Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 190727 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 327 AM EDT Tue Jul 19 2016 .SYNOPSIS... A cold front will stall out overhead before passing through the entire area tonight. High pressure will build overhead for Wednesday and Thursday. High pressure over the Atlantic will control the weather pattern for Friday through Sunday. A cold front may impact the area early next week. && .NEAR TERM /THROUGH TONIGHT/... Light winds and breaks in the cloud cover combined with the fact that a lot of areas saw rain on Monday has resulted in patchy fog. Fog may be locally dense early this morning...especially across central Virginia into the Shenandoah Valley. Do think that there is enough high and mid-level clouds early this morning to prevent widespread dense fog...but this will have to be monitored over the next few hours. Also...a few showers/isolated thunderstorms are expected along the cold front early this morning near the Allegheny Highlands. Isolated showers may make it into portions of the Shenandoah Valley...but instability is limited east of the Appalachians so most activity should dissipate. A weak cold front is currently located near the Mason-Dixon Line early this morning. A dewpoint gradient is noticed along the boundary with lower to middle 60s to the north and upper 60s and lower 70s south of the boundary. The cold front will drop south into the area today...but it is likely to get hung up across the Potomac Highlands into northern Virginia and portions of the Washington Metropolitan area. Little temperature difference will be associated with the front...but a dewpoint gradient is expected to develop. Therefore...areas across northern Maryland into northern Virginia and eastern West Virginia will turn out less humid today. However...higher dewpoints are expected to remain in place across central Virginia into southern Maryland and even close to Washington DC. Max temps will top off in the upper 80s to lower 90s across most locations and this will cause some instability to develop. The most instability is expected to be along and south of the boundary...with mixed layer CAPE around 2000 J/KG possible. The highest amounts of instability will be across central Virginia where dewpoints will remain in the upper 60s close to 70 degrees. Farther north...instability will be more limited. A few showers and thunderstorms are expected near and to the south of the boundary this afternoon into this evening. Coverage will remain isolated to scattered since the forcing mechanism will be relatively weak. An upper-level trough over New England will dip south into the Mid-Atlantic today and this will cause shear profiles to increase a bit. Marginal shear profiles and moderate to perhaps high instability will cause the potential for a couple thunderstorms to become severe...with locally damaging winds and large hail being the primary threats. The best chance for stronger storms will be across central Virginia perhaps into extreme southern Maryland where instability will be highest. The cold front will stall out just to the south of the area tonight and a northerly flow will allow for drier and more comfortable conditions overnight. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... High pressure will remain over the region for Wednesday through Wednesday night...bringing dry and seasonably warm conditions along with lower humidity. The high will shift offshore Thursday and a return flow will develop. Humidity will begin to increase during this time. A popup shower or thunderstorm cannot be ruled out over the Potomac Highlands into central Virginia during the afternoon and evening hours...especially over the higher terrain. Thursday night will turn out warm and humid as high pressure settles off to the south and east. && .LONG TERM /FRIDAY THROUGH MONDAY/... The return and amplification of heat and humidity will be the main story for the end of the week and into the weekend. Much of the time period will feature northwest flow aloft with warming temperatures and a gradual return in humidity. Core of large upper ridge will be centered over the central US with the Northeastern US/Mid Atlantic on the periphery with largely west/northwest flow. With this type of pattern would expect a risk of some showers/thunderstorms with daytime instability and potential MCS`s moving along the periphery of the ridge. Expecting widespread 90s for high temperatures Friday through Monday. Humidity should be initially tempered in the westerly flow, but as northwest flow weakens Sunday and possibly turns southwest ahead of an approaching front, will see humidity values rise. Thus the combination of heat/humidity will likely lead to heat index values in excess of 100F, and heat headlines may be necessary. The most oppressive days currently appear to be Saturday and Sunday. As frontal system approaches Monday, will see the chances for synoptically driven showers/thunderstorms increase. && .AVIATION /07Z TUESDAY THROUGH SATURDAY/... Patchy fog is expected early this morning. Vsbys should remain at VFR levels and perhaps MVFR levels for most of the terminals...but fog may be more dense across KCHO. MVFR to IFR vsbys are expected early this morning. Any Fog will lift by 12-13z. VFR condtions are expected most of the time today through Wednesday night. A cold front will slowly drop south through the area this afternoon into tonight. A few showers and thunderstorms are possible this afternoon and evening. Coverage will be isolated to widely scattered. The best chance for a thunderstorm will be across KCHO...and any thunderstorms that do develop may contain gusty winds or hail. High pressure will build overhead for Wednesday through Wednesday night. The high will move off the coast for Thursday and Thursday night...and a return southerly flow will develop. However...VFR conditions will likely persist. Generally VFR expected from Friday and through the weekend. A few showers/thunderstorms are possible which may bring a brief period of sub-IFR conditions. Patchy fog also possible during overnights, but coverage not likely to be widespread. && .MARINE... A cold front will slowly drop through the waters today into tonight. A few showers and thunderstorms will develop along and ahead of the boundary. Coverage of convection will be isolated to scattered...but thunderstorms will be capable of producing locally gusty winds. A pressure surge will develop behind the cold front late tonight into early Wednesday. Winds may approach SCA criteria for a brief period during this time. However...the pressure surge will be weak so confidence was too low for an SCA headline at this point. High pressure will remain over the waters Wednesday through Wednesday night before moving offshore Thursday. A return southerly flow will develop for Thursday into Thursday night. Mostly sub-SCA conditions expected for the end of the week and weekend, however brief intervals of gusts up to around 20 knots are possible. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...MM AVIATION...BJL/MM MARINE...BJL/MM is the U.S. government's official web portal to all federal, state and local government web resources and services.