Area Forecast Discussion
Issued by NWS Wakefield, VA

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000 FXUS61 KAKQ 272359 AFDAKQ Area Forecast Discussion National Weather Service Wakefield VA 759 PM EDT Thu Jul 27 2017 .SYNOPSIS... Low pressure affects the region Friday through Saturday...with a cold front pushing across the region Friday night. Low pressure lingers near the Mid Atlantic Coast Sunday through at least Monday night. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Latest regional radar mosaic depicts a line of showers and thunderstorms pushing into the Piedmont, just on the edge of the forecast area. This area of convection is associated with a shortwave pushing into the Mid-Atlantic region. Air mass remains warm and unstable over the local area, with mixed-layer CAPE values of 1000-1500 j/kg. Latest hi-res guidance continues to indicate the line of showers/thunderstorms will diminish in coverage/dissipate as it pushes across the Piedmont into central Virginia as the instability wanes, the convection outruns the deepest moisture coming over the mountains, and downslope flow impacts the storms. Have lowered PoPs central Virginia toward the coast this evening so 30-50%, keeping likely PoPs confined to the Piedmont. Some re-development is possible late this evening through the overnight period near the coast, as outflows interact with increasing moisture and the shortwave drops over the region. Based on the warm/moist air mass, and energy embedded in the northwest flow, will keep chance PoPs through the overnight period inland. Mild tonight with plenty of cloud cover and a light southwest breeze. Lows in the low to mid 70`s. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH SUNDAY/... Potent shortwave/compact upper low currently over southwest Ontario, digs across the Great Lakes region into the Ohio Valley Friday. An associated area of low pressure deepens and tracks from the Ohio Valley into Northern Virginia Friday afternoon. The trailing cold front is expected to sweep across the central Appalachians late Friday, with a pre-frontal trough over the Piedmont Friday afternoon. The air mass over the local area will be warm and quite moist with dewpoints in the low to mid 70`s and precipitable waters in excess of 2 inches (+2 standard deviations). While mid-level lapse rates again will be rather modest, theta-e advection and temperatures in the 80`s result in mixed layer CAPE values of 1000-15000 j/kg. Mid-level flow increases ahead of the shortwave, resulting in deep layer shear of 25 to 30 knots. Expect scattered to numerous showers and thunderstorms to spread over the northern local area Friday morning through the afternoon as the low tracks into the region. Some mid-level drying expected Friday morning into the afternoon across central and southern Virginia, resulting in some breaks in the clouds and plenty of opportunity to warm into at least the mid 80`s. Latest short term guidance indicates scattered to numerous showers developing ahead of the pre- frontal trough mid afternoon, and quickly pushing eastward. Another area of scattered to numerous showers and thunderstorms possible along the cold front as it pushes through the region Friday evening. Given the increasing mid level flow, broken/bowing convective segments are possible with damaging wind gusts possible. SPC maintains a slight risk for severe weather across the region. Meanwhile, an extended period of showers/thunderstorms expected across the northern local area as the low stalls and deepens. There still remains some spatial differences in the short term guidance with respect to the surface low, but general consensus is across the Delmarva. The Canadian remains a southern outlier. Given the pivot point and strong 850mb u components over the Delmarva, expect periods of heavy rainfall Friday afternoon through Friday night over the Maryland Eastern Shore. Given 3 hour flash flood guidance of 2-3 inches and inter-office coordination, have hoisted a flash flood watch for the Maryland Eastern Shore for Friday afternoon through Saturday afternoon. Flooding could be exacerbated during periods of high tide. Mid-level dry air entrainment across inland Virginia late Friday night will result in drying conditions inland. However, likely PoPs persist along the coast. The surface low slides just offshore Saturday as the upper low digs over the Mid-Atlantic. Deepest moisture pushes offshore Saturday morning, but the combination of strong upper level dynamics and plenty of remnant moisture (precipitable waters of 1.5 to 1.75 inches) will result in ongoing showers and embedded thunder through the day Saturday. Best coverage again from the Northern Neck to the Maryland Eastern Shore. Below normal temperatures forecast due to clouds, showers, and a northwest winds of 10-15 mph. Highs only in the upper 70`s to mid 80`s. Model consensus centers the upper low along the Mid-Atlantic coast early Sunday morning, with ongoing chances for showers along the coast, especially the Maryland Eastern Shore. Cool Saturday night, with lows in the low to mid 60`s. Sunday temperature forecast will be a challenge given the uncertainty in the guidance. Strong cold advection expected as 850mb temps drop to around 10-12C. Add in considerable cloudiness, and daytime temperatures will be near 2 standard deviations below normal. Highs forecast now in the mid to upper 70`s. If the GFS/MEX guidance pans out, a few record low max temperatures may be in jeopardy. North to northeast winds could gust to 25-30 mph near the coast Sunday afternoon. Chance PoPs linger along the coast. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... Low confidence forecast early next week as the models continue to offer up different solutions on just how slow and far off the coast the cut off low tracks. Favoring a ECMWF track ivof the Gulf Stream, combined with lingering upper level energy, decided to hold onto chc pops generally along and east of I95 Sun night/Mon as the low slowly lifts NE along the coast. Rather breezy over eastern sections with continued marine issues early next week. Otw, high pressure over the mts keeps the rest of the fa dry. Lows Sun nite from the upr 50s NW to near 70 SE. Highs Mon in the upr 70s-lwr 80s except mid 70s at the beaches. Lows in the 60s to nr 70 SErn beach areas. High pressure builds across the region for the mid week period. Highs Tue in the mid-upr 80s except upr 70s at the beaches. Lows in the mid 60s-lwr 70s. Highs Wed/Thus 85-90. Moisture returns from the west Thurs afternoon. Added slght chc diurnal pops to the Piedmont. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... Rather unsettled weather can be expected over the next 24-48 hours as a series of disturbances move across the region. Showers and thunderstorms currently over the mountains will move into the piedmont and coastal plain this evening and tonight. A few light showers may affect RIC early while there will be a chance for some patchy heavier rain during the overnight. Expect only a slight chance for a thunderstorm at any of the TAF sites tonight. MVFR cigs will be possible late tonight into early Fri, then a chance for MVFR/IFR cigs and rain as a series of weak disturbances move through during the day. A stronger disturbance moves into the area from the NW by Friday afternoon/evening, potentially bringing strong to severe storms into the region. This next disturbance/low pressure system will slow down just off the coast for the weekend. This will allow for steady rain especailly at SBY and expected MVFR or IFR cigs for much of the weekend. Improving weather for early next week. && .MARINE... Some southerly channeling noted across the bay tonight but will keep wind speeds below SCA levels for now. SW winds aob 15 KTS continue through Fri nite ahead of a cold front. A fairly strong cold front (by mid summer standards) will cross the area early Sat shifting the winds to the N by the afternoon then NE at nite. A decent surged noted behind this feature which will result in SCA level winds late Sat/Sat nite. Meanwhile, another area of low prs progged to develop along this boundary but uncertainity remains as to this features speed and track. Will hold off on any headline attm given a late 4th period starting time. Although some of the data across the northern coastal waters Sun and Sun nite is rather impressive for the summer, have capped gusts below gale force for now. Plenty of time to adjust (higher or lower) if needed. Seas build into the SCA range as well, but capped heights across the northern coastal waters at 8 feet. Contitions slowly improve by mid week but lingering NE swell will likely result in marine issues through mid week. && .TIDES/COASTAL FLOODING... Water levels in the upper Bay (mainly eastern side adjacent to the MD eastern shore) approached minor flooding thresholds early this morning and will continue the coastal flood statement mainly for Cambridge through 10 am. An additional statement may be needed for tonight. && .AKQ WATCHES/WARNINGS/ADVISORIES... MD...Flash Flood Watch from Friday afternoon through Saturday afternoon for MDZ021>025. NC...None. VA...None. MARINE...None. && $$ SYNOPSIS...BMD/SAM NEAR TERM...SAM SHORT TERM...SAM LONG TERM...MPR AVIATION...MRD/JEF MARINE...MPR TIDES/COASTAL FLOODING...

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