Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 170817 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 417 AM EDT Thu May 17 2018 .SYNOPSIS...
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A front will remain nearly stationary across the region through Saturday. Ample moisture, from a non-tropical low pressure system over the northeast Gulf of Mexico and Bermuda high pressure off of the Southeast U.S. Coast, will bring the potential for heavy rain and flooding this afternoon into Friday evening. As that stationary front slowly dissolves overhead early next week, a cold front will advance from the northwest before again stalling over our region during the middle of next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Persistent moderate rain has been occurring across northwest Virginia, eastern West Virginia Panhandle, and central Maryland during the past several hours. Frederick County Maryland has been receiving the most rainfall in this region. There are currently a couple of flood warnings across this region of the CWA. The culprit of the flooding has been and will continue to be a stationary boundary bisecting our CWA during the past few days. A Flood Watch remains in effect for the northern two- thirds of our CWA through Friday evening. Additional flood warnings are anticipated today and tonight as more rainfall is on the way along a stationary front and ahead of a non-tropical low pressure system. Actually, the moisture surging north during the next 24 hours will interact with the stationary front across our CWA to fuel showers and perhaps a thunderstorm or two. Additional rain amounts today and tonight will average one to two inches with isolated locations receiving four inches. Obviously, with an already saturated soil in most locations, especially in north-central CWA, more localized flooding would be imminent through tonight.
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&& .SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
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Through the period Friday into Friday night, the stationary boundary should wiggle a little farther north to allow a 700 mb low and a 500 mb low pushes into the Ohio and Tennessee Valleys from the Arklatex region. These areas of low pressure will add to the chance for more rain showers and a few thunderstorms Friday and Friday night. Thus, more rainfall could lead to more flooding. As for now and due to low confidence of coverage, we will not be extending the Flood Watch beyond Friday evening. We will have to see how the coverage and intensity of showers and thunderstorms evolves through tonight and Friday to determine additional flood watches. As for Saturday and Saturday night, we may be adding or slowly transitioning to more convective-based rainfall rather than stratiform-based rainfall to the big picture. Hence, more rainfall in the forecast, which could lead to more flood watches and warnings. There are coverage uncertainties for this period, as well.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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Drier conditions are expected on Sunday as a trough axis lifts north and east of our region, but some lingering light showers will be possible. The drier conditions will be short lived as a frontal boundary drops down from the north, increasing shower and thunderstorm activity Monday into Tuesday as the boundary slows and stalls across southern Virginia. Depending on where this boundary stalls will have implications on temperatures and precipitation chances through mid week. For now, will continue advertising chance POPs Mon-Wed. Model guidance is in agreement with keeping the boundary just south of our area, maintaining near normal temperatures with a north northeasterly wind flow off the Atlantic.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/...
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Terminals have IFR visibility and LIFR ceilings the rest of the overnight and possibly early today. Expect IFR conditions for the most part today into tonight due to moderate to at times heavy rainfall. IFR to MVFR conditions Friday through Saturday with improvement Saturday night. Mainly VFR conditions forecast Sunday and Monday with the only threat of sub VFR conditions being a passing shower over the terminals. Winds will be light and favor a southerly trajectory. A frontal boundary will drop southward through the terminals late Monday, before stalling south of the terminals through mid week. A moist easterly flow north of the aforementioned boundary will bring the potential for sub VFR VIS/CIGs through mid week.
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&& .MARINE...
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Below small craft advisory conditions the rest of the night through today and tonight. Patchy fog over the coastal waters could reduce visibilities below 1SM at times. Winds may increase near or above the SCA threshold for Friday, and expected to remain below criteria the rest of the period. Generally sub SCA conditions expected Sunday through Monday with winds remaining light. A frontal boundary will cross the waters late Monday which will help increase shower activity, resulting in the potential for periodic gusty winds.
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&& .HYDROLOGY...
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Ongoing Flood Warnings this morning for portions of northern Virginia, eastern panhandle of West Virginia, and northern Maryland. These areas will continue to see flooding of small creeks/streams and low lying areas this morning as moderate to heavy rain continues to fall. The Opequon Creek at Martinsburg has reached flood stage (10 feet) early this morning thanks to continuing rains across the area. The creek is forecast to continue to rise early this morning before subsiding below flood stage later today. Precipitable water is forecast to remain 1.6 inches or greater through Friday and we generally remain on the cool side of the boundary through Saturday, leading to an overrunning feed of that moisture into the cooler Atlantic moisture-laden airmass. It`s still too early to specify exactly where heavy rain axes will set up, but it`s reasonable to assume one or more will be over this service area sometime in the next 72 hours. An additional three to five inches of rain, with locally higher amounts, are expected through Saturday. Minor flooding is now expected on many of the larger streams, and a Flood Watch is in effect this morning for locations along and west of the Blue Ridge. Locations to the east of the Blue Ridge will see the watch go in to effect this afternoon through Friday evening to cover this potential as well as potential for flooding of smaller streams and other areas. Depending on plumes/axes of heavier rainfall, there is definitely potential for more significant flooding, especially during the Flood Watch period.
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...Flood Watch from 2 PM EDT this afternoon through Friday evening for DCZ001. MD...Flood Watch from 2 PM EDT this afternoon through Friday evening for MDZ005-006-011-013-014-501>508. Flood Watch through Friday evening for MDZ003-004. VA...Flood Watch from 2 PM EDT this afternoon through Friday evening for VAZ053-054-506. Flood Watch through Friday evening for VAZ026>031-505-507. WV...Flood Watch from 2 PM EDT this afternoon through Friday evening for WVZ503-504. Flood Watch through Friday evening for WVZ050>053-055-501-502- 505-506. MARINE...None.
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&& $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...BKF AVIATION...BKF/KLW MARINE...BKF/KLW HYDROLOGY...JE/BKF

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