Area Forecast Discussion
Issued by NWS Wakefield, VA

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649 FXUS61 KAKQ 202028 AFDAKQ Area Forecast Discussion National Weather Service Wakefield VA 428 PM EDT Thu Jul 20 2017 .SYNOPSIS... High pressure builds toward the Mid Atlantic today into the weekend, bringing increasingly hot and humid conditions to the area. The hottest weather can be expected from Friday through Sunday. && .NEAR TERM /THROUGH FRIDAY/...
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Current analysis indicates a weak sfc/lee trough of low pressure located from NJ...SSW to south central VA and central NC. The trough of low pressure aloft is now situated south of the CWA and centered along the SC/GA coast. Skies avg partly to mostly sunny across the area this aftn with temperatures generally ranging from 90 to 95 F along and near the coast with mid to upper 90s over metro RIC. Dew pts have again mixed out enough over the hottest locales such that heat indices this aftn are genly only around 100 F or slightly higher overall. Seeing a few isolated showers/tstms across mainly the far N and will keep 20% PoPs there through 7pm, otherwise mainly dry overnight (earlier convection over OH/IN has weakened and seems unlikely to hold together as a weak wave aloft crosses N of the area overnight in the WNW aloft. Warmer than last night with lows staying 70-75 F or higher tonight. Model consensus has the upper ridge axis centered from the TN Valley to NC on Fri into early Sat, with an upper trough over the eastern Great Lakes and NE states. The core of the highest 850 temperatures on Fri (21-22C) will be over much of the local area of fri and with dew pts expected to be higher (only mixing out to around 70 F inland and to the lower-mid 70s near the coast). Expect heat indices to be more solidly into advisory criteria (105 to 109 F) and have therefore issued a heat advisory for most of the CWA (left coastal Worcester/NC Outer Banks out for now with cooler temperatures as well as much of the far SW where it will be slightly drier). Actual highs will be around 100 F metro Richmond to the mid/upper 90s elsewhere (locally cooler/lower 90s immediate coast). As far as precip chances, airmass appears too stable overall for much in the way of pop-up storms during the day, but with WNW flow aloft and some weak shortwave energy moving into the area by late aftn, will have ~20% PoPs by mid/late aftn over the NW 1/2 of the CWA. Enough shear moves into the far NW for a marginal risk for severe with wind being the main threat is any type of convective complex can hold together upon crossing the Mtns.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
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Ramped PoPs up a bit more for Fri night/especially N/NE zones closest to upper level shortwave energy tracking through the WNW flow aloft. Very warm/humid Fri night with lows mainly 75 to 80 F. Slightly cooler most areas for Sat due to potential for more clouds and as core of 850 mb heat shifts a bit S. However, it will be even more humid so expect at least southern portions of the CWA to need a heat advisory even with highs on avg only in the mid 90s. As airmass destabilizes more by Sat aftn/early evening, and as heights aloft drop, convection developing from the NW will be more likely to hold together for at least 30-50% PoPs across most areas (20% NE NC) from late Sat aftn through Sat night. Far N/NE portions of the CWA are in a slight risk where higher shear will reside with marginal risk for the remainder of the CWA. Again wind will be the primary threat. Lows mainly in the mid to upper 70s Sat night and continued hot an humid with additional heat headlines likely needed for some of the region on Sunday. Highs mainly in the mid to upper 90s Sunday.
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&& .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
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Long term period will feature near normal temps on avg and occasional chances for rain. In general expect lwr temps than this weekend as the broad upr-level ridge across the eastern CONUS breaks down in response to an approaching upr-level trough. The trough and associated low-level front will bring an increasing chance for rain Mon, but will cap PoPs at 40% for now due to uncertainties regarding timing and available moisture. Further model divergence occurs Tue and Wed, with the 12Z ECMWF keeping the low-level boundary in the vicinity and the 12Z GFS pushing things offshore, so will maintain low end PoPs (20-40%) through midweek, with the best chance over far SE areas. As for temps, after one more hot day Mon with highs up to the mid 90s, expect mid/upr 80s for Tue through Thu.
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&& .AVIATION /20Z THURSDAY THROUGH TUESDAY/... VFR conditions will persist today into Fri morning. Some scattered CU between 4-5kft this afternoon then clear this evening with a few high clouds. Any fog tonight will be this and patchy at best and left out of forecast for now. S-SW winds below 10 kts. OUTLOOK...Mainly VFR conditions expected Fri and Sat. But, isolated storms Fri aftn into Fri night acrs NW or nrn counties, and isolated to sctd storms Sat aftn into Sat night could produce MVFR conditions. && .MARINE...
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Extended period of sub-SCA conditions continues this week into the weekend. Pattern is stagnant with Bermuda high pres centered well offshore and typical summertime late day thermal troughs with S/SW winds averaging 5-10 kt, occasionally up to 15 kt especially tonight due to a slight increase in the pressure gradient. Waves over the Bay 1-2 ft with seas over coastal wtrs 2-3 ft. The next (weakening) cold front approaches the region late Mon, dropping into the area Mon night/Tue. Mainly sub-SCA conditions expected to continue however with weak CAA behind the front, and just some 4-5 ft seas psbl out 20 nm. Flow becomes N/NEly then Tue through Thu as the frnt slides offshore and dissipates.
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&& .CLIMATE...
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Heatwave is expected to develop, mainly Friday through Sunday. The 2nd half of July is climatologically the hottest few weeks of the year so we still may not set any daily records at our main climate sites. For reference, record highs today through Sunday are listed below: * Date: Today(7/20) Fri(7/21) Sat(7/22) Sun(7/23) * RIC: 103/1930 104/1930 103/1952 103/1952 * ORF: 102/1942 101/1926 102/2011 103/2011 * SBY: 104/1930 106/1930 104/1930 103/2011 * ECG: 104/1942 102/1987 104/1952 104/1952
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&& .AKQ WATCHES/WARNINGS/ADVISORIES... MD...Heat Advisory from 10 AM to 8 PM EDT Friday for MDZ021>024. NC...Heat Advisory from 10 AM to 8 PM EDT Friday for NCZ012>017- 030>032. VA...Heat Advisory from 10 AM to 8 PM EDT Friday for VAZ048-061-062- 064-068-069-075>078-080>090-092-093-095>100-509>525. MARINE...None. && $$ SYNOPSIS...LKB/TMG NEAR TERM...LKB SHORT TERM...LKB/TMG LONG TERM...MAS AVIATION...TMG/JAO MARINE...MAS/MAM CLIMATE...AKQ

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