Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 171906 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 306 PM EDT Wed Aug 17 2016 .SYNOPSIS...
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A weak cold front will gradually move south of the area through Friday before dissipating. Another cold front will cross the area early next week. High pressure returns for the middle of next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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An active weather evening is ahead with multiple threats. Severe Thunderstorm Watch already out west of the Blue Ridge until 8 PM. Due to relatively little activity expected to the east this afternoon, SPC has suggested doing a second watch if warranted farther east at a later time. A Flash Flood Watch has also been issued through 3 AM for a stripe across the central CWA where best QPF signal overlaps with pockets of lower FFG, including the Baltimore-Washington metro. Heavy rain will be possible elsewhere, but have held off on a larger watch due to coverage uncertainty and/or higher FFG. The severe weather threat will mainly come in the form of damaging downburst winds. Multicell clusters will be possible given moderate shear, but given forcing from approaching wave, an MCS could also be possible. High PWAT values, moisture transport into the stalled boundary, and favorable upper jet positioning suggest activity could last well into the evening, if not after midnight near the Bay. While some guidance suggests one main push of storms, mergers and regenerative processes are not out of the question, as demonstrated by a recent HRRR run. If such processes occur, localized point amounts of 3-4 inches are within the realm of possibility. Activity should diminish (at least to showers) by late tonight as the main wave/vort max moves to the east.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY/...
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Deep moisture will shunt south of most of the area Thursday with WNW flow establishing. This will lead to slightly drier and cooler air. However, the washed out boundary will not be far away with potential for weak disturbances in the flow, so the best chance of a shower and storm will be across the south. Had to introduce POPs a little to the north on Friday, as there are some indications a wave could pass along the front to the south and result in a chance of showers and storms by afternoon or evening.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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High pressure will build to the north Friday night. Drier and slightly cooler, yet seasonable, air will funnel southward into our region. We can`t rule out a scattered shower Friday night due to a few week disturbances that may try to sneak under the building high pressure. Most of the activity will be across the Virginia Piedmont or lower Southern Maryland. The area of high pressure will weaken some over our region Saturday and Saturday night, allowing for additional weak disturbances to move in from the west and bring us a chance for additional showers or perhaps a thunderstorm. A moderate cold front is expected to move slowly eastward across the Great Lakes region Sunday. This front could spawn a few thunderstorms, mainly in the afternoon and along and west of the Blue Ridge Mountains. Showers and thunderstorms could linger Sunday night and Monday as the front moves toward the Chesapeake Bay and possible stalls just east of the bay. High pressure will build in behind the front Monday night and linger through Wednesday. Drier air and slightly cooler temperatures are expected.
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&& .AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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Main threat period to the terminals will be between 5 PM and midnight. There will likely be a two hour period or so when thunderstorms are rather widespread (except maybe CHO) and restrictions will be likely in heavy rain and gusty winds. This window will be narrowed down with amendments. With the rain, some fog or low clouds aren`t out of the question late tonight, but have not placed in the TAFs yet due to uncertainty. A stray storm may cross near CHO both Thursday and Friday, but otherwise VFR should prevail. VFR conditions Friday night through Saturday night. Winds light and variable.
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&& .MARINE...
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W/NW flow this afternoon 10-15 kt. Fairly widespread thunderstorms will impact the waters this evening (possibly slightly earlier near DC) with a strong wind threat. Special Marine Warnings are likely. W to NW flow will continue through Friday, generally 10 kt or less. While a stray storm can`t be ruled out on southern waters, coverage will be much less than today. No marine hazards expected Friday night through Saturday night.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flash Flood Watch through late tonight for DCZ001. MD...Flash Flood Watch through late tonight for MDZ011-013-014- 503>506. VA...Flash Flood Watch through late tonight for VAZ026>031-038>040- 050>054-501-502-505>507. WV...None. MARINE...None.
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&& $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...KLW AVIATION...ADS/KLW MARINE...ADS/KLW

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