Area Forecast Discussion
Issued by NWS Wakefield, VA

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000 FXUS61 KAKQ 270752 AFDAKQ Area Forecast Discussion National Weather Service Wakefield VA 352 AM EDT Thu Jul 27 2017 .SYNOPSIS... High pressure slides off the Northeast coast overnight. Low pressure affects the region Friday through Friday night, with a cold front pushing across the region Friday night. High pressure builds into the region through early next week. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... High pressure cntrd right along the srn New England coast will slide ewd overnight, and should maintain dry wx acrs the region. SE winds will become S, resulting in warmer dewpoints. Moisture return and modest isentropic upglide over the Piedmont could result in an isolated shower twd morning, but best coverage expected to remain west of the local area. Otherwise, the sky will average mostly clear to partly cloudy, with some patchy stratus/ground fog possible late. Lows will range fm the mid 60s to around 70. && .SHORT TERM /6 AM THIS MORNING THROUGH SATURDAY/... Increasing confidence for widespread beneficial rainfall through the short term, especially Thursday night through Friday night. Initial shortwave drops over the Ohio Valley Thursday and into the Mid-Atlantic region late Thursday into Thursday evening in west to northwest flow aloft. Pressure falls ahead of a frontal boundary over the Ohio Valley will induce a lee/thermal boundary over the Piedmont Thursday afternoon. Modest mid-level lapse rates and dewpoints in the mid to upper 60`s result in only marginal instability, with mixed layer CAPE values progged around 500-1000 J/kg. Moisture returns in earnest, with guidance indicating precipitable water values increasing to around 2 inches by Thursday afternoon. The result will be isolated to scattered afternoon showers and thunderstorms over the Piedmont. Highs Thursday generally in the mid to upper 80`s. Attention turns upstream late Thursday for convection developing over the Ohio and Tennessee Valleys. Guidance indicates this activity should organize ahead of the boundary and cross the central Appalachians into the Mid-Atlantic region Thursday evening. Energy in the northwest flow aloft in tandem with increasing moisture and elevated instability (negative showalter values along with an elevated mixed layer) will aid the showers/thunderstorms dropping over the higher elevation into the local area Thursday evening through Thursday night. Strong winds are possible along the leading edge of the convection due to deep layer shear around 25-30 knots. SPC has downgraded the slight risk to a marginal risk. There still remains some uncertainty in the guidance, especially in the placement of the upstream low pressure system, so will continue to monitor future model trends for severe weather risk Thursday evening. Mild, with lows in the 70`s under a mostly cloudy to cloudy sky. Potent shortwave/compact upper low digs over the Ohio Valley into the northern Mid-Atlantic region Friday into Friday night, and then drops over the region Saturday before pushing offshore Saturday night. At the surface, low pressure lifts over the Kentucky Valley into the northern Mid-Atlantic region Friday. Height falls will result in modest deepening of the surface low over the northern Mid-Atlantic region Friday night, before the low pushes offshore Saturday. A trailing cold front pushes across the region Saturday night. There remains a great deal of uncertainty with the overall evolution of convection Friday, not only due to model uncertainty but due to the potential of convection Thursday evening. A fair amount of clouds are expected Friday, which along with modest mid-level lapse rates will limit overall instability. If any dry air wraps into the system Friday morning, we could see better instability. However, given the warm/moist air mass and strong dynamics, scattered to numerous showers and thunderstorms are expected Friday afternoon and evening. POPs ramp up to categorical late Friday into Friday night inland with the arrival of the strongest height falls. Increasing mid-level flow will result in deep layer shear around 30 knots, which will result in better organization and stronger storms. Gusty winds and heavy rainfall remain the main threat. SPC maintains a slight chance of severe weather across the region. There is also a slight risk for excessive rainfall across the northeast local area Friday, near the strongest onshore 850mb anomalies. Highs Friday generally in the low to mid 80`s. Front pushes south of the region Saturday as the surface low lifts along the Northeast coast. Strong height falls over the region in tandem with 1000-500mb relative humidity values around 80-90 percent will result in afternoon showers. Instability wanes behind the front, but perhaps a few rumbles of thunder are possible near the coast. Cooler, with a north wind of 10-15 mph (gusts 25 mph Eastern Shore). Highs generally in the low to mid 80`s. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... Low pressure pushes farther NE and away from the area Saturday night with the trailing front stalling ivof the Gulf Stream early next week. Lingering moisture keeps chc pops going along the coast Sat eve. Otw, pt cldy with lows from near 60 NW to around 70 SE as high pressure approaches the mts. 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 AKQ 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 ivof 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 /08Z 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 would keep strong winds 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) look to approach minor flooding thresholds early this morning and will continue the coastal flood statement, should be able to drop this prior to 7 am). && .AKQ WATCHES/WARNINGS/ADVISORIES... MD...None. NC...None. VA...None. MARINE...None. && $$ SYNOPSIS...SAM NEAR TERM...SAM/TMG SHORT TERM...SAM LONG TERM...MPR AVIATION...BMD MARINE...LKB/MPR TIDES/COASTAL FLOODING...AKQ

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