textproduct: Raleigh

WHAT HAS CHANGED

* Heat Advisory cancelled due to widespread rain-cooled air from showers and thunderstorms, with associated marked decrease in temperatures and heat index values

* Convective chances raised for today ahead of a mid-level disturbance (MCV) approaching from the nrn Midlands of SC and Charlotte Metro vicinity

* Hottest conditions occurred Fri-Sat and will undoubtedly be trending less so in the coming days

KEY MESSAGES

As of 225 PM Sunday...

1) Continued hot, though trending less so, with scattered to locally numerous convection this afternoon-evening and with at least a few strong to damaging downbursts probable

2) Chances for diurnally driven convection each day through the week and into the weekend.

DISCUSSION

As of 225 PM Sunday...

KEY MESSAGE 1...Continued hot, though trending less so, with scattered to locally numerous convection this afternoon-evening and with at least a few strong to damaging downbursts probable

In the mid/upr-levels, the previously, anomalously strong lwr mid- latitude anticyclone responsible for this now waning heat wave has weakened 5-6 decameters since it peaked over the Virginias at 00Z- 12Z Fri; and it has also drifted well offshore and east of cntl NC, now about midway of Bermuda and the coast of the Carolinas. Mid- level flow behind the departing anticyclone and downstream of a shortwave trough over the mid MS Valley has to veered to swly/wly over cntl NC per 12Z upr air and more recent regional VWP data. A mid-level trough, convectively-amplified from convection over the TN Valley and srn Appalachians on Sat, has been evident in 700 mb charts and visible satellite data this morning over the Upstate and now Midlands of SC. This feature will track through the swly/wly flow regime and probably focus ascent over the NC Piedmont this afternoon, amid more-broadly and weakly falling heights that have been evident through most of the Middle Atlantic and Southeast for the past couple of days. Another, higher-level and more-subtle perturbation evident in water vapor satellite data near the Atlanta Metro is forecast to deamplify as it tracks into the Upstate of SC this evening and across cntl NC tonight-early Monday.

At the surface, the pattern remains very similar to Sat in and around cntl NC, with an Appalachian-lee trough extending across the srn Middle Atlantic Piedmont and high pressure stretching across the Appalachian Plateau and lwr OH Valley. A weak lee low may form within the lee trough over VA, while a wavy frontal zone, stretching from srn New England wwd and across nrn PA and to a 1013 mb, convectively-amplified low over nrn OH, may become augmented by outflow swd across PA but otherwise move little through tonight. It will then probably be drawn swd into the lee low and associated convergence over VA later Mon and Mon night.

Cumulus downstream of the aforementioned mid-level trough has already developed and deepened considerably over the Sandhills and adjacent portions of the Piedmont of the Carolinas since 15Z, with locally numerous cells near and northeast of CAE. These showers and storms will likely continue to grow in coverage and intensity early this afternoon while spreading generally enewd across the Piedmont and Sandhills. Continued high DCAPE, with observed values between 1000-1200 J/kg at GSO and MHX this morning and corroborated by values a couple of hundred J/kg higher downstream of the developing convection in more recent SPC meso/objective analysis data, will provide a favorable environment for damaging downbursts.

By most-typically-used measures of high temperature forecasting (eg. temperatures at 850 and 925 mb and 1000-850 mb thickness), the airmass over NC has already cooled relative to previous days. Indeed, 24-hour surface temperature changes today have been minus 2- 5 F through the morning. Diabatic cooling from convection and outflow this afternoon will further cool the airmass in place. Heat index values from the most accurate/reliable sources (ie. ASOS that directly measure surface dewpoint vs AWOS, Econet, and RAWs instrumentation that indirectly derive dewpoint by measuring temperature and RH) will nonetheless still likely reach the 102-106 F range through the Advisory area. For as hot as the ambient temperatures have been during this heat wave, the highest heat index measured by any ASOS in cntl NC has been 108 F (RDU).

Temperatures will be less hot yet on Mon owing to cooling from today and additional, scattered to locally numerous convection and overturning on Mon, amid continued, swly and convectively-perturbed mid-level flow. Slightly less hot temperatures may be offset by slightly higher surface dewpoints, so heat index values will still reach the upr 90s to near 105 - mostly below Heat Advisory criteria of 105 F. DCAPE is forecast to be much less on Mon than recent days, which suggests downburst potential will be less after peaking today.

KEY MESSAGE 2... Chances for diurnally driven convection each day through the week and into the weekend.

Central NC will be on the nw periphery of the mid-level anticyclone off the Southeast US coast, with a s/w trough to the west over the mid-MS Valley Tue and Wed. The s/w will weaken as it slowly shifts ewd across the TN/OH-Valley Wed night/Thu, then continue across the Carolinas/mid-Atlantic Thu night/Fri. Another trough may amplify over the OH Valley as a s/w approaches from the west late week, drifting slowly ewd over the weekend. At the surface, a front may move into the area Tue/Tue night as the parent low shifts off the Northeast US coast, with a secondary area of low pressure lingering over the area Wed/Wed night along the old boundary. General Piedmont troughing expected through at least Thu and Fri, with model spread increasing thereafter. PWATs of 1.8 to 2.1 inches are expected through the week, with SBCAPE of roughly 1000-2000 J/Kg and Bulk Shear of 20-30 kts (via GFS forecast soundings). Given the continued moist, unstable environment, expect diurnally driven showers and storms each day, with a focus along surface boundaries. With little appreciable airmass change, temperatures will generally remain above normal, with Friday expected to be the warmest day and Wednesday perhaps the relative coolest.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

As of 115 PM Sunday...

Convection with strong wind gusts will continue to grow in coverage, to scattered to locally numerous, and intensity ahead of a mid-level trough now progressing across the NC Piedmont and Sandhills. PROB30 and/or TEMPO thunder will cover such occurrences, most likely at Piedmont sites and FAY. Light sswly surface winds and VFR conditions are otherwise expected except for a small risk of stratus and mist/fog mainly at RWI Mon morning.

Outlook: Scattered to locally numerous convection will probably result again each afternoon through mid-week, as a series of mid- level disturbances overspread a persistent, Appalachian-lee/Piedmont surface trough and eventually a backdoor cold front likely to settle into the area by Wed. Morning stratus/mist may also result in areas that receive convective rain the previous afternoon-evening.

CLIMATE

Records within reach this forecast period...

Record High Temperatures:

July 5: KFAY: 101/2024 KGSO: 98/2012 July 6: KFAY: 100/2024 July 10:KRDU: 99/1993

Record High Minimum Temperatures:

July 5: KRDU: 78/2024 July 6: KGSO: 76/1999 KRDU: 79/1900 KFAY: 77/2024 July 7: KGSO: 73/2024 KRDU: 77/1900 KFAY: 78/2017

RAH WATCHES/WARNINGS/ADVISORIES

None.