Area Forecast Discussion
Issued by NWS Sterling, VA

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511 FXUS61 KLWX 270911 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 511 AM EDT Tue Sep 27 2016 .SYNOPSIS... A cold front will pass through early this morning before stalling just to our south and east later today. Weak high pressure will build overhead tonight. A strong upper-level low will impact the area later Wednesday through Thursday night...bringing the potential for heavy rain. The upper-level low will slowly move away from the area Friday through the weekend. && .NEAR TERM /THROUGH TONIGHT/... A cold front will slowly pass through the area early this morning and it is expected to stall out just to our south and east across the eastern shore of Maryland into central Virginia. A band of showers along and ahead of the front near the I-95 corridor will slowly slide south into southern Maryland by mid-morning. Patchy fog has also developed ahead of the boundary early this morning. Vsbys should improve behind the boundary as drier air filters in. Weak high pressure will build in behind the boundary for most areas today...bringing dry and seasonable conditions along with increasing sunshine. The one exception will be across southern Maryland where more clouds and showers will persist through much of the day...closer to the boundary. Max temps will range from the lower 70s in extreme southern Maryland due to more clouds to the mid and upper 70s across most other locations. Weak high pressure will remain overhead tonight while the boundary remains stalled just to our south and east. Most areas will remain dry and seasonably cool. Min temps will range from the lower and middle 40s in the the Potomac Highlands to near 60 in Washington and Baltimore as well as southern Maryland. The one exception will be across extreme southern Maryland into central Virginia. More clouds and even a few showers are possible closer to the boundary. Patchy fog is expected due to radiational cooling. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... A strong cutoff upper-level low will drop south from the Great Lakes into the Ohio Valley and Midwest Wednesday and the upper-level low will continue to drop south toward Kentucky and Tennessee Wednesday night through Thursday. This system will remain nearly stationary Thursday night. This system will be strong enough to cause surface low pressure to develop nearby over the Ohio Valley into the southeastern Conus. Latest 00z deterministic guidance has come into much better agreement and the 00z GEFS has also come to better agreement. This increases the confidence for heavy rainfall and potential flash flooding. For Wednesday: A southerly flow aloft will allow for warm and moist air to overrun the relatively cooler air in place. This will cause clouds to quickly increase along with an increasing chance for showers. The boundary that stalled to our south and east is also expected to move into the area. This means that warm and more humid air to the south and east will also move overhead. Therefore...a few thunderstorms are possible as well especially near and east of the Blue Ridge Mountains during the afternoon and evening. For Wednesday night through Thursday night: The upper-level low will continue to slowly drop to our south and west during this time. This will allow for subtropical air to move into the area. Overrunning and a strengthening low-level jet will be the primary lifting mechanisms for showers and possible thunderstorms. Locally heavy rain is likely as subtropical moisture gets drawn into the area. Also...with a south to southeast flow aloft there is the potential for training convection over the same areas. Several inches of rain are possible during this time. The exact location of the heaviest rain remains uncertain at this time...but as of now it appears that northern and central Maryland into northern Virginia toward the Washington and Baltimore metropolitan areas has the best chance for the heavier rainfall. Again...timing and exact of location of heaviest rainfall remains uncertain so please stay tuned to latest forecasts for updates. Despite the recent dry conditions...flash flooding is possible during this time for reasons mentioned above. && .LONG TERM /FRIDAY THROUGH MONDAY/... GEFS family continues to converge on ECMWF solution...with upper- level low to our west at 12z Friday. Low becomes nearly vertically stacked Friday into Saturday as it begins to weaken and slowly retreat to the north. While the heaviest rain should be winding down during this time...shower chances (possibly still locally heavy at times) continue until dry slot works into the area. When this occurs at any one location will partially depend on the longitude of upper level low as it retreats back northward (e.g., a further east/west solution would mean an earlier/later end to rainfall). Thus, locations across northeast Maryland have the best chance of rain lingering into Saturday. OOz ECMWF/GFS is further west/east and ends QPF Saturday/Friday. Improving conditions across all areas late Saturday into early next week with high pressure building into the area. Temperatures near seasonable normals. && .AVIATION /09Z TUESDAY THROUGH SATURDAY/... A cold front will slowly pass through the terminals early this morning. Patchy fog and areas of low clouds are expected ahead of the boundary. Showers across the eastern terminals will gradually shift south and east early this morning as well. IFR conditions are possible until the boundary passes through. However...confidence is low. The boundary should clear KMRB by 9z and the other terminals between 9 and 11z. VFR conditions are expected later this morning through this afternoon. High pressure will remain overhead tonight...but patchy fog is possible. An upper-level low will impact the terminals Wednesday through Thursday night. Increasing clouds along with possible showers are expected Wednesday. Widespread showers and possible thunderstorms are expected to impact the terminals Wednesday night through Thursday night. Exact details remain uncertain at this time...but locally heavy rainfall along with subvfr conditions are expected. Sub-VFR conditions remain possible Friday and perhaps into Saturday as as upper level disturbance brings showers to the region. Conditions becoming VFR from south-to-north by late Saturday and into Sunday as high pressure builds in. && .MARINE... A cold front will pass through the waters early this morning. Will continue with the Small Craft Advisory until 6 am. Winds should diminish behind the boundary due to a weakening gradient. High pressure will briefly build overhead later today into tonight. Low pressure will impact the waters later Wednesday through Thursday night. Widespread showers and isolated thunderstorms are expected during this time which will cause locally lower visibilities. A small craft advisory may be needed for portions of the waters Wednesday night through Thursday night. Potential for Small Craft Advisory continues Friday. Winds fall below SCA criteria sometime Saturday and remain below criteria into Sunday. && .TIDES/COASTAL FLOODING...
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Winds will turn to the northwest early this morning behind a cold front. This should cause tidal anomalies to decrease. An onshore flow will develop tonight into Wednesday and it will strengthen Wednesday night through late this week. Tidal anomalies will increase and minor flooding is possible.
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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 AM EDT early this morning for ANZ530>534-536>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...MSE/BJL

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