Area Forecast Discussion
Issued by NWS Wakefield, VA

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784 FXUS61 KAKQ 220841 AFDAKQ Area Forecast Discussion National Weather Service Wakefield VA 441 AM EDT Tue Aug 22 2017 .SYNOPSIS... High pressure remains centered off the Mid Atlantic coast through tonight as a cold front begins to approach from the west. The front crosses the area late Wednesday afternoon through Wednesday night, with strong high pressure building north of the area from Thursday into the upcoming weekend. && .NEAR TERM /THROUGH TONIGHT/... Latest surface analysis centers ~1021mb high pressure from eastern NC to well off the coast, resulting in a light southerly flow over the local area. A few spots of interior VA and NE NC seeing patchy fog per latest obs, but the coverage is minimal compared to yesterday morning at this time. With a south wind of 3-5 mph most locations, do not expect dense fog other than patchy/shallow ground fog through 12Z. For today, increasing return SSW flow will prevail on the back side of departing high pressure offshore and ahead of an approaching cold front. A weak thermal/lee trough by this aftn will provide a bit more of a pressure gradient than the past few days, resulting in SSW winds of 10-15 mph this aftn. Highs today should rise about 2 to 5 degrees above Monday`s values, with low-mid 90s for most locations (and only a little cooler near the coast). With dew pts genly staying in the mid 70s SE to around 70 F NW, heat indices this aftn may approach 105 F for a few hrs, especially over NE NC and SE VA where dew pts over the past few weeks have genly verified higher than guidance. Given that it will be marginal, did not issue a heat advisory but will mention in the HWO. While the atmosphere will be quite moist with precipitable waters around 1.75 inches, warm air aloft, weak lapse rates, and a lack of appreciable forcing will limit the convective potential this afternoon. Will retain silent PoPs for most areas, did add a 20% over a few spots across interior NE NC. Warm/humid tonight with little chance for any rain under a SW wind of 10 mph. Lows mainly in the mid to upper 70s. Clouds increase across the north ahead of the approaching cold front, but will have a dry forecast through 6 am/Wed. && .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... Better forcing arrives along the front Wednesday as a lead wave drops over the Mid-Atlantic. The cold front progged to slowly push across the area Wednesday afternoon into Wednesday night. Latest models generally depict somewhat weaker forcing with the lead wave as the vort gets sheared out, and have dropped PoPs a little from previous forecast through 18Z. PoP`s increase more during the 18Z/Wed to 00Z/Thu period, with some sharpening of the upper trough looking to focus best lift over southern/SE zones. Will have high chc PoPs N/NW zones to likely over the S/SE late in the day. The air mass ahead of the front will become modestly unstable, with mixed- layer CAPE values progged around 1000-2000 J/kg. Deep layer flow also progged around 25-30 knots as mid level flow increases, resulting in better organization and overall storm strength. Mid level lapse rates also improve to around 6 to 6.5 C/km late. SPC maintains a marginal risk for severe weather for most of the CWA, with damaging winds and heavy rainfall the main threats. The NW, where PoPs are a little lower and where the lift is weaker is not included in the marginal risk at this time. Highs generally in the mid-upper 80`s north to around 90 F south. Strong Sfc cold front drops south across the area Wednesday night into Thursday morning. Modest ascent accompanies the front within the RRQ of a ~110kt jet over New England, along with PW values to around 2.0" (around +1 st dev). By Wed evening, have maintained highest (70%/Likely) POP across srn VA/NE NC, with slight to low end rain chances by Wed night NW of RIC, where drier air arrives first. PoPs across SE portions gradually diminish to high chance by daybreak Thu morning. Thunder will be maintained into Wednesday evening, with showers thereafter as stable NE flow prevails in the wake of the front. Drier air gradually filters in from the NW late Wednesday night into Thursday with partial clearing across the nrn tier counties. Lows lower 70s SE to mid-upper 60s NW. && .LONG TERM /THURSDAY THROUGH MONDAY/... The pattern will be characterized by the gradual development of a longwave upper level trough over the eastern United States for the late week period into next weekend. Meanwhile, broad 1024+ mb surface high pressure builds south over the interior northeast from Great Lakes Thursday and Friday, eventually setting up over New England and ridging down the east coast over the weekend into early next week. Resultant E-NE flow will result in markedly cooler and drier conditions over the region, with early morning lows in the 50s to lower 60s inland to mid-upper 60s along the coast. Maintained a dry forecast through the forecast period. Overall, high temperatures Thu-Mon will be below normal for late August, mainly in the upper 70s to low 80s. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... Some patchy fog/low stratus will be possible through 12Z this morning, but with a little more mixing than past few nights, it should be less widespread and of minimal impact at TAf sites. Have included SCT low clouds at most terminals, but no cigs and only MVFR vsbys at SBY/ECG. Otherwise, for today any low cloud/fog will have lifted by 13Z, with mainly dry conditions thereafter. SCT cumulus expected to develop for the aftn, along with a S to SW wind of 10-15 kt. Not much chance for any tstms today or this evening. A cold front approaches the region late tonight, but with a continued SW flow, expect VFR conditions tonight/early wed morning with fog/low stratus unlikely. Flight restrictions will be possible with the front later on Wednesday... mainly in the afternoon and through Wednesday night due to showers and thunderstorms. The front should exit the coast Thursday morning with lingering showers possible across far SE VA/NE NC. Conditions are anticipated to improve Thursday afternoon into Friday as the front shifts farther southeast and high pressure builds north of the region. Breezy NE winds expected near the coast Thursday and Friday. && .MARINE... Early this morning, sfc high pressure was cntrd off the Mid Atlc coast. Winds were SSW 5 to 15 kt over the waters with Waves 1 to 2 ft, and Seas 2 to 3 ft. The high will slide farther out to sea during today. A cold front will approach the waters from the NW tonight, then crosses the area Wed thru Wed night. High pressure will then gradually build down into the region from the NW Thu aftn into Sat. SSW winds will increase later today into tonight, as the front approaches the waters fm the NW. Have kept wind speeds at 15 kt for the Ches Bay tonight, and 15 to 20 kt for the coastal waters, with a few gusts up to 25 kt possible over the nrn two coastal zns. Waves will build to 2 to 3 ft in the Ches Bay, and Seas will build to 3 to 5 ft over the nrn coastal zns. Thus, have issued a SCA for the nrn two zns (650-652) fm late this evening into early Wed morning. Winds shift to the N/NE behind the front fm later Wed aftn into Thu morning, mainly 10 to 15 kt. Post-frontal, look for onshore (NE flow) for the remainder of the forecast period, as high pressure builds in fm the NW. Expect wind speeds at 10 to 15 kt (could be gusts to 20 kt) acrs most of the waters, with the highest winds over the SE VA/NE NC waters. Waves will be 1 to 3 ft, and Seas will be 2 to 4 ft. && .AKQ WATCHES/WARNINGS/ADVISORIES... MD...None. NC...None. VA...None. MARINE...Small Craft Advisory from 10 PM this evening to 6 AM EDT Wednesday for ANZ650-652. && $$ SYNOPSIS...LKB NEAR TERM...LKB SHORT TERM...LKB/SAM LONG TERM...MAM AVIATION...LKB MARINE...TMG

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