Area Forecast Discussion
Issued by NWS Sterling, VA

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273 FXUS61 KLWX 181904 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 304 PM EDT Fri Aug 18 2017 .SYNOPSIS...
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A cold front will cross 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.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Latest surface analysis depicts the cold front entering western West Virginia and western Pennsylvania, heading southeast. Aloft, a broad trough covers the eastern and central US, with several embedded shortwaves. One of these is located over West Virginia at present and is heading east. Ahead of these sources of lift, we have a truly soupy air mass, with dew points in the mid 70s to around 80 widespread, and air temperatures near to just above 90. This is resulting in CAPEs estimated to be well in excess of 2000 J/kg. The associated shortwave is helping increase shear to around 30 knots, with areas near the Mason-Dixon expected to reach 35 knots. Suffice it to say, we have a pretty good setup (as August goes) for severe weather, and with the high PW`s, flash flooding. Best flash flood threat will depend on the front slowing and cells starting to train later on, but any storm can produce some extremely heavy rainfall rates given the high CAPE and high available moisture. Watches for severe and flash flooding extend west to near the Blue Ridge, but an isolated instance early west of there is not impossible. However, best odds are near the I-95 corridor on east to the bay as storms intensify and tap into the excessive CAPE and moisture. Timing looks primed with late aft/early evening for most of the region, and while CAPE should diminish this evening, cells may continue to regenerate well into the evening as the front slows, which is why the flash flood watch extends to 2 AM. Besides all of that, heat remains a concern until the storms reach the area, and the heat advisory remains in effect. It may be cancelled early as the storms cool the region down by early evening. Later tonight, front will slowly but surely clear us out storm- wise, but moisture may be slow to dwindle, with patchy fog possible. Lows will be a bit cooler than last night, with 60s and low 70s expected.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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Other than patchy fog, Saturday should start out tranquil with some sun. Humidity will be lower, though temperatures will only be a few degrees less than today. However, potent upper level trough will move into the region late in the day and at night, and it appears enough moisture will remain to result in isolated showers and t-storms late in the day into the evening. High pressure dominates Sunday for the most part, with temperatures expected to drop a bit further once again - the most comfortable day of the weekend. However, warm front will start poking back north late in the day and at night, and that may result in a return risk of showers in central VA.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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High pressure will move offshore Monday and settle into Tuesday, allowing for southerly flow to settle over our area. Moisture advection could enhance diurnally driven showers and thunderstorms Monday and Tuesday afternoon/evening. An upper trough and strong cold front will approach from the northwest on Tuesday night into Wednesday, bringing additional chance of showers and thunderstorms on Wednesday. The front should move across on Wednesday night with a high pressure returning Thursday into Friday. High temperatures Monday through Wednesday will be in the mid to upper 80s in Northern Maryland... to low 90s in Central Virginia at times... and 70s at higher elevations... highs in the 70s and 80s Thursday and Friday.
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&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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Primary TAF concern next few hours will be potential for gusty thunderstorms with brief IFR cigs and vis moving across the terminals. Tried to peg timing as best as possible in TAFs but still some uncertainty exists. Overnight, more rural spots, including MRB and CHO, could see patchy fog development. Otherwise, expecting VFR Saturday, though an isolated shower or t-storm is possible late. Mainly VFR again Sunday with high pressure dominant. VFR conditions expected Monday into Wednesday, with periods of sub-VFR conditions mainly Monday and Tuesday afternoon/evening and Wednesday due to possible showers and thunderstorms.
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&& .MARINE...
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SCA in effect through the evening for southerly channeling and approaching t-storms. Special Marine Warnings are likely over the next several hours as strong storms move in with approaching cold front. Then, expect northerly channeling overnight, so SCA continues for main channel until early morning. For balance of Saturday, should be sub SCA, but an isolated gusty t-storm is possible late in the day as an upper trough moves on through. Returning to tranquil conditions Sunday. Mainly dry conditions expected Monday into Wednesday, with periods of showers and thunderstorms possibly developing Monday and Tuesday afternoon/evening and Wednesday. Wind gusts are expected to stay below the small craft advisory threshold.
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&& .HYDROLOGY...
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Wet antecedant conditions combined with very high PW`s and potential for training storms have prompted us to issue a flash flood watch for eastern half to two thirds of CWA. Biggest risk is Maryland near bay and the urban centers. Storms may continue to train well into the evening, so have watch going until 2AM.
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&& .TIDES/COASTAL FLOODING...
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Southerly flow will continue to cause elevated water levels through tonight. CFA for St. Mary`s through tonight, and may need to add other sensitive sites of Annapolis and DC Southwest Waterfront for next cycle tonight. Will need to watch the less sensitve sites as well as Baltimore also touched in the last high tide cycle.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch until 2 AM EDT Saturday for DCZ001. Heat Advisory until 8 PM EDT this evening for DCZ001. MD...Flash Flood Watch until 2 AM EDT Saturday for MDZ004>006-011- 013-014-016>018-503>508. Heat Advisory until 8 PM EDT this evening for MDZ011-013-014- 016-504-506. Coastal Flood Advisory until 3 AM EDT Saturday for MDZ017. VA...Flash Flood Watch until 2 AM EDT Saturday for VAZ039-040- 050>057-501-502-505-506. Heat Advisory until 8 PM EDT this evening for VAZ052>057. WV...None. MARINE...Small Craft Advisory until 9 AM EDT Saturday for ANZ531>534- 537-539>541-543. Small Craft Advisory until 1 AM EDT Saturday for ANZ530-535- 536-538-542.
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&& $$ SYNOPSIS...RCM NEAR TERM...RCM SHORT TERM...RCM LONG TERM...IMR AVIATION...IMR/RCM MARINE...IMR/RCM HYDROLOGY...RCM TIDES/COASTAL FLOODING...RCM

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