Area Forecast Discussion
Issued by NWS Wakefield, VA

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000 FXUS61 KAKQ 271614 AFDAKQ Area Forecast Discussion National Weather Service Wakefield VA 1214 PM EDT Thu Jul 27 2017 .SYNOPSIS... High pressure remains off the Northeast Coast today as a cold front tracks into the Ohio Valley. 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 /THROUGH TONIGHT/...
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Latest water vapor imagery and RAOB plots depict a series of small amplitude shortwaves over the Midwest into the Ohio Valley. At the surface, high pressure has slid offshore with a stationary boundary aligned from the Southeast northward into the Ohio Valley. A cold front is also located from the Midwest into the Great Lakes. Upstream convection appears to be intensifying along the boundary over eastern Kentucky and West Virginia per cooling cloud tops. This convection is associated with the Ohio Valley shortwave and located along a leading edge of deep moisture. Per latest hi-res guidance, this activity will continue to push eastward into a favorable air mass over the central Appalachians through mid afternoon. Over the local area, the atmosphere is expected to become increasingly moist as precipitable waters climb to 1.75 to 2 inches by mid/late afternoon. Return flow also pushes dewpoints into the low 70`s, which combined with warm afternoon temperatures will help overcome modest mid level lapse rates for mixed layer CAPE values of 1000-1500 J/kg. Pressure falls ahead of the cold front will result in a sharp lee/thermal trough over the Piedmont. Hi-res guidance is struggling to handle the upstream convection as it comes across the mountains and drops into the Piedmont. The main limitation for the local area will be westerly flow from 1-3km and modest mid level lapse rates. However, given the amount of instability and deep layer shear values of 25-30 knots, anticipate the air mass over the local area will be able to support additional development ahead of the convection by 3-5 pm over the Piedmont, nearest the surface trough and approaching shortwave energy. Thereafter, guidance continues to struggle with just how far east to push the convection through this evening. The NAM and associated hi-res guidance keep the precip confined to the Piedmont, but the more course resolution guidance pushes the showers/thunderstorms across the region through this evening. Given the air mass and shearing wave, have trended toward the latter solutions, with pushing scattered to numerous showers and thunderstorms across the local area late today through this evening. Given the deep layer shear, downdraft CAPE, and negative showalter values, some stronger storms with strong wind gusts are possible. SPC maintains a marginal risk for severe weather. Locally heavy rainfall is also possible. Showers/thunderstorms diminish in coverage late tonight toward the coast as instability wanes and the shortwave pushes offshore. Warm and more humid today with highs in the upper 80`s to low 90`s. Heat indices warm into the mid 90`s. Mild tonight with plenty of cloud cover and a light southwest breeze. Lows in the low to mid 70`s.
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Low pressure develops along this boundary on Friday as the cold front pushes into far nw counties late aftn/early evening. Widespread thunderstorms should be anticipated Friday night as the front finally moves through the region, and especially as an upper trough digs over the Ohio Valley as it intensifies. This will enhance instability/lift and some storms could become severe. Pwats increase to around 2.00 inches, and MBE vectors in Bufkit confirm slow storm motion. This will create the potential for very heavy rainfall and localized minor flooding. Therefore the primary impacts with thunderstorms Fri/Fri night will be very heavy rainfall, locally strong to severe wind gusts, and localized minor flooding. Models are just now keying in on the upper trough becoming a more organized and intensifying low, which brings caveats to the overall forecast. If the trough digs deeper into the Ohio Valley, this could slow the ewd progression of the cold front into the Mid Atlantic Region, and/or it could focus heavier precipitation along nrn periphery of sfc low/deformation axis farther south within the forecast area instead of keeping the bulk of heavy rains north of the area. Due to timing and placement inconsistencies/uncertainty between model solutions, have opted to hold off on issuing a flood watch but will continue to mention potential within HWO. The cold front is expected to stall over far SE VA/NE NC on Saturday before shifting southeast of the area late Saturday night. Sfc low lingers near the Mid Atlantic Coast through Saturday night with the upper trough sitting overhead. This will keep showers/storms focused along the coast with possible wraparound moisture moving into the area from the north on Saturday. Precip chances expected to wane Saturday night as the upper low shifts ewd. Once again, primary threats from any thunderstorms will be moderate to heavy rainfall and locally strong wind gusts. Widespread precipitation, cloud cover, and cooler Canadian air with the upper trough will keep temperatures 5-10 degrees below normal Fri/Sat. Highs in the low-mid 80s Fri...lower 80s Sat. Low temperatures gradually cool off with each passing night. Warm and muggy Thu night with lows in the low-mid 70s. Lows Fri night lower 70s (upper 60s possible far NW). Lows Sat night lower 60s NW to mid-upper 60s SE.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/... Models have a shortwave riding ENE along the boundary, but the latest data suggests the high pressure over the mountains suppresses the bulk of the moisture SE of the Wakefield FA. This low is progged to deepen out over the Gulf Stream by Tue, which will bear watching this time of the year. Kept the forecast dry through the period except for some slight chc pops invof the Albemarle Sound Mon night and Tue where a limited amount of moisture spreads north. Temps below normal to start with readings returning to near normal by mid week. Highs 80-85 Sun, low-mid 80s Mon, mid-upr 80s Tue and upr 80s-lwr 90s Wed. Lows in the 60s Mon/Tue, mid 60s-lwr 70s Wed. && .AVIATION /16Z THURSDAY THROUGH MONDAY/... A leeside trough develops over the region this afternoon as a cold front crosses the Midwest into the Ohio Valley today. Shortwave energy/moisture interacting with the lee trough will result in the development of thunderstorms primarily this evening into the overnight hours. Main impacts will be heavy rainfall, locally strong wind gusts, and brief periods of CIG/VIS reductions to MVFR/IFR. With storms not expected to impact TAF sites until closer to 28/0000Z, have kept thunder and CIG/VIS reductions out of 06Z TAF issuance and will likely do the same for the 12Z issuance as well. The cold front slowly crosses the Mid Atlantic Region on Friday with more widespread thunderstorms anticipated. Very heavy rainfall, locally strong wind gusts, and rapid CIG/VIS reductions to IFR (possibly LIFR) will be the primary impacts. Some storms could become severe with gusts in excess of 50kt due to additional instability/lift from an upper trough digging over the Ohio Valley as it intensifies. The cold front is expected to stall over far SE VA/NE NC on Saturday before shifting southeast of the area late Saturday night. This will keep showers/storms focused along the coast with possible wraparound moisture moving into the area from the north. Once again, primary threats from any thunderstorms will be moderate to heavy rainfall and locally strong wind gusts. with rainfall intensity waning, any brief CIG/VIS reductions will likely be MVFR. Upper trough sits over the Mid Atlantic Region Sunday through Monday...exiting the coast late Monday night/Tuesday morning. Periods of clouds and showers/storms should be anticipated. && .MARINE... No headlines in the short term, winds are southerly and 10-15 kt on avg early this morning with seas 2-3 ft and waves around 2 ft in the Bay. S/SW flow around 10 kt through early this aftn will back slightly in direction to S/SE with speeds increasing to 15 kt later this aftn and tonight. Probably have a few hrs where occasional gusts to 20 kt develop, but appears too marginal to raise headlines for the Bay. Waves avg 2-3 ft in the Bay with 3-4 ft seas for coastal waters. Similar conditions Fri, except winds will tend to be a few Kt less. A fairly strong cold front (by mid summer standards) will affect the region over the weekend and at some pt SCA headlines will likely be needed. The timing remains rather uncertain however, as the models are quite different with respect to the evolution of sfc low pressure along the Delmarva and slowly translating offshore Sun/Mon. Overall, the models are a little slower at the arrival of the stronger NW winds Sat and the bulk of the SCA conditions could hold off until late Sat night or Sunday as winds shift to the N. Latest ECMWF/GEM would keep strong winds of 20-25 kt going through Mon. Have blended the solutions for now, but went higher than Superblend since it has too much averaging and never really brings winds much higher than 15 kt at any given time. Have seas building to 5-6 ft and Bay waves to 3-4 ft Sat night through Sun night. && .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...None. NC...None. VA...None. MARINE...None. && $$ SYNOPSIS...BMD NEAR TERM...SAM SHORT TERM...BMD LONG TERM...MPR AVIATION...BMD MARINE...LKB/MPR TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.