Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC
Issued by NWS Raleigh/Durham, NC
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577 FXUS62 KRAH 051150 AFDRAH Area Forecast Discussion National Weather Service Raleigh NC 750 AM EDT Fri Jul 5 2024 .SYNOPSIS... A subtropical ridge will continue to extend across the South Atlantic states ahead of a cold front that will move into and stall over the southern Middle Atlantic and Carolinas this weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 430 AM Friday... ...Dangerously hot, with record temperatures and scattered afternoon- evening showers/storms... The Heat Advisory has been upgraded to an Excessive Heat Warning for the ne Piedmont, nrn Coastal Plain, and the nrn Sandhills, where heat index values around 110 F will be likely this afternoon. The Heat Advisory remains otherwise unchanged in time and location, for peak heat index values generally in the 105 to 109 F range. A subtropical ridge will migrate slowly ewd and across the South Atlantic states through 12Z Sat, while a shortwave trough and accompanying compact, mid-level cyclone now over the upr MS Valley will lift newd across the upr Great Lakes and into srn ON. Wly, quasi-zonal mid-level flow of recent days over the srn Middle Atlantic will back to swly. Within that swly flow regime, a well- developed MCV from an ongoing MCS over sern MO and nrn AR will move newd across the OH Valley today and across the cntl Appalachians and nrn Middle Atlantic tonight. A trailing mid-level trough may glance nrn and wrn VA and nwrn NC this evening. At the surface, hot sswly flow will persist across the Carolinas and srn Middle Atlantic, downstream of a convectively-reinforced cold front that will extend from a ~1010 mb low over ern PA/MD and NJ swwd into the srn Appalachians and TN Valley by 12Z Sat. An Appalachian-lee trough will slightly precede the front and extend across the Foothills and Piedmont of the srn Middle Atlantic/Carolinas. The trough may be reinforced by a wind shift and convergence axis left behind by outflow from Thu evening`s convection, which was apparent in 08Z surface observational data along an arc from DAN-HBI-CLT. The parent outflow boundary has lost character at the surface (no longer discernible as a wind shift or with any apparent cooling) but has continued sewd as an undular bore and extended along an arc from near EWN-EYF-LBT. That feature will likely exit Sampson Co. and reach sern NC and nern SC by 12Z this morning. 850 mb temperatures that were observed at 21 C at GSO last evening are forecast to increase another degree or two C and reach near 3 sigma above average over the Carolinas, beneath the subtropical, mid- level ridge axis today. The corresponding area of 1440-1450 meter 1000-850 mb thicknesses that were observed in a small area over the wrn Piedmont and at GSO last evening is forecast to expand in coverage throughout cntl NC today. As such, already hot temperatures of Thu will likely tick higher today, with widespread upr 90s to lwr 100s likely. Additionally, the upr 60s surface dewpoints that were common Thu afternoon are likely to be replaced by lwr 70s dewpoints this afternoon, except for a small area of upr 60s likely to reappear with afternoon mixing over the srn Piedmont and wrn Sandhills. The combination of hotter temperatures and higher surface dewpoints on average will favor widespread heat index values of 105 to 109, with 110-112 F likely in the counties included in the Excessive Heat Warning. That increased heat and humidity will quickly erode morning convective inhibition, amid ~5.5 to 6 C/km mid-level lapse rates already in place per 00Z regional RAOB data. Widely scattered showers and storms will likely develop by midday-early afternoon along the surface features noted above, which may yield strong to isolated damaging wind gusts while also locally/briefly cooling localized areas. Somewhat greater concentration/multi-cell clustering will be possible into the nw Piedmont this evening, where upstream cells over nwrn NC may be focused by the glancing mid-level trough and subsequently propagate into those areas. It will otherwise remain very mild and muggy tonight, with lows in the mid to upr 70s, to around 80 F in urban areas. && .SHORT TERM /SATURDAY AND SATURDAY NIGHT/... As of 330 AM Friday... * Hot and humid conditions continue Saturday with heat indices climbing to 100 to 105 degrees * Isolated to scattered flash flooding possible as storms will be slow moving and efficient rainfall producers. A compact, but flattening, shortwave embedded within the broad troughing will lift through the Lower Great Lakes into southern Quebec by Sun morning. The related surface pattern will shift a surface low into the Northeast with a trailing cold front extending into the Ohio and Tennessee Valley. Weakening dynamics at the surface will likely prevent the surface cold front from traversing the Appalachian mountains. Ahead of the front and east of the mountains, a continued hot and humid airmass will be in place as early afternoon temperatures quickly rise into the 90s to around 100 degrees with surface dew points in the upper 60s to mid 70s, resulting in heat indices of 100 to 105 degrees. Heat indices over the western Piedmont may be closer to the mid 90s as increasing/thickening cloud cover spreads in from the west and proximity to greater chances for showers/storms and convective outflow slowing rise in afternoon temps. Although details will be dependent on convective activity Fri/Fri night, another heat advisory will likely be needed for at least the Coastal Plain and eastern Sandhills for Sat. Showers and thunderstorms are expected to initiate along a lee trough that develops with diurnal heating over the western Piedmont with the a hot and humid airmass in place along and east of the trough. Moderate instability is expected to develop by early afternoon as MLCAPE values rise to 1500-2500 J/kg and MLCIN erodes. Flooding will likely be the primary hazard with these storms with anomalous deep layer moisture (PWAT >2.25), warm cloud layer between 10,000 to 15,000 and weak cloud layer flow and Corfidi vectors. Additionally, deterministic Hi-Res guidance depicts initial storm outflow to congeal creating basically an effective front draped SW to NE across central NC that slowly progresses eastward with time. This orientation would align cloud-layer flow parallel to the front which would bring a risk for training storms. 00z HREF aligns with theses thoughts depicting of 1" to 2"/hr rain rates and the strongest storms capable of over 3"/hr. Current forecast has widespread rain of 0.5 to 1.5 inches, but as finer details become clear expect the range between min/mix to increase with isolated locations capable of seeing over 3 inches. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 330 AM Friday... A rather persistent pattern is expected through the remainder of the extended as central NC will be tucked between broad reinforcing troughing through the central CONUS with the stationary sub-tropical high broadly draped across the western Atlantic in the eastern GOM. NWP guidance has been consistent with a closed low drifting across the western Atlantic and retrograding towards the Carolina coast, but is expected to remain over the Atlantic. At the surface a diffuse pressure gradient will feature a lee-trough redeveloping over the region each afternoon with continued anomalous deep layer moisture in place as PWAT values remain at-or-above 2 inches. This pattern will favor continued above normal temperatures with highs in the mid 90s in the urban areas and overnight lows only reaching into the mid 70s. With continued heat related stresses heat related illnesses will be possible especially among individuals sensitive to heat and do not have access to effective cooling and/or adequate hydration. Diurnally favored showers/storms and storms will also be possible each afternoon into the evening, likely focused along the lee-trough and any subsequent outflow boundaries thereafter. Organized convection appears to be at a minimum with deep layer shear remains weak around 10 to 20 kts, strongest across the NW Piedmont, and result in mostly pulse summer-time showers/storms. Greatest coverage could be Tues into Tues night with proximity of mid/upper troughing swings through the Great Lakes region. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... As of 745 AM Thursday... Patchy MVFR visibility restrictions around GSO will become VFR by ~12Z. VFR conditions are otherwise expected away from scattered showers/storms that are expected to redevelop with afternoon heating. Some clustering and more-prolonged risk of showers/storms will be possible at INT/GSO this evening-early tonight, where lift will be maximized in the vicinity of a lee surface trough, outflow from upstream convection, and glancing influence of a mid-level trough. A slightly better chance (than recent mornings) of stratus and fog will exist late tonight-Sat morning, especially at INT/GSO, as low-level moisture and humidity values gradually increase. Outlook: A good chance of morning stratus and fog and mainly afternoon-evening showers/storms will exist through early-mid next week, as a surface trough/front settle into the region and interact with an unseasonably moist air mass in place. && .CLIMATE... Record High Temperatures: July 5: KGSO: 98/2012 KRDU: 102/1999 KFAY: 101/2002 July 6: KRDU: 102/2022 KFAY: 99/1990 Record High Minimum Temperatures: July 5: KGSO: 74/1999 KRDU: 78/1902 KFAY: 77/2005 July 6: KGSO: 76/1999 KRDU: 79/1900 KFAY: 76/2017 July 7: KGSO: 73/2019 KRDU: 77/1900 KFAY: 78/2017 && .RAH WATCHES/WARNINGS/ADVISORIES... Heat Advisory from 11 AM this morning to 8 PM EDT this evening for NCZ007-021>023-038-039-073>075-078-083>086-088-089. Excessive Heat Warning from 11 AM this morning to 8 PM EDT this evening for NCZ008>011-024>028-040>043-076-077. && $$ SYNOPSIS...MWS NEAR TERM...MWS SHORT TERM...Swiggett LONG TERM...Swiggett AVIATION...MWS CLIMATE...RAH