Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 181020 AAA AFDLWX Area Forecast Discussion...UPDATED National Weather Service Baltimore MD/Washington DC 620 AM EDT Fri Aug 18 2017 .SYNOPSIS... A cold front will approach from the west today before crossing the region tonight. Surface high pressure will build in Saturday, but an upper level disturbance will cross the region Saturday night. A warm front will advance back north across the region early next week, followed by a strong cold front in the middle of the week. && .NEAR TERM /THROUGH TONIGHT/... Latest analysis shows high pressure over the Atlantic while a cold front moves into the Ohio Valley. A southerly flow between these systems has allowed for warm and quite humid conditions. Dewpoints in the mid 70s for many areas will make it feel uncomfortably humid early this morning. The high moisture will cause areas of low clouds and fog early this morning underneath the nocturnal inversion. Also cannot rule out a shower or thunderstorm due to elevated instability...but there is little in the way of forcing so most areas should remain dry through at least mid-morning. The cold front will approach our area from the west later today while a surface trough develops overhead...most likely between the Appalachian Mountains and Blue Ridge Mountains. A southerly flow ahead of the cold front will continue to usher in hot and very humid conditions. Dewpoints are likely to be in the upper 60s and lower 70s this afternoon west of the Blue Ridge Mountains with mid to upper 70s most likely east of the Blue Ridge Mountains. Low-level temps are progged to be about a degree celsius higher compared to yesterday. Therefore...max temps are forecast to be a couple degrees hotter for most locations. This will cause max temps to top off in the mid to upper 80s for valley locations west of the Blue Ridge MOuntains with lower 90s near the Washington and Baltimore Metropolitan areas into central Virginia and southern/central Maryland. The combination of heat and very high humidity will cause heat indices to top off near 105 degrees near the urban Corridor from Baltimore through Washington DC toward Fredericksburg. A Heat Advisory has been issued for these areas. The high heat and humidity will cause moderate to high amounts of instability to develop this afternoon. Showers and thunderstorms are likely to develop along and ahead of the cold front...especially this afternoon and evening. Shear profiles will strengthen a bit as an upper-level trough digs over the Great Lakes. The strengthening shear profiles along with the moderate to high instability suggests that some storms may become severe...with damaging winds and large hail being the primary threats. An isolate tornado cannot be ruled out...especially toward the waters where surface winds may be backed a bit. Confidence is low since the magnitude of low- level shear profiles should be weak. PWATS are progged to be near or even above 2 inches...especially east of the Blue Ridge Mountains where the higher dewpoints are. Therefore...thunderstorms will contain torrential downpours and isolated flash flooding is a concern. Did consider a Flash Flood Watch with the update this morning. There is a chance for the outflow boundary to become parallel to the storm motion and this could enhance the threat for flash flooding. The farther east...the better the chance. Latest hires guidance (HRRR and NCAR ensembles) suggest that the highest rainfall amounts will be near or even just east of the Bay. This combined with the fact that storms should be moving due to the increasing shear are reasons why we held off on a watch with this cycle. will have to be monitored closely throughout the day. The best chance for storms will be during the middle afternoon hours west of the Blue Ridge Mountains...late this afternoon into early this evening for the Washington and Baltimore Metropolitan areas...and this evening across southern Maryland. Drier air should move in behind the cold front tonight...but winds will be light. Therefore...patchy fog is possible overnight. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Drier and less humid conditions are expected behind the front Saturday...but an upper-level disturbance will pass through late in the afternoon into the evening. An isolated shower/thunderstorm cannot be ruled out. However...much of the day Saturday will turn out dry with sunshine. High pressure will build overhead overnight Saturday through Sunday...bringing dry conditions along with lower humidity. The high will begin to shift offshore Sunday night and a return flow will develop. && .LONG TERM /MONDAY THROUGH THURSDAY/... Ridge of surface high pressure will slide offshore on Monday allowing for the development of return southerly flow and increasing low level moisture. Thus, we`ll some increase in chances for isolated-scattered afternoon/evening showers/thunderstorms. In addition, warmth/humidity will also trend back upward, with highs back to near or above 90F and dew points in the 70s for both Monday and Tuesday. A potent upper trough and cold front will then move through the Great Lakes on Tuesday and across the Mid-Atlantic and Northeast on Wednesday, with additional chances for showers/thunderstorms. Cooler and drier high pressure will follow the front for later next week. && .AVIATION /10Z FRIDAY THROUGH TUESDAY/... Low-level moisture is progged to get trapped underneath the subsidence inversion early this morning. Areas of low clouds and fog are expected. Did allow for IFR Cigs across the eastern Terminals and MVFR to IFR vsbys across the western terminals. The best chance for IFR conditions will be between about 09z and 13z. Cigs/Vsbys will improve later this morning. However...showers and thunderstorms are likely later this afternoon into this evening. Some thunderstorms will produce gusty winds and torrential rainfall that can briefly reduce cigs/vsbys to IFR or below. Drier air will move in behind a cold front tonight...but winds may become light and this will allow for patchy fog to develop toward morning. VFR conditions are expected later Saturday through Sunday night for most of the time. Cannot rule out an isolated shower/thunderstorm late Saturday afternoon and evening...but most areas should be dry. Mainly VFR is expected for Monday and Tuesday, although some brief reductions are possible in isolated to scattered showers/thunderstorms each afternoon/evening. There is also the possibility for patchy late night/early morning low clouds/fog. && .MARINE... Southerly winds continue to channel up the middle portion of the Bay and lower Tidal Potomac River early this morning. A Small Craft Advisory remains in effect. Will continue with the SCA for later this morning into this afternoon across the middle portion of the Bay and lower Tidal Potomac River...but there may be a lull in the gusts during this time. Winds should pick up ahead of a cold front late this afternoon. The cold front will pass through tonight and an SCA is in effect for most of the Bay and lower Tidal Potomac overnight into early Saturday for northwest winds. will be marginal. Showers and thunderstorms are likely ahead of the cold front...especially late this afternoon and evening. Thunderstorms will be capable of producing gusty winds. An isolated waterspout or thunderstorm with large hail cannot be ruled out as well. Special Marine Warnings may be needed. High pressure will build toward the waters later Saturday through Sunday night. upper-level disturbance will pass through the waters Saturday evening. An isolated shower/thunderstorm cannot be ruled out...but most areas will be dry. Sub-SCA conditions likely to continue Monday, although southerly return flow will be on the increase, and SCA conditions then become possible Monday night and Tuesday. Additionally, a few afternoon/evening showers/thunderstorms are possible both Monday and Tuesday. && .TIDES/COASTAL FLOODING...
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Southerly flow will continue to cause elevated water levels through today. A second round of minor flooding is possible with tonight`s high tide cycle at our three more sensitive sites of Straits Point, Annapolis, and DC Southwest Waterfront.
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DC...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for DCZ001. MD...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for MDZ011-013-014-016-504-506. VA...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for VAZ052>057. WV...None. MARINE...Small Craft Advisory from 1 AM to 9 AM EDT Saturday for ANZ531- 532-539-540. Small Craft Advisory until 9 AM EDT Saturday for ANZ533-534- 537-541-543.
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&& $$ SYNOPSIS...BJL NEAR TERM...BJL SHORT TERM...BJL LONG TERM...MM AVIATION...BJL/MM MARINE...BJL/MM TIDES/COASTAL FLOODING...MM is the U.S. government's official web portal to all federal, state and local government web resources and services.