Area Forecast Discussion
Issued by NWS Sterling, VA

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777 FXUS61 KLWX 241350 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 950 AM EDT Sun Sep 24 2017 .SYNOPSIS... High pressure will remain in place through the first part of the week, then slowly weaken its grip over the area by the midweek as Hurricane Maria moves northward through the Western Atlantic. A cold front will move through late in the week, bringing more seasonable temperatures. && .NEAR TERM /THROUGH TONIGHT/...
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High pressure over the Great Lakes this morning. 925 mb temps and low- level thickness values are both forecast to be higher than yesterday. Still anticipate it to be just a touch warmer. No changes made to the going forecast. Tonight, low-level moisture will start to creep up as winds turn from northerly to northeast/easterly. This should keep lows in the 60s in most areas, with the urban areas staying above 70.
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&& .SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/... High pressure remains in control Monday, but weakens slightly, along with a continued increase in low-level moisture. This should hold high temps down a degree or two lower than today. Still remaining dry. By Monday night, Hurricane Maria will be somewhere southeast of the North Carolina outer banks. Right now it appears the storm will be well southeast of us, with little if any impacts. But it`s still a few days away, so consult the latest advisories from the National Hurricane Center regarding Maria`s expected track. Depending on the storm`s proximity to us, and ability to generate a large rain shield, a stray shower or thunderstorm is possible east of the Blue Ridge late Monday night through Tuesday or Tuesday night. Otherwise, it remains unseasonably warm and humid. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/... The start of the long term period will continue to be dominated by attention to Hurricane Maria. At the start of the period, many models plot the hurricane as hovering just off the Outer Banks of North Carolina. This would likely bring some increased (but not particularly strong) winds to our southernmost zones, perhaps some gusts in the 30 mph range in southern MD and along the bay shore, along with a few showers. Impacts further north and west would likely be less. As we will remain in a warm air mass, temperatures will remain above normal, with 80s appearing likely, unless there is more cloud cover and precip that currently anticipated. Maria appears to stall due to the erosion of the ridge to its north, which would allow westerlies in association with an approaching shortwave to influence the storm. The timing of this will be critical, as a late arrival could allow the storm to continue northwestward. While most guidance does not support this occurring, everyone should continue to monitor the progress of the storm over the next few days. Refer to the latest statements from the National Hurricane Center for up-to-date information and forecasts on Maria. By Thursday, the approaching shortwave and attendant surface cold front should turn the hurricane eastward and it should begin to increase speed away from the coast. This same system will put an end to our late September warm spell, with a much more seasonable air mass arriving behind it for Friday. While the front will bring a fairly noticeable air mass change, limited dynamics and moisture mean that any rain with this system will likely be spotty and light. By Saturday, uncertainty increases. While some models show high pressure overhead, the EC has continued to bring a sharp shortwave southeast from Canada, which then tries to induce cyclogenesis in our vicinity. Each subsequent run has had varying locations and strengths with this, so confidence on it occurring is low and confidence on it affecting our weather significantly is lower. && .AVIATION /14Z SUNDAY THROUGH THURSDAY/...
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VFR with light winds expected through the valid TAF period. Only potential exception would be MVFR vsby due to fog primarily at CHO (perhaps at MRB also?) tonight. Clouds increase Monday night and Tuesday out ahead of Hurricane Maria, with a potential isolated shower or thunderstorm at BWI/IAD/DCA, but exact details and extent are track-dependent and will become clearer in the days ahead. Attention will be on Hurricane Maria Wednesday into Thursday. While sub VFR conditions are not likely, a gusty northwest breeze of 20-25 knots is possible, as are a few showers. A further deviation northwest in the track of Maria, though not likely, could bring worse conditions. Refer to latest NHC statements for up-to-date information on Maria.
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&& .MARINE... Light winds continue over the waters through Monday night. Perhaps a stray shower or thunderstorm on Tuesday as winds begin to increase due to enhanced gradient between high pressure and Hurricane Maria. How strong those wind gusts get depends on the ultimate track of the storm, but the most likely outcome yields Small Craft Advisory level gusts starting Tuesday afternoon. SCA winds looking likely across the waters on Wednesday and Thursday as Hurricane Maria approaches and we get enveloped by its expanding wind field. A few showers will also be possible. If the storm`s track deviates further northwest, more significant impacts are possible (though not likely). Refer to latest NHC statements for up-to-date information on Maria. && .TIDES/COASTAL FLOODING...
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Water levels running about a foot above astronomical normals. Annapolis scraped by just under minor flood threshold. SW DC likely to peak right around the minor threshold. Have an Advisory for DC til 2pm. The afternoon/evening cycle is the higher of the two. Confidence exists for an Advisory for St Marys. (For that matter, confidence exists for St Marys through Monday, and have made that extension.) Elsewhere, it will be close. Would rather have another run of model guidance and have the latest anomaly trends before making decisions. Those will come this afternoon. Given light winds, things should remain status quo through Monday. As Hurricane Maria moves northward in the Atlantic, the increasing winds should start piling water up in the lower Bay, which will gradually seep northward, similar to what happened when Jose passed last week. Tidal anomalies could approach two feet by midweek; ironically though, the astronomical tide becomes less favorable, so it could be a net result of status quo even then.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Coastal Flood Advisory until 2 PM EDT this afternoon for DCZ001. MD...Coastal Flood Advisory until 10 AM EDT Monday for MDZ017. VA...None. WV...None. MARINE...None.
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&& $$ SYNOPSIS...JE NEAR TERM...JE/HTS SHORT TERM...JE LONG TERM...RCM AVIATION...JE/RCM MARINE...JE/RCM TIDES/COASTAL FLOODING...HTS/JE

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