Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 180119 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 919 PM EDT Mon Jul 17 2017 .SYNOPSIS... An upper level trough will affect the weather today and Tuesday. After a period of hot but mostly dry weather during the middle of the week, another cold front may approach the region by the end of the week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Most of the convection has dissipated due to the loss of daytime heating. However...the remnants of an outflow boundary remain over northern and central Virginia...and latest mesoanalysis shows some instability along with a weak capping inversion. Isolated showers and thunderstorms are possible across these areas late this evening. A southerly flow will cause more warm and humid conditions overnight. A stray shower cannot be ruled out along a weak surface trough near and west of the Blue Ridge and Catoctin Mountains...but most areas will be dry. Low-level moisture will get trapped underneath the nocturnal inversion...resulting in areas of low clouds and fog. Fog may be locally dense...especially in areas that received heavier rainfall and also rural areas and sheltered valleys. Min temps will range from the lower to middle 60s in the Potomac Highlands to the middle 70s in downtown Washington and Baltimore. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY NIGHT/... Upper trough will remain overhead on Tuesday, and another round of scattered afternoon/evening showers/thunderstorms is anticipated. Could once again be an isolated severe and heavy rain risk with atmospheric conditions similar, although mid level lapse rates are a bit weaker. High temperatures in the upper 80s to around 90F. The upper trough will continue to slowly weaken and gradually pull away Tuesday night into Wednesday. So a relatively calm night expected again Tuesday night after any evening convection ends, with the potential for patchy fog. By Wednesday, there could still be an isolated shower/storm but coverage should be less than today and Tuesday. Temperatures rise a bit into Wednesday with highs from 90-95F. This combined with dew points in the upper 60s to around 70F will give heat indices 95-100F locations east of the Blue Ridge and 90-95F west. Warm and muggy conditions Wednesday night, with lows 68-78F. && .LONG TERM /THURSDAY THROUGH MONDAY/... The main story for the end of the week will be the heat. High temperatures Thursday and Friday will be in the mid to upper 90s with dew points in the lower 70s, heat advisories will be possible. Temperatures over the weekend may be more variable due to storms and a wavering front, but still above normal with high humidity. In terms of synoptic patterns, a mid/upper level trough will be just east of the Mid-Atlantic coast on Thursday. Forecast instability is meager Thursday afternoon, so the best chance of an isolated shower or storm will be over the mountains. The subtropical ridge then will be able to exert influence on the weather through the remainder of the period, though it will largely remain centered to our south and west. As such, a cold front will be able to work down from the north on Friday, but fail to make it completely through the forecast area. With flow a little north of west aloft and the front in the vicinity, thunderstorm chances will increase Friday, but especially Saturday into Sunday, with MCS activity (or remnants) possible. Model spread increases later Sunday into Monday with how to handle next piece of energy moving across Canada, but with potential for the front to still be in the area, forecast will have a chance of thunderstorms. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/... Areas of low clouds and fog are expected late tonight into Tuesday morning. IFR conditions are possible...especially with CIGS in the eastern terminals and possibly from VSBYS in the western terminals. Scattered showers/thunderstorms are expected Tuesday afternoon/evening which may bring brief gusty winds and heavy rain resulting in reductions to ceiling/visibility. Patchy low clouds and fog are possible overnight Tuesday into early Wednesday as well. A thunderstorm cannot be ruled out Wednesday afternoon and evening...but coverage should be isolated. It looks like any isolated thunderstorms will remain away from the terminals (in the mountains) on Thursday, though there could be more scattered coverage across the area on Friday. Otherwise VFR conditions are likely. && .MARINE... Southerly winds will channel up the lower and middle portion of the Maryland Chesapeake Bay tonight and the lower Tidal Potomac River. A Small Craft Advisory is in effect for these areas until 2 am. will be marginal with gusts around 18 knots. Otherwise...most of the time winds should remain below SCA criteria through Wednesday night. Main weather concern will be for thunderstorms, which could pose a risk for Tuesday afternoon and evening. Gusty winds are possible. Thunderstorms are possible Wednesday afternoon and evening...but coverage should be isolated. Light westerly winds on Thursday with high pressure in control. A cold front will approach from the north on Friday, but will likely not push completely through the area. While there are no clear opportunities for SCA conditions, scattered thunderstorms could affect the waters on Friday. && .TIDES/COASTAL FLOODING... South flow persists across the Chesapeake Bay region through midweek. Water levels at the preferred high tide (late afternoon/evening) will approach minor coastal flooding thresholds at particularly sensitive sites like Straits Point (St. George Island in St. Mars county), Annapolis and SW DC Waterfront today through Wednesday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 2 AM EDT Tuesday for ANZ532>534-537- 540-541-543. && $$ SYNOPSIS...MM NEAR TERM...BJL/MM SHORT TERM...MM LONG TERM...ADS AVIATION...BJL/ADS/MM MARINE...BJL/ADS/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.