Area Forecast Discussion
Issued by NWS Sterling, VA

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888 FXUS61 KLWX 161910 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 310 PM EDT Wed May 16 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 Thursday 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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1830Z regional radar mosaic shows showers increasing, likely due to daytime heating. Several cloud-to-ground lightning strikes over the past hour with the cluster of convection heading north through the Virginia Piedmont. Not anticipating thunder to get much past the tidal Potomac, which is essentially where the stalled frontal boundary exists. South of the front, MLCAPES up to 500 J/kg exist, with effective bulk shear values nearing 30kts per the SPC mesoanalysis. This combination will keep just a minimal severe thundestorm threat through the evening hours. Rainfall rates of these showers from dual pol estimates up to 2" per hour are occuring, but the cluster is moving along at a good clip. These showers and storms will be providing more moisture to existing soils that are already saturated, making the antecedent conditions more favorable for additional rainfall Thursday-early Saturday.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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12Z GFS has extended period of accumulating rainfall through the next 72 hours for most portions of the CWA, as an example, the BUFKIT sounding from the 12Z GFS has nearly 3.5" falling through Saturday morning. While it is difficult to pinpoint the exact forcing mechanism, certainly daytime heating will cause an increase in activity in the late afternoon through early evening hours both Thursday and Friday. Additionally, deep moisture in the entire vertical up to the tropopause becomes apparent Thursday afternoon and continues through Friday evening, along with a warm-cloud depth around 11kft. Based on this, WPC, 12Z GFS & ECMWF guidance, 12-hour flash flood guidance, and collaboration with our neighboring WFOs, we have issued a Flood Watch for Thursday afternoon through Friday evening. Even though rainfall rates generally will reach 0.25-0.50" per hour at times through this period, the chances for flooding will increase with time as the soils continue to saturate and run-off increases. That is, the rainfall rates we are experiencing today may not produce flooding, but the same rates come Friday will likely cause flooding. Flooding is even possible beyond Friday evening, but feel that their is not enough certainty in forecasts beyond Friday night to continue the watch past Friday evening at this time.
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&& .LONG TERM /SATURDAY THROUGH WEDNESDAY/...
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Conditions look to be not as wet on Sunday as the flow turns more southwesterly in advance of a cold front pushing down from the northwest. Precip should lower to only scattered showers and thunderstorms for Sun and Mon, and temps should rise back to the 80s during the afternoon. By mid next week that advancing cold front will stall over the region as high pressure passes east across Canada and New England. Most of our region is expected to be north of that stalling front, turning winds off the cooler Atlantic for mid week.
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&& .AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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Most terminals will have MVFR for CIGS through the remainder of the daylight hours today, with KBWI and KMTN already IFR. There will be a period of IFR and even LIFR CIGS tonight after midnight, continuing through daybreak Thursday. Conditions will improve a bit in the late morning, but will once again drop to IFR again Thursday afternoon. VFR conditions expected for most of Sunday and Monday with only scattered showers and thunderstorms to reduce visibility and cigs at times. Winds should be light southwest. By mid next week a cold front will advance and then stall over the region setting up easterly winds over our region, and potentially lower cigs and vsbys at times as cool moist air comes in from the Atlantic.
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&& .MARINE...
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Surface dewpoints nearing 70F will cause formation of fog over the coastal waters, reducing visibilities below 1SM at times tonight and early Thursday morning. No small craft threshold winds are expected throught this period. Other than thunderstorms and fog, particularly south of the Bay Bridge, no marine hazards are expected over the next several days.
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&& .HYDROLOGY...
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Our ongoing hydrologic event is in the process of taking a turn after the significant flash flooding from last night. Radar estimated 5 to 6.7 inches of rain in a swath from extreme southeastern Washington County up through the city of Frederick. The highest ground truth total we`ve had in through the mesonet was 6.56 inches just WSW of Frederick. That kind of extreme rainfall in a very short period of time is becoming less of a threat as we transition to just a continuous moisture plume from the Gulf and Atlantic. But with this continuous plume comes steady, occasionally heavy, rain that will add up more over time and could still cause significant flooding. 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 we are now issuing a Flood Watch from early Thursday 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 Thursday afternoon through Friday evening for DCZ001. MD...Flood Watch from Thursday afternoon through Friday evening for MDZ003>006-011-013-014-501>508. VA...Flood Watch from Thursday afternoon through Friday evening for VAZ026>031-053-054-505>507. WV...Flood Watch from Thursday afternoon through Friday evening for WVZ050>053-055-501>506. MARINE...None.
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&& $$ SYNOPSIS...CS NEAR TERM...Lee SHORT TERM...Lee LONG TERM...CS AVIATION...Lee/CS MARINE...Lee/CS HYDROLOGY...JE

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