Area Forecast Discussion
Issued by NWS Wakefield, VA

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000 FXUS61 KAKQ 201817 AFDAKQ Area Forecast Discussion National Weather Service Wakefield VA 217 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 /UNTIL 6 PM THIS EVENING/... Current analysis indicates a weak sfc trough of low pressure located from Long Island NY...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. Made a few minor tweaks to the forecast for current temperatures and dew pts mainly for the eastern shore where climatologically temps and dew pts tend to surge through the morning hrs then steady off w/ the seabreeze in the aftn. Could touch a 105 F heat index ~18Z for the interior MD eastern shore at places like SBY due to combination of highs in the mid 90s and sfc dew pts that will not mix out as much as areas W of the Bay, but have decided to hold off on a issuing a heat advisory as it will be marginal and tomorrow will be worse. Otherwise, with rising heights aloft all areas and increasing 850 mb temperatures, expect highs today to avg in the mid to upper 90s W of the Bay and 90 to 95 F closer to the coast (locally upper 80s immediate coast). Expect enough mixing of dew pts in the aftn for the WNW 2/3 of the CWA (into the 60s to keep heat indices below 105 F on avg. While dew pts will tend to remain in the 70-75 F range across the ern 1/3, temperatures will be a little cooler there, so resultant heat indices look to peak in the upper 90s to 104 F at reputable sites. Will mention heat indices of 100 to 104 in the HWO. Except for isolated sea breeze boundary induced showers or tstms this aftn, expect mainly dry conditions and will keep PoPs of 14% or less except near the coast (15-20% there). && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/... Model consensus has the upper ridge axis centered from the TN Valley to the Carolinas Fri into early Sat, with an upper trough over the Great Lakes and NE states on Sat. The core of the highest 850 temperatures on Fri (22-24C) will be over the NW portion of the CWA, shifting to the southern zones on Sat (22-23C). Fri may be the hottest day across the N, with high temps ranging fm the mid 90s to around 100 degrees. A heat advisory will be likely on Fri, as heat index values will be 103 to 108 degrees over most of the region. Sat may be slightly cooler and a bit more unstable, as heights aloft drop, but should once again see highs of 95 to 100 F. Dew pts may be a little higher, so heat indices should be at least as high if not higher. At least a heat advisory will likely be needed, possibly even excessive heat warning (especially for SE VA/NE NC). This is certainly not unprecedented heat for this part of the country during this time of year (see climate section), but it will be very hot nonetheless, and potentially dangerous for those not prepared for it. There will be isolated storms across the NW or nrn counties Fri aftn into Fri night, and isolated to sctd storms across the nrn half of the region Sat aftn into Sat night. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... An upper level trough tracks across the Great Lakes and Northeast through the extended period, pushing a weakening cold front into the region during the Monday night to Tuesday morning time frame. Another hot day is forecast Sunday, but modest height falls and lower thickness values will knock a few degrees off. Highs in the mid to upper 90`s with heat indices back in the 105 to 108 range. Pressure falls ahead of the approaching from will sharpen the thermal gradient over the Piedmont, but a general lack of appreciable forcing, warm temperatures aloft, and westerly flow will result in only a slight chance POP over the Piedmont into central Virginia. Better chances (albeit 30-40% at this time) for diurnally driven showers/thunderstorms expected Monday as deep layer moisture increases ahead of the front and the upper trough tracks into the Northeast. Few degrees colder again, with highs generally in the mid 90`s and heat indices 100 to 105. Chance POPs continue into Monday evening as general model consensus drops the front over the local area Monday night. Strongest height falls progged north of the region, which result in cyclogenesis off the Northeast coast. The front likely stalls along the coast Tuesday, with chance POPs continuing across the southeast forecast area. Cooler, with highs in the upper 80`s to low 90`s. Thereafter, medium range guidance is at odds with handling the upper trough over southeast Canada. Prefer the more progressive ECMWF solution, which builds high pressure across the Northeast Tuesday, resulting in onshore flow and unsettled conditions over the local area. However, confidence is not high at this time, so have trended toward model consensus. Highs Wednesday in the mid to upper 80`s. Overnight lows through the period will generally be in the 70`s. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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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.
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&& .MARINE... Latest obs/buoy reports reflect W-SW flow ~10-15 kt across the waters this morning. Early morning surface analysis depicts high pressure over the western Atlantic with a surface trough persisting inland over the piedmont of central Virginia. Little change in the surface pattern expected today and through the weekend as high pressure remains entrenched over the western Atlantic and the storm track remains well north of the region. Thus, given a weak pressure gradient, resultant south to southwest wind will persist at or below 15 knots is expected through Monday. Winds increase slightly late each day into the overnight hours, and becoming a bit gusty at times in the aftn/early evening w/developing seabreeze. Seas generally 2-3 feet and waves 1-2 feet. NWPS showing SE Swell developing Friday, which could result in a moderate rip current risk for Fri/Sat. Forecast will remain low for today. A weakening cold front approaches the region late Monday, dropping across the region Monday night into Tuesday morning. Flow backs to the northwest to north Tuesday at or below 10 knots due to a lack of cold advection. High pressure builds across the Northeast into Wednesday, resulting in onshore flow Wednesday. && .CLIMATE... 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 Thursday 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 && .AKQ WATCHES/WARNINGS/ADVISORIES... MD...None. NC...None. VA...None. MARINE...None. && $$ SYNOPSIS...TMG NEAR TERM...LKB/TMG SHORT TERM...LKB/TMG LONG TERM...SAM AVIATION...TMG/JAO MARINE...MAM CLIMATE...

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