Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 271852 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 252 PM EDT Thu Jul 27 2017 .SYNOPSIS...
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An upper-level disturbance will pass through the area this evening. Upper level low pressure system will intensify as it moves into the area Friday. This will slowly pass through the area Friday night into Saturday before stalling near the Mid- Atlantic coast through Monday. High pressure is expected to build overhead during the middle portion of next week.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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Surface analysis shows high pressure off the southeast coast with a cold front extending from Quebec to Illinois. Isolated to scattered showers and thunderstorms are possible. Shear profiles will strengthen a bit in response to the shortwave energy moving through and this may cause a few thunderstorms to become severe. However...widespread severe weather is not expected since the forcing will be weak and shear profiles will be marginal. Locally damaging wind gusts are the primary threat. The shortwave energy and surface trough will move to the east tonight while a stronger upper-level low digs through the Great Lakes. Our area will remain in between these systems...so any showers will likely remain isolated to scattered. Min temps will range from the 60s in most areas west of the Blue Ridge Mountains to the lower and mid 70s near Washington and Baltimore.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY NIGHT/...
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Upper-level low will track from near Lake Winnepeg into the Ohio Valley Friday, and into our area Friday night. This system will interact with a cold front to our north, causing surface low pressure to develop along the boundary. The upper-level low and surface low will strengthen as it moves into our area Friday night. Coastal low pressure is expected to develop near the Delmarva Peninsula Friday night. This system is expected to remain nearly stationary around this area through Saturday night. As the low approaches the area Friday...showers will become more widespread along with some thunderstorms. Deep moisture in place suggests that showers and thunderstorms will be locally heavy. The widespread showers and thunderstorms are expected to continue through Friday night as the low pressure strengthens overhead. A flash flood watch has been issued for the northern 2/3 of the forecast area. The heaviest rain is likely to be just north of the surface low and near the 850mb warm front...where the most frontogenetical forcing is expected. Latest guidance seems to paint this area across northern and central Maryland...northern Virginia...eastern West Virginia and toward the Washington and Baltimore metropolitan areas. Most likely rainfall amounts for Friday through Friday night are around 1 to 3 inches...again with the best chances of the highest amounts being across the areas mentioned. However...locally higher amounts are possible due to the convective nature of the precipitation. The low is expected to stall near the Delmarva Peninsula Saturday through Saturday night. A gusty north to northeast flow is likely along with more rain. Unusually cool conditions are expected due to the cloud cover and rain. The trend over the past couple days has been slower and farther south/west with this system for Saturday through Saturday night.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/... The cut-off low will likely be moving off the east coast Sunday while coastal low pressure will be near Delmarva. Showers will be tapering off from west to east across the Mid- Atlantic region and northerly flow will keep temperatures on the cool side for late July. The cut-off low oscillates along the Mid- Atlantic coastline through Monday resulting in cool and cloudy conditions across portions of the Mid-Atlantic region. This system finally moves out to sea Tuesday and high pressure moves into the region. Temperatures will slowly climb back to near normal by next Wed-Thu. && .AVIATION /19Z THURSDAY THROUGH TUESDAY/... Although plenty of clouds around celings are VFR. Isolated to scattered thunderstorms are expected this afternoon and evening. A couple storms may be severe...with damaging winds the primary threat. Low pressure will approach the terminals Friday before moving overhead and intensifying Friday night. Widespread showers and thunderstorms are expected to develop and rain may be locally heavy. A few storms may also be severe. More rain is likely Saturday and Saturday night along with Subvfr cigs and vsbys as the low stalls out near the Delmarva Peninsula. Gusty north to northeast winds are also possible during this time. Rainfall may linger Sunday (and perhaps into Monday) as low pressure meanders near the area. Improving conditions expected once low moves further out to sea by the middle of the week. && .MARINE...
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High pressure will remain offshore through tonight and an onshore flow is expected. Marginal SCA is effect. Low pressure will approach the waters Friday before strengthening overhead Friday night. The low will likely stall out near the Delmarva Peninsula Saturday through Saturday night. Gusty winds are possible with the low nearby...especially Friday night through Saturday night. Northerly winds continue on the waters Sunday-Monday. SCA may be needed.
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&& .HYDROLOGY... A flash flood watch has been issued for Friday afternoon through Saturday afternoon for the northern 2/3 of the forecast area. Most likely rainfall amounts during this time will be around 3 to 5 inches. This may cause minor flooding...particularly across the Potomac River Basin. && .TIDES/COASTAL FLOODING...
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Persistent southerly flow has allowed tidal anomalies to increase to over a foot. Coastal Flood Advisories this evening at Straits Point. Low pressure is expected to impact the area Friday into the weekend, bringing the potential for elevated tidal anamolies with a risk of coastal flooding.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch from Friday afternoon through Saturday afternoon for DCZ001. MD...Flash Flood Watch from Friday afternoon through Saturday afternoon for MDZ003>006-011-013-014-016>018-501>508. Coastal Flood Advisory until 9 PM EDT this evening for MDZ017. VA...Flash Flood Watch from Friday afternoon through Saturday afternoon for VAZ027>031-040-051>055-057-501-502-505>507. WV...Flash Flood Watch from Friday afternoon through Saturday afternoon for WVZ050>053-055-501>504. MARINE...Small Craft Advisory until 2 AM EDT Friday for ANZ532>534-537- 540-541-543.
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&& $$ PRODUCTS...WOODY!/HSK

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