Area Forecast Discussion
Issued by NWS Wakefield, VA

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044 FXUS61 KAKQ 280747 AFDAKQ Area Forecast Discussion National Weather Service Wakefield VA 347 AM EDT Fri Jul 28 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 THIS MORNING/... Expect just sctd showers and isolated tstms overnight into Fri morning, due to the combination of increasing low level moisture and shortwave energy dropping acrs the region. Will have Pops at generally 30-40%. Mild overnight with plenty of cloud cover and light SSW winds. Lows in the lower to mid 70s. && .SHORT TERM /6 AM THIS 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 /08Z 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 especially at SBY and expected MVFR or IFR cigs for much of the weekend. Improving weather for early next week. && .MARINE... Relatively quiet in the near term for the marine area, some enhancement of the winds with showers/tstms with brief gusts to 20-25 kt, but outside of any convection winds are mainly 15 kt or less. Similar conditions through today into this evening with S/SW winds mainly 10-15 kt outside of convection, slowly shifting to the W/NW late tonight/early Sat. Seas avg 2-4 ft and Bay waves up to 2 ft. The main story continues to be a strong cold frontal passage Sat/Sat night (by mid summer standards). Latest NAM/GFS/ECWMF/GEM have now trended much closer with respect to timing and the overall position of the primary features, lending to increasing confidence in the marine forecast. Headlines will be needed, but given that it will be a late 3rd/4th period (and beyond) event, its too early to issue any SCA headlines with the morning package. We could get close to Gales Sat night for the coastal waters, but confidence in reaching Gales is fairly low so have opted against a Gale watch for now (and if it were to occur this would be of the marginal Gale variety (i.e. sustained winds 25-30 kt with gusts to 35 kt). In any event, a decent surged noted to develop by Sat aftn from N to S across the waters as sfc low deepens along/just off the Delmarva coast, followed by strong CAA,especially Sat night. Seas build to 7-9 ft for coastal waters by Sat night/Sun, with 4-5 ft waves for the Bay. Conditions slowly improve Sun night/Monday, but ongoing SCA`s will persist due to lingering NE swell will likely result in marine issues through at least late Mon or Tue. && .TIDES/COASTAL FLOODING... Water levels in the upper Bay at Bishop`s Head will be within a few tenths of a foot of minor flood thresholds with the upcoming high tide early this morning. Otherwise water levels will slowly fall over the next 24 hrs with no additional statements needed. Some minor flooding may be possible Sunday/Mon along the coast and in the lower Bay with the strong N/NE flow behind the departing sfc low and associated cold front. && .AKQ WATCHES/WARNINGS/ADVISORIES... MD...Flash Flood Watch from 2 PM EDT this afternoon through Saturday afternoon for MDZ021>025. NC...None. VA...None. MARINE...None. && $$ SYNOPSIS...SAM NEAR TERM...SAM/TMG SHORT TERM...SAM LONG TERM...MPR AVIATION...JEF MARINE...LKB TIDES/COASTAL FLOODING...AKQ

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