Area Forecast Discussion
Issued by NWS Sterling, VA

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685 FXUS61 KLWX 180800 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 400 AM EDT Fri May 18 2018 .SYNOPSIS... 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 through Saturday morning. A cold front will advance from the northwest late Sunday, before becoming stationary over the mid-Atlantic at the beginning of next week. && .NEAR TERM /THROUGH TONIGHT/... Radar shows light to moderate steady rain across northern Maryland, eastern West Virginia Panhandle, and parts of northern Virginia. Rain rates will average one quarter to one half inch per hour. Periods of moderate to heavy rain and a few embedded thunderstorms are also pushing across southern Maryland close to where the stationary front crosses the Tidal Potomac and central Chesapeake Bay. Rain rates will average one half to one inch per hour. Flood Warnings are in effect for some areas over eastern West Virginia and eastern and southern Maryland. A Flood Watch remains in effect. As for today into tonight, a Flood Watch remains in effect through tonight. Periods of rain, heavy at times with a possible embedded thunderstorm or two, is expected today into tonight. Additional rain amounts will average 1 to 3 inches with higher amounts across southern Maryland. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Deep moisture will still be present from the surface to the tropopause Saturday and Saturday night. During this time we will see an increase in both the local area streams and mainstem rivers. Lingering showers will continue on Sunday, as cold front drops through the area from the northwest. High temperatures will be in the low 80`s. The dry pattern will not last too long, as a low pressure system moving across the northeast will drag a cold front across the region late Sunday into Sunday night. && .LONG TERM /MONDAY THROUGH THURSDAY/... 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. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... All terminals will encounter IFR to LIFR conditions the rest of the overnight and into today and tonight as more rain is on the way that will obviously keep ceilings low and visibility relatively low. We could see a little improvement in ceilings Saturday and Saturday night, but primarily only to MVFR for brief periods of time and mainly at MRB and CHO terminals. Clearing skies Sunday afternoon, with another approaching cold front late Sunday into Monday. Low ceilings will be possible during this period. A chance for some scattered thunderstorms on Monday as 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. && .MARINE... Increasing easterly flow today will bring small craft conditions, predominantly in the northern portion of the Chesapeake Bay, but extending to all waters midday today. Areas of fog will continue giving visibilities at or below one mile at times, particularly tonight. A southerly flow will develop Saturday and Saturday night ahead of an approaching cold front. Flow should be generally light with some gusts Saturday afternoon due to diurnal heating. Winds approaching SCA criteria will be possible on Sunday afternoon as a cold front is exiting the region. Calmer winds expected Monday, but the potential exists for some thunderstorms. 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. && .HYDROLOGY... Conditions continue to worsen as this slow-evolving hydro event continues. A pretty widespread three inches or more of rain has fallen across the northern half of the forecast area already, with amounts that high more spotty to the south. (Parts of Culpeper and Fauquier Counties haven`t even seen an inch so far.) The evolution is so far going along similar to model projections, with light to moderate rain across the north, and more showery and heavy rain developing this morning to the south. This activity will pivot northward, and it appears will be followed by multiple additional rounds of precipitation. Confidence has increased in the rainfall prolonging into Friday night, and accordingly the Flood Watch was extended through that time. Also, the area has been expanded to include more counties. We still expect another 1 to 3 inches across most areas with 3 to 5 inches in southern Maryland and the Virginia Piedmont between now and Saturday. Streams (including the mainstem Potomac) are already elevated or in flood, so flood potential is high. Forecasts now call for moderate flooding on the mainstem rivers downstream from Point of Rocks and minor flooding at several tributary locations. River Flood Warnings will be in effect for these locations based on a high confidence of at least minor flooding. It should be noted that depending on where heavy rain axes set up during the course of the event, there could be even more significant flooding than we are forecasting and interests near any river should heed warnings and keep themselves and their belongings away from the rivers. && .TIDES/COASTAL FLOODING... An onshore flow will increase today and this will cause anomalies to continue to increase. However, confidence is not high enough for any advisories across the Bay and lower Tidal Potomac River. The persistent onshore flow will continue through Saturday and this will increase the threat of minor tidal flooding for the more sensitive locations starting Saturday morning. Additionally, freshwater inundation will pose a threat to the DC/Alexandria tidal zone, and a Coastal Flood Watch is in effect from Friday through Sunday, with a focus really starting Friday afternoon/evening, for moderate flooding or greater. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flood Watch through Saturday morning for DCZ001. Coastal Flood Watch from Saturday morning through Sunday afternoon for DCZ001. Coastal Flood Advisory until 6 AM EDT Saturday for DCZ001. MD...Flood Watch through Saturday morning for MDZ003>006-011-013- 014-016>018-501>508. VA...Flood Watch through Saturday morning for VAZ025>031-036>040- 050>057-501>508. Coastal Flood Watch from Saturday morning through Sunday afternoon for VAZ054. Coastal Flood Advisory until 6 AM EDT Saturday for VAZ054. WV...Flood Watch through Saturday morning for WVZ050>053-055- 501>506. MARINE...Small Craft Advisory until 3 AM EDT Saturday for ANZ530>533- 538>541. Small Craft Advisory from 2 PM this afternoon to 8 PM EDT this evening for ANZ534>537-542-543. && $$ SYNOPSIS...KLW NEAR TERM...KLW SHORT TERM...KLW LONG TERM...BKF/KLW AVIATION...BKF/KLW MARINE...BKF/KLW HYDROLOGY...JE/BJL TIDES/COASTAL FLOODING...JE/BJL

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