Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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524 FXUS62 KRAH 210820 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 420 AM EDT Fri Jul 21 2017 .SYNOPSIS...
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Hot and mainly dry sub-tropical high pressure will expand across the mid-South and southern Appalachians through the weekend, then gradually weaken by early next week.
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&& .NEAR TERM /TODAY THROUGH TONIGHT/... As of 415 AM Friday... Ridging continues to build east across the Ohio Valley today, with ensuing northwest flow potentially steering emanations originating from ongoing MCS over the Ohio Valley into the area tonight. In addition, HRRR hints of outflow originating from convection due to differential heating on the slopes migrating into the foothills (perhaps enhanced by the Ohio Valley activity) and northwest Piedmont from around sunset into the overnight. As such, will have a slight chance PoP through the overnight to account for this possibility. Meanwhile, temperatures will be a degree or two warmer than Thursday...mostly 96-99 degrees with potential for a few spots touching triple digits. Mixing in the vicinity of the surface trof axis over the western Piedmont lowered dew points into mid 60s and kept heat indices to near 100 on Thursday. Trof will have a similar influence today in the western Piedmont, albeit with slightly warmer temperatures, so heat indices will range mostly from 101 to 104 degrees in the west, while the east should see 104-107. As such, delineation of the ongoing heat advisory looks good. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
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As of 415 AM Friday... Mid level ridging begins to flatten on Saturday as a short wave and associated front drops out of the Ohio Valley and grazes the area Saturday night. Very warm airmass in place Saturday modifies further, with low level thicknesses inching up to near 1450M (a modest 4M warmer than today = perhaps a degree warmer), so will basically maintain persistence highs from 97-99 and anticipate our heat advisory expanded perhaps a county to the west. The front will be moving across the area late in the day, and will have 30% PoPs across the northern tier from mid afternoon through midnight, with potential for showers and isolated storms to persist overnight due to outflow propagation. Persistence mins in the mid 70s Saturday night.
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&& .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 400 AM Friday... A pair of sub-tropical ridges will initially be centered over TN/KY and the west-central N. Atlantic (just se of Bermuda), with an intervening weakness/break in the ridge over the Carolinas. Both ridges are forecast to weaken through early next week, at the base of a nrn stream trough forecast to migrate across the Great Lakes and Northeast. Central NC will consequently become increasingly susceptible to passing perturbations, some resulting from upstream convection, in wly to nwly flow. At the surface, a trough will remain anchored in the lee of the srn and central Appalachians through until a synoptic cold front, one related to the aforementioned passing nrn stream trough, settles south into the Carolinas late Tue-Wed, where it will likely stall and become increasingly diffuse through Thu. The weakening and suppression of the sub-tropical ridge will result in both a gradual waning of the very hot conditions that will encompass central NC for the next few days, and also an increase in diurnally-maximized convection Sun-Tue. If medium range guidance solutions prove correct, the aforementioned frontal zone may settle far enough swwd to yield dry and near, to even slightly below average temperatures, by the middle of next week over all but the far srn tier of central NC counties/in the vicinity of the frontal zone. however, such an aggressive surge of the front to near the NC/SC line during the hottest time of year would be a somewhat atypical feat. While a Heat Advisory will probably be needed for at least a portion of central NC again Sun (Ie. the srn and ern piedmont, Sandhills, and Coastal Plain), the models indicate a good chance for the arrival of considerable high level moisture that may keep temperatures from reaching full heating potential that would otherwise favor 95 to 100 degree temperatures once again. Thereafter, increasing clouds, convective coverage, and steadily decreasing influence of the sub-tropical ridge, should collectively favor a continued downward trend into the upper 80s by the middle of next week. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... As of 135 AM Friday... 24 hour TAF period: VFR conditions under high pressure will generally persist through the TAF period. However, a brief period of sub-VFR visbys at KRWI will be possible, mainly at RWI...from 08-12Z this morning. Isolated convection is expected this afternoon/early evening...too random to include in the TAFs. An outflow boundary will be crossing the mountains towards evening and may make it into the northwest Piedmont after sunset...will reevaluate this scenario for increased confidence in its convective potential for the 12Z issuance. Outlook: Generally VFR conditions are expected Saturday. Scattered mainly diurnally driven convection is possible Saturday and Sunday, with MVFR to IFR conditions. A better chance of afternoon/evening storms are expected by early next week as a cold front moves into the area, along with morning stratus and/or fog. && .CLIMATE... RDU Records: Date | High Year | High Min Year ------------------------------------------ 21 July | 102 2011 | 78 1932 22 July | 103 2011 | 78 2011 23 July | 105 1952 | 78 2011 24 July | 101 2011 | 79 2011 GSO Records: Date | High Year | High Min Year ------------------------------------------ 21 July | 102 1926 | 76 2011 22 July | 101 1926 | 77 2011 23 July | 99 1952 | 78 2010 24 July | 99 1914 | 78 2010 FAY Records: Date | High Year | High Min Year ------------------------------------------- 21 July | 105 1932 | 79 1977 22 July | 107 1932 | 79 2011 23 July | 103 2011 | 79 1998 24 July | 105 1952 | 79 1945 && Heat Advisory from noon to 8 PM EDT Friday for NCZ010-011-025>028- 040>043-075>078-084>086-088-089. && $$ SYNOPSIS...MWS NEAR TERM...mlm SHORT TERM...mlm LONG TERM...MWS AVIATION...mlm CLIMATE...RAH

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