Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 251631 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 1230 PM EDT MON JUL 25 2016 .Synopsis... Strong high pressure aloft will extend across the region through much of the work week, bringing continued hot and humid conditions to central NC. The chances for afternoon storms will increase by mid week. && .Near Term /through Tonight/... As of 930 AM Monday... ...Dangerously hot conditions continue today across much of central NC... Little change needed to existing forecast. Hot/humid weather will persist, with warm and dry mid levels resulting in low CAPE this afternoon despite highs in the mid-upper 90s, and very weak deep layer shear as deep high pressure ridging stays parked overhead. Latest CAM runs including the 3km NAMRR, SPC SSEO, HRRRX, and WRF- NMM/ARW suggest that isolated convection will occur over central NC late today, mainly over the southeast CWA in late afternoon and in the NW CWA early in the evening, a reflection of sea breeze activity and convection easing off the higher terrain, respectively. Given the dry air in place noted on water vapor imagery and 12Z RAOBs, though, I would expect any coverage to be under 10%, and will keep a pop-free forecast for the rest of the day. High confidence in highs in the mid-upper 90s, agreed upon by high-res guidance, but heat index forecasts will still be tricky as they will depend greatly on dewpoints. A delay in the mixing-out of these dewpoints by even a couple of hours will make a big difference in HI. Will lean toward yesterday`s dewpoint trends, with readings slipping into the upper 60s to around 70 in the NW CWA this afternoon, while in the east readings will hold in the mid 70s for several more hours before dropping into the lower 70s late. Today`s slightly higher surface winds from the SW should facilitate a bit earlier mixing-out than we saw yesterday. Nevertheless, with these temps and dewpoints, HI values should peak in the 100-106 range across the heat advisory area. Considering the successive days of hot/humid weather with little recovery at night, exacerbating the risk of heat illnesses, will keep the heat advisory as is. -GIH Previous discussion from 305 AM: An area of high pressure centered aloft over the Carolinas will maintain the hot and dry conditions today. Low level thicknesses around 1440m Sunday are projected to inch upward to 1443/44m this afternoon, supportive of max temps in the mid-upper 90s. Surface dewpoints mixed out quite a bit in the northwest Piedmont Sunday afternoon with mid-late afternoon dewpoints in the mid 60s. While expect sfc dewpoints to mix out again, do not think that they will go as low as Sunday afternoon. Thus, should see heat index values around 105 degrees over most of central NC, excluding the nw Piedmont. Thus, little change required to the current heat advisory. Cannot rule out the potential for an isolated t-storm, mainly across the far south. However, considering the pocket of warm air aloft(as per 00Z GSO sounding), convective inhibition will be high. Thus,will refrain from mentioning an isolated t-storm for now. Tonight, very warm and muggy conditions will persist with most places not dropping below 80 degrees until after midnight. Min temps generally in the mid 70s. && .SHORT TERM /Tuesday and Tuesday night/... As of 305 AM Monday... Tuesday, a minor s/w approaching from the west will weaken the upper high overhead. This lowering of heights, the approach of the slightly cooler air aloft associated with the s/w, and a sfc tough over the Piedmont may be enough to allow for the development of isolated-scattered afternoon convection, mainly north of the I-40/85 corridor late Tuesday afternoon-evening. Elsewhere, presence of the upper ridge should inhibit convective development south of highway 64. Low level thicknesses projected to be just as warm Tuesday as today. Thus, will forecast max temps comparable to today in the mid to upper 90s. Another heat advisory may be warranted for most of the region. Any convection occurring early Tuesday evening across the north should quickly dissipate after sunset. Continued warm and muggy with overnight temps only cooling into the mid-upper 70s. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 3 AM Monday... Wednesday through Wednesday night: The remnants of a weak cold front will settle southward into northern portions of the area on Wednesday into Wednesday night eventually merging with a surface trough over the area. Meanwhile, the strong mid level ridge currently over the area will be located off the Southwest U.S. Coast by Wednesday. This will leave central NC on the southern fridges of the better westerlies for Wednesday. Given the the weak boundary will reside across northern portions of the area and potential for weak impulses to move across the area in the westerly mid level flow, expect we will see scattered to locally numerous showers and storms Wednesday afternoon/evening. Locations across the north will stand the best chance of seeing convection (closer to the actual boundary where better coverage is expected). With central NC on the southern fringe of the better westerly mid level flow (maybe around 20-25 kts at 500 mb across the north) we may see some multi-cell clusters of storms. Given this and expected mlcape values of around 1500 J/KG or higher during the afternoon into the evening and expected frontal zone placement we may see a better chance for severe storms on Wednesday than past days (especially across the northern half of the area). Though hard to time disturbances/mcv`s from upstream convection can and will have and impact on the eventual convective chances and placement. High temps on Wednesday are again expected to be above normal despite increasing chances for storms. Expect high temps to range from the lower to mid 90s north/northwest to the mid to upper 90s elsewhere, with heat index values from around 100 north to around 103-105 degrees southeast. Lows Wednesday night are expected to generally be in the mid 70s. Thursday through Sunday: Isolated to widely scattered showers and storms are expected each afternoon/evening through Saturday as the mid level ridge to our south/southeast is expected to bulge northward a bit late week into the first half of the weekend. This should result in high temps continuing to run in the mid to upper 90s for highs, with perhaps a few sites touching 100 by late week. This combined with a continued moist low level air mass (especially to the east of the lee side trough expected over the area) will allow for heat index values of around 100+ across the southern/southeastern half of the area which may result in additional heat advisories even if we dont reach heat index values of 105 degrees, thanks to the prolonged stretch of heat and humidity. The mid/upper level ridge is expected to shift eastward as s/w energy is expected to track across the Great Lakes into the Northeastern U.S next weekend. This should result in another weak frontal zone approaching central NC from the north by Sunday, yielding and increasing chance of mainly afternoon/evening convection. High temps by Sunday are expected to generally be in the lower to mid 90s. Low temps for the period are expected to generally be in the mid to upper 70s. && .Aviation /18Z Monday through Saturday/... As of 1230 PM Monday... VFR conditions are likely to dominate through Tue afternoon. Strong, dry, and deep ridge of high pressure in the middle and upper levels of the atmosphere will hold overhead for the 24 hours and beyond, leading to minimal cloudiness (scattered daytime cumulus based above 4 kft) and very low chance of storms. Light fog (MVFR) is possible late tonight into Mon morning mainly at RWI/FAY, and isolated banks of shallow, more dense fog may occur, mainly near RWI. Surface winds will be light from the southwest at 6-10 kts through Tue. Looking beyond 18z Tue: Mostly VFR conditions are anticipated. One exception will be in the late night and early morning hours each day this week, when pockets of sub-VFR fog are possible. Storm chances will remain fairly low through Tue night, then increase Wed through the end of the work week, but remain scattered in coverage. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... Heat Advisory from noon today to 8 PM EDT this evening for NCZ007>011-024>028-040>043-074>078-083>086-088-089. && $$ SYNOPSIS...Hartfield NEAR TERM...Hartfield/WSS SHORT TERM...WSS LONG TERM...BSD AVIATION...Hartfield

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