Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 181817 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 217 PM EDT Tue Jul 18 2017 .SYNOPSIS...
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An upper level low across the Piedmont of North Carolina will drift east and then southeast across the Carolinas through Thursday. A strong ridge of high pressure across the Central Plains will expand into our region for Friday through the weekend.
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&& .NEAR TERM /THROUGH TONIGHT/...
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As of 215 PM Tuesday... A well defined upper trough axis that extended southwest from KDCA to just west of KCLT to near KATL at midday will shift slightly east through tonight and will be the primary player in convective evolution during the next 12 hours. The trough will close off this afternoon as a couple of waves of vorticity rotate about the center. Deeper moisture is present east of the trough axis and the widespread area of morning stratus has largely cleared out from U.S. 1 westward while a good deal of cloud cover holds on to the east in the Coastal Plain and the eastern Piedmont. Should seek an increase in the number of breaks of sunshine as the afternoon wears on. Mid-level temperatures near the trough axis at 500 hPa are relatively cool and range between -11 and -12C resulting in mid level lapse rates exceeding 6 deg C/km. In addition, the exit region of a 60+kt jet across northeastern NC at 250 hPa will contribute to some broader forcing for ascent. The air mass across central NC has become moderately unstable with MLCAPE values ranging between 1000 and 2000 J/KG this afternoon with the instability axis extending near the U.S. 1 gradient in the sweet spot where some sunshine expanding from the west overlaps the deeper moisture across the east. Isolated convection is developing this afternoon resulting from low-level convergence in a couple of locations: in a region of differential heating near the instability axis from Roxboro southwest through Asheboro to Charlotte, across the I-95 corridor and across the eastern slopes of the Appalachians. The greatest coverage should be located across the east and southeast ahead of the upper trough and in a region of deeper moisture and greater instability. The wind field today is very weak with the flow below 500 hPa generally less than 12 kts, with stronger flow above that across the east in proximity to the upper jet. Organized severe convection is not expected with the weak shear and pulse or convective mode although the strongest storms could produce a localized strong wind gust or even some small hail. The convection should wane this evening with the loss of heating and limited forcing for ascent. Overnight lows will range within a few degrees of 70. -Blaes
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&& .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
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As of 215 PM EDT Tuesday... The upper trough will have become closed by Wednesday morning and will be centered near Goldsboro with a northeast-southwest trough axis. Convective coverage on Wednesday afternoon and evening will be more limited than previous days as the deep moisture axis will have shifted offshore and much of the area will be located on the subsident side of the trough. Will include slight chance PoPs for much of the area with slightly higher Pops across the Coastal Plain, east of the trough axis. The convection should dissipate early Wednesday evening. High temperatures on Wednesday will be a few degrees higher as morning low-level thickness values increase to 1410m or greater, supporting highs in the 89 to 94 range. Lows on thursday morning will range in the lower 70s. -Blaes
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&& .LONG TERM /THURSDAY THROUGH MONDAY/... As of 355 AM Tuesday... A mid-upper level evolving now over the middle Atlantic states will move slowly east, to just off the sern NC coast by 12Z Thu, before drifting swwd along the sern U.S. coast and to near the FL panhandle by Sat. This is a change in model run-to-run continuity from previous days, which indicated the low would move off DelMarVa and out to sea through the end of the week. The retrogression of this feature would occur in a break, or weakness, in a pair of sub- tropical ridges that will otherwise span from the central Plains/mid MS Valley to the wrn N. Atlantic, and may result in slightly less hot conditions than earlier forecast, particularly along the sern U.S. coast where a relative minimum in heights/thicknesses would result. At the surface, a trough will become established in the lee of the srn and central Appalachians, on the wrn periphery of high pressure over the central and wrn N. Atlantic. The presence and influence of the sub-tropical ridge will likely restrict diurnal convection to differential heating zones over the mountains and along the sea breeze. Despite the possibility of the slightly less hot temperatures noted above, heat and high heat indices may yet become hazardous, as temperatures beneath the ridge climb into the middle 90s to around 100 degrees - hottest Fri-Sat. When combined with surface dewpoints ranging from the middle to upper 60s west of the Appalachian-lee trough (over the nw NC piedmont) to lower to middle 70s to the east of that feature (over the Coastal Plain), heat index values between 100 and 110 degrees are expected to result. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... As of 140 PM Tuesday... 24 hour TAF period... Areas of stratus this morning with widespread IFR/MVFR restrictions has given way to a mix of VFR and MVFR conditions in stratocumulus early this afternoon. Isolated to scattered convection will develop during the afternoon, especially from interstate 85 south and especially eastward. Coverage will be more limited than a typical summer day but across the east we will include temporary restriction of an MVFR shower and a mention of a vicinity shower with VFR conditions across the west. Will hold off mention of thunder at this point given low predictability. Mainly VFR conditions are expected this evening and overnight. Another period of stratus is expected to redevelop late tonight although the coverage will be more limited and generally confined to the eastern Piedmont and especially the Coastal Plain. Looking further ahead... Mainly afternoon and evening showers and isolated storms are expected again on Wednesday with a possibility for fog/low stratus in the morning across the east. Chances of diurnal showers and storms decrease a great deal by Thursday into the weekend with hot weather and generally VFR conditions expected. -Blaes && .CLIMATE... RDU Records: Date | High Year | High Min Year ------------------------------------------ 18 July | 103 1887 | 79 1887 19 July | 101 1902 | 80 1942 20 July | 102 1942 | 76 1981 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 ------------------------------------------ 18 July | 97 1986 | 76 1986 19 July | 98 1977 | 75 2016 20 July | 100 1977 | 75 1986 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 ------------------------------------------- 18 July | 103 1942 | 76 2013 19 July | 101 2002 | 77 1988 20 July | 104 1932 | 80 1996 21 July | 105 1932 | 79 1977 22 July | 107 1932 | 79 2011 23 July | 103 2011 | 79 1998 24 July | 105 1952 | 79 1945 && .RAH WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...BLAES NEAR TERM...BLAES SHORT TERM...BLAES LONG TERM...KCP AVIATION...BLAES CLIMATE...RAH

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