exits to the northeast, clouds will quickly diminish from southwest to northeast this evening. An area of high pressure over the northeastern Gulf of Mexico will slide across the Florida Peninsula and into the Atlantic Ocean tonight. As this happens, surface ridging nosing northward into the area will cause winds to go light across much of the Shenandoah Valley and central Virginia, allowing temperatures to drop. Dew points in the middle 20s suggest lows in the upper 20s to around freezing are likely where skies remain clear and winds do in fact go light. The gradient will remain elevated over northeastern Maryland closer to the departing surface low moving into the Canadian maritimes, but temperatures should still drop to around freezing due to the cold airmass. Meanwhile, a developing warm front over southwestern Virginia will cause an increase in clouds across the southern and central Shenandoah Valley after midnight. Considering the dry air and narrow window (spatially and temporally) for clear skies and light winds, believe physical frost formation is unlikely except in patches tonight, so have only issued a Freeze Warning where I expect temperatures to drop below freezing, with no Frost Advisory surrounding it yet.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
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Wednesday should wind up pretty warm (60s to near 70). Went a little above guidance with downsloping westerly flow and skyrocketing 850 mb temps (+10-12 C by 00Z). Clouds will increase through the day as a warm front continues to develop over central/southwest Virginia and moves northeastward, with a few sprinkles possible over the higher terrain. 24 hours after freezing temperatures, Wednesday night should prove mild with clouds and mild downsloping breezes. Lows in the middle 40s to lower 50s are expected. Most showers should be confined to the Allegheny Highlands as low pressure passes to the north, with very little moisture return east of the mountains on westerly flow. Cold advection begins by about midday Thursday as low pressure to the north departs across southern New England. Downsloping flow should result in mild temperatures at the surface, though, especially in the I-95 corridor. The cooling temperatures should cause precipitation to change to snow showers over the Allegheny Highlands with a coating to an inch or two of snow expected. Cooler temperatures return Thursday night with lows in the 30s.
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&& .LONG TERM /FRIDAY THROUGH MONDAY/...
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At the start of the period Friday, a departing area of low pressure will be located over the Canadian Maritimes. In the wake of that low, breezy northwesterly winds will continue ushering colder air into the region. Other than a few lingering upslope snow showers along the Allegheny Front, no precipitation is expected. High pressure centered over the Great Lakes and Ohio Valley will build in for the weekend, maintaining cool and dry conditions over the area. Winds will weaken and shift from light northerly to light easterly as high pressure slides off to our north. By Sunday Night, models have a cut-off upper level low and associated surface low tracking through the Lower Mississippi Valley. As that system moves east on Monday, models keep it suppressed off to our south as it interacts with a southeastward translating, positively tilted trough located over the Mid-Atlantic. As a result, it looks like the precipitation associated with that low will stay south of our area on Monday.
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&& .AVIATION /18Z TUESDAY THROUGH SUNDAY/...
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Breezy conditions will linger into this evening on west flow. VFR deck (OVC050) should clear quickly early this evening. Winds gradually shift to southerly AOB 10 kts Wednesday with VFR. Low pressure will pass north of the area Wednesday night into Thursday. Widespread showers are not expected to reach east of the mountains, so no restrictions are expected. BKN VFR deck likely as the low passes, with gusty W flow (G 30-35 kts) expected in its wake. VFR conditions are expected Friday through the weekend. A gusty northwest wind on Friday should diminish by Saturday.
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&& .MARINE...
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SCA gusts linger until at around midnight for all waters, then begin to retreat east across the open waters of the Chesapeake Bay as the gradient relaxes from west to east. Winds should be below SCA criteria by daybreak. A few gusts around 20 knots are possible during the day, but mixing down gusts in warmer air over cooler water makes this more uncertain. Small Craft Advisories will become more likely late Wednesday night as the gradient increases further after low pressure passes over Pennsylvania, with gale conditions likely (especially in any scattered shower or sprinkle activity) on Thursday. Small Craft Advisories may be needed Thursday night into Friday as departing low pressure moves into the Canadian Maritimes. Winds should be light over the weekend.
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&& .FIRE WEATHER...
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Drier air advecting into the region Thursday on gusty west winds could result in an enhanced risk of fire spread Thursday, especially if fuel moistures decrease appreciably by then.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...Freeze Warning from midnight tonight to 9 AM EDT Wednesday for MDZ003>006-503-505-507-508. VA...Freeze Warning from midnight tonight to 9 AM EDT Wednesday for VAZ026>031-039-040-051-501-502-505>507. WV...Freeze Warning from midnight tonight to 9 AM EDT Wednesday for WVZ051>053. MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ530>534- 537-539>543. Small Craft Advisory until midnight EDT tonight for ANZ535-536- 538.
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&& $$ SYNOPSIS...DHOF NEAR TERM...DHOF SHORT TERM...DHOF LONG TERM...KJP AVIATION...DHOF/KJP MARINE...DHOF/KJP FIRE WEATHER...DHOF

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