Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 260747 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 347 AM EDT Mon Sep 26 2016 .SYNOPSIS...
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High pressure to the north will move off the coast today. A cold front will pass through the area tonight into Tuesday. An upper- level low will impact the area during the middle to perhaps latter portion of the week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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High pressure over New England extends southwest toward the Mid- Atlantic Coast early this morning. A southeast flow has developed with the ridge axis to our east. Warm and moist air is overrunning the cooler air in place this morning...resulting in plenty of clouds. Patchy drizzle is expected as well due to the isentropic lift. A cold front will approach from the west later today while high pressure continues to move off to the east. A south to southeast flow will continue over our area...and weak isentropic lift will continue during this time. This will likely keep clouds in place throughout the day. Cannot rule out a few breaks of sun...but much of the day will turn out cloudy. A few popup showers and isolated thunderstorms are expected west of the Blue Ridge Mountains this afternoon where there may be a little more instability closer to the cold front. The cold front will slowly cross through the area tonight. More widespread showers and possible thunderstorms are expected across all areas. The best chance for widespread precipitation will be early this evening near the Allegheny Highlands...late this evening across the Shenandoah Valley toward the Blue Ridge Mountains...and overnight across the Washington and Baltimore Metropolitan areas and points south and east. There will be enough moisture for moderate to locally heavy rainfall at times...but this system will be progressive. Average rainfall amounts around a quarter to one-half inch are expected with locally higher amounts.
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&& .SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
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The cold front will get hung up just to our south and east Tuesday while high pressure builds overhead northern and western areas. However...a cutoff upper-level low will be tracking through the Great Lakes and our area will likely remain in the right entrance of the upper-level jet over New England. This may cause clouds and precipitation to linger into the morning hours near the I-95 corridor and throughout the day across southern Maryland. Weak high pressure will build overhead for most areas Tuesday night into Wednesday...allowing for dry and seasonable conditions. Showers may remain hung up across extreme southern Maryland...closer to the stalled boundary. Cutoff upper-level low drop south into the Ohio Valley later Wednesday through Wednesday night. Guidance still differs in some of the details...but confidence of this system impacting our area has increased since guidance has come into better agreement. This system will be strong enough to have surface low pressure associated with it over the Ohio Valley. This will allow for the boundary just to our south and east to return north and west as a warm front. A southwest flow aloft will also usher in more moisture across our area...bringing more clouds along with the chance for showers later Wednesday and Wednesday night.
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&& .LONG TERM /THURSDAY THROUGH SUNDAY/...
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Uncertainty remains rather high with the evolution of upper low as it drifts southward and becomes cutoff from the main belt of westerlies Thursday and possibly into the weekend. While ensemble guidance appears to be in the early stages of converging on a solution that keeps the upper low to our southwest/west through at least Friday...there are still several members (especially of the GEFS) that quickly move it off to our north. Ultimately, spatiotemporal evolution of upper low will have a significant impact on sensible weather elements through much of the long term. This forecast leans heavily on a forecast blend, with greatest weight placed on the ECMWF (and EPS)...as it has exhibited the greatest run-to-run consistency. Thus, showers and possibly a few thunderstorms appear likely Thursday...with shower chances continuing through at least Saturday. Temperature forecast prone to large errors...as amount of insolation remains in question.
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&& .AVIATION /07Z MONDAY THROUGH FRIDAY/...
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High pressure will continue to move off to the east and a southeast flow will increase today with gusts around 15 to 20 knots this afternoon. Low clouds have developed across most areas early this morning due to the southeast flow. The low clouds will continue to expand north and east this morning and lower as well. Most terminals will have MVFR conditions along with patchy drizzle...but will continue with IFR conditions this morning for KCHO. Low clouds will likely hang around throughout the afternoon and possibly even through most of tonight. MVFR conditions are most likely...but cigs may be close to 3kft this afternoon into this evening. A cold front will pass through the terminals late this evening through the overnight. Showers are likely during this time with isolated thunderstorms possible. A northwest flow behind the boundary should clear things out overnight across the northwest terminals and early Tuesday morning farther south and east. Weak high pressure will build overhead for later Tuesday into early Wednesday. An upper-level low will impact the terminals later Wednesday through Wednesday night...bringing the chance for showers. Subvfr conditions are possible...especially Wednesday night. Sub-VFR conditions probable Thursday with showers and perhaps a few thunderstorms. Periods of Sub-VFR conditions could linger into Saturday as upper low remains near the area...though, uncertainty beyond Thursday remains high.
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&& .MARINE...
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A southeast flow has developed over the waters early this morning as high pressure moves off to the east. A cold front will approach the waters later today through this evening before passing through overnight into early Tuesday. The southeast flow will strengthen ahead of the boundary...and a small craft advisory is in effect for the waters this afternoon through tonight. Will have to monitor winds over the waters this morning. Gusts around 15 knots are expected and winds may even approach SCA levels across the Bay and lower Tidal Potomac River later this morning. Showers are expected late tonight into early Tuesday along with a possible thunderstorm. Weak high pressure will build over most of the waters later Tuesday into Wednesday...but clouds and showers may get hung up across the middle portion of the Bay Tuesday into Tuesday night since the boundary will stall just to our south and east. An upper-level low will drop through the Ohio Valley later Wednesday into Wednesday night. Surface low pressure will develop over the Ohio Valley as well...and the gradient between high pressure over New England and the lower pressures over the Ohio Valley will strengthen. A Small Craft Advisory may be needed for portions of the waters Wednesday night. Small craft advisory possible Thursday into Friday as coastal low possibly develops off the Mid-Atlantic coast.
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&& .TIDES/COASTAL FLOODING...
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Tidal anomalies have dropped closer to one-half foot above normal early this morning due to the light flow. However...southeast winds will increase today into tonight and tidal anomalies will increase as well. The next high tide cycle this afternoon will be the lower of the two...so confidence is too low at this time for an advisory. There is a better chance for minor flooding during the high tide cycle tonight into early Tuesday. Tidal Anomalies should drop later Tuesday behind the cold front.
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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 noon today to 6 AM EDT Tuesday for ANZ530>543.
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&& $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...MSE AVIATION...BJL/MSE MARINE...BJL/MSE TIDES/COASTAL FLOODING...BJL

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