Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 160803 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 403 AM EDT Sun Jul 16 2017 .SYNOPSIS... Weak high pressure over the midwest will control the weather today. An upper level trough will affect the weather Monday and Tuesday. After another period of tranquil weather during the middle of the week, another cold front may approach the region by the end of the week. && .NEAR TERM /THROUGH TONIGHT/... Surface front is located across the Carolinas this morning, while high pressure is located over the Ohio Valley. The shortwave trough which will have some effect on our weather during the early part of the week is diving across Ontario. Satellite indicates there may be a few patches of fog in the western valleys, but it`s more pronounced west of the Appalachians where dew point depressions are smaller. Overall tranquil conditions are expected today as the weak high influences the weather: light winds, scattered cumulus, and near seasonable temperatures. Some instability will develop over the Appalachians as steep lapse rate plume spreads eastward, but soundings indicate a stout low level cap. Have limited slight chance POPs to the highest elevations where terrain circulations will at least have a chance to be a trigger. Return flow will commence tonight and start advecting in higher dew points. This could lead to more of a low cloud or fog threat, though specifics are not clear at this time. While steep lapse rates will still be found aloft, am not seeing much of a trigger to overcome stable low levels, so have tapered POPs back some. Several degrees will be added to the lows...which will be in the mid 60s to lower 70s. && .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... The upper level trough will be closed off north of Lake Ontario by Monday morning, with lower heights having expanding down the Appalachians. There are still a range of solutions as to how convection will unfold Monday, though the consensus is that the best chance will be in the mountains, where terrain circulations will help serve as a trigger since other low level features will be lacking. Better mid level forcing will be north of the region, though can`t rule out some small perturbations in the trough. Meanwhile the front to our south will be washing out, though some models hint that some remnant troughing or even weak surface lows could try to work north Monday or Tuesday. These features could provide a focus for convection as well. By Tuesday, the closed low will be filling and lifting to the east as broader troughing approaches from the west. With gradual height rises, would expect less thunderstorm coverage for Tuesday, with the trough axis moving to our east overnight and ending the precipitation threat. Monday will have the better chance of having a few strong storms when lapse rates will be steepest aloft, resulting in around 2000 J/kg of MUCAPE. The problem will be a stable layer between 5-10kft AGL, though if it can be broken, pulse storms could briefly produce hail and strong winds in an environment with around 20 kt of deep layer shear. Temperatures will remain near normal Monday, but rise a few degrees Tuesday as 850 mb heights begin to rise. Humidity will also return with dew points into the lower 70s. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... High pressure over the western Atlantic will overall dominate the weather Wednesday, and it should linger into Thursday. This will provide hot, increasingly humid weather, but overall, it should be dry, with only isolated thunderstorms possible. Some uncertainty regarding the end of the week exists with regard to a shortwave crossing the region, which looks likely to send a cold front from Canada southward into our area. Timing and push southward are both uncertain, but a greater chance of showers and thunderstorms exists by Friday, with an increased chance of cooler air moving in by Saturday. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... Weak high pressure will provide VFR conditions with light winds today. Return flow commences tonight and rising dew points could result in low clouds or fog. Model solutions are split at this time so will be conservative with 12Z TAFs. Upper level disturbance will approach Monday but thunderstorm coverage still doesn`t look especially widespread, with MRB, CHO, and IAD perhaps having slightly higher chances. Any stronger storms could contain gusty winds and hail though. After a lull overnight, some scattered storms may redevelop on Tuesday. VFR overall in the long term, but risk of showers and thunderstorms increases Friday. && .MARINE... High pressure will result in light winds today, becoming southerly by tonight. With southerly flow established Monday and Tuesday, will have to monitor for periods of channeling, though the overall wind flow in the atmosphere is forecast to be light, so am not seeing much potential for a SCA. Isolated to scattered thunderstorms will be possible Monday and Tuesday as well, primarily during the afternoon and evening. Sub SCA overall in the long term, though risk of thunderstorms will increase Friday. && .TIDES/COASTAL FLOODING... Southerly flow developing across the region as high pressure shifts offshore will start to build anomalies today, but since anomalies have fallen substantially since yesterday, the threat of any flooding today appears reduced. By Monday afternoon, minor flooding at high tide is possible at our most vulnerable sites (most likely Straits Point). This would likely continue into Tuesday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...RCM NEAR TERM...ADS SHORT TERM...ADS LONG TERM...RCM AVIATION...ADS/RCM MARINE...ADS/RCM TIDES/COASTAL FLOODING...RCM is the U.S. government's official web portal to all federal, state and local government web resources and services.