Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 201536 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 1036 AM EST Mon Nov 20 2017 .SYNOPSIS... High pressure will build to our south through tonight. The high will move offshore Tuesday and a cold front will pass through Tuesday night into Wednesday. High pressure will build back overhead later Wednesday through Thanksgiving Day. The high will build to our south for Friday and Saturday before a cold front moves through Saturday night. && .NEAR TERM /THROUGH TONIGHT/...
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Upper-level trough moving east of us this morning while surface high pressure builds to the south. Rising heights will allow plenty of sunshine this afternoon...but temps will still be below average for this time of year. Max temps will range from the upper 30s along the ridges of the Allegheny Highlands...to the mid and upper 40s in northern Maryland...to the middle 50s in central Virginia. The high will remain to the south tonight. A weak shortwave in a southwest flow aloft may bring a few high clouds...but dry conditions will persist. Min temps will range from the mid and upper 20s in the colder valleys to the mid and upper 30s in downtown Washington and Baltimore.
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&& .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/... High pressure will shift offshore Tuesday while a cold front moves through the Great Lakes. A southwest flow in between these systems will allow for milder conditions. Max temps will be in the upper 50s to lower 60s across most locations. There will be some high clouds in a southwest flow aloft...but the cloud deck should be thin enough for a partly to mostly sunny sky much of the day. The cold front will approach our area from the north and west Tuesday evening before passing through overnight Tuesday into early Wednesday morning. There will be separation between the lift associated with the cold front and deeper moisture to our south and east. Latest guidance shows that some of this moisture can get drawn up into the front...bringing the chance for showers. The best chance for showers will be across eastern areas where moisture has a better chance to move in before the frontal passage. Any showers early Wednesday morning should dissipate as the cold front moves off to the south and east. A northwest flow will cause chillier conditions despite increasing sunshine. High pressure will settle overhead and it will turn out dry and cold. Did also want to note that a few snow showers are possible along and west of the Allegheny Front Tuesday night into early Wednesday. However...as discussed earlier moisture should be limited so any accumulation would be light. && .LONG TERM /THURSDAY THROUGH SUNDAY/...
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Longwave western US ridging and east coast troughing will continue to dominate the weather pattern over the United States through the holiday weekend. While no significant storm systems are currently forecast, several upper level impulses will move through the large-scale troughing. Sprawling surface high pressure is expected to be in place on Thanksgiving Day from the southern Plains extending northeastward through the Appalachians. At the same time, weak shortwave energy in both the northern and southern branches of the jet stream will be moving towards the east coast and may lead to surface cyclogensis off the southeastern US coastline. At this time, the surface ridging and flow aloft is expected to keep anything that can develop well to our south and east, leading to a dry and uneventful weather forecast for Thanksgiving Day. Surface ridging is expected to persist into Friday as well, keeping the forecast dry. By Friday night and Saturday, a more potent northern stream system will move into the upper Midwest, with its associated cold front barrelling towards the Mid-Atlantic Saturday night. Once again, southern stream energy will be present as well near the southeastern states, and the potential exists for surface cyclogenesis to occur. At this time, fast flow aloft and orientation of upper level trough suggests that a track well offshore is favored. However, chances for rain showers increase, either with the offshore low or with the cold front itself. Following the front Sunday, blustery and colder conditions are likely, along with the potential for snow showers over the higher terrain. Temperatures will start out below normal on Thanksgiving (40s), moderate back towards normal or even a bit above for Friday and Saturday (50s), before falling back below for Sunday (40s).
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&& .AVIATION /15Z MONDAY THROUGH FRIDAY/...
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VFR conditions will persist through Tuesday. Winds are mostly below 15 knots at present and will gradually turn to the southwest this afternoon and continue like so tonight. On Tuesday, southwest winds will continue, with gusts around 20 knots in the afternoon. A cold front will pass through Tuesday night into early Wednesday. A few showers are possible along with subVFR cigs. The best chance for this will be across the eastern terminals. VFR conditions will return behind the cold front for later Wednesday through Wednesday night. VFR and light winds expected Thursday and Friday with surface high pressure
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&& .MARINE...
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West winds this morning will gradually turn to the southwest this afternoon. Wind gusts should gradually diminish throughout the day. SCA continues across portions of the Bay thru the day, but winds may drop below SCA criteria on all waters late in the afternoon. However, southerly channeling will develop this evening as high pressure shifts east, with SCA returning, so the period of sub SCA will likely be less than 6 hours where the SCA remains in effect through the period. High pressure will move offshore Tuesday. South to southwest winds will increase during this time ahead of the next cold front. Therefore, SCA for all the waters Tuesday. The SCA was extended into Tuesday evening for portions of the waters. A cold front will pass through the waters late Tuesday night into Wednesday morning. Northwest winds will pick up behind the front and an SCA may be needed for portions of the waters later Wednesday into Wednesday night. Sub-SCA conditions expected Thursday and Friday with surface high pressure.
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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 6 PM EST this evening for ANZ530- 531-539. Small Craft Advisory from midnight tonight to midnight EST Tuesday night for ANZ531-539-542. Small Craft Advisory from 6 AM to 6 PM EST Tuesday for ANZ535- 536-538. Small Craft Advisory until midnight EST Tuesday night for ANZ532>534-540-541-543. Small Craft Advisory from 6 AM Tuesday to midnight EST Tuesday night for ANZ530. Small Craft Advisory until noon EST today for ANZ537. Small Craft Advisory from 6 PM this evening to midnight EST Tuesday night for ANZ537.
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&& $$ SYNOPSIS...BJL/RCM NEAR TERM...BJL/RCM SHORT TERM...BJL/RCM LONG TERM...MM AVIATION...BJL/MM/RCM MARINE...BJL/MM/RCM

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