textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Details were added for a potential severe weather threat on Monday evening into the first part of the overnight.

Guidance is becoming more consistent in showing a cooling trend next weekend.

The aviation forecast was updated to reflect the recent 06Z TAF issuance.

KEY MESSAGES

1. Another Heat Advisory is in effect Monday afternoon for the SC Upstate, northeast GA, and the Charlotte Metro. Hot and humid conditions persist most of this week, with little relief until next weekend. 2. There is a Slight Risk for Severe Weather across western North Carolina this evening. More seasonally-typical diurnal convection continues for most of the week.

DISCUSSION

Key message 1: Another Heat Advisory is in effect Monday afternoon for the SC Upstate, northeast GA, and the Charlotte Metro. Hot and humid conditions persist most of this week, with little relief until next weekend.

An unbearable week is on tap this fine Monday morning. The most recent water vapor imagery depicts broad subtropical ridging extending across most of the southern CONUS, with the core of a broad h50 anticyclone located over eastern Texas. This configuration shouldn't change much through Wednesday morning, though the core of the anticyclone may retrograde westward into the Four Corners region, allowing heights to fall subtly over the Carolinas.

A protracted pattern change will play out from Wednesday through Saturday, as a deep upper low migrates from Manitoba eastward into the Hudson Bay and northern Quebec, gradually wearing down the subtropical ridge over the Southeast. A weak cold front may cross the area by Saturday, helping to further weaken the ridge.

As a result, hot and muggy conditions are expected for several more days. Today's afternoon heat index forecast has most of the low terrain seeing triple digits once again (though by mid-afternoon, mixing of dry air into the PBL may result in highly-variable dewpoints, so some locations may not see heat index values quite as high as others); the remainder of the week should feature afternoon heat index in the upper 90s or lower 100s - still unbearable, but not quite what we've seen the last two days. Should a cold front actually take place Friday or Saturday, next weekend could turn out significantly cooler.

Key message 2: There is a Slight Risk for Severe Weather across western North Carolina this evening. More seasonally-typical diurnal convection continues for most of the week.

As of around midnight, a few pesky convective cells have cropped up over the Triad, within an area of 1500+ J/kg sbCAPE. Another weak cell has begun producing lightning over Polk County. Surface obs are suggestive of an axis of weak surface convergence extending along and east of the NC Blue Ridge Escarpment...likely responsible for triggering this late-breaking initiation. With instability on its last leg and winds forecast to weaken (thus resulting in weakening convergence), don't really expect much additional activity, but will continue to monitor...

Otherwise, looks like a mostly benign morning and afternoon period Monday, with a relatively suppressed solution from most of the CAMs. The action will get going later than the typical diurnal period on Monday, as a weakening cold front sweeps across the Tennessee Valley, arriving in Appalachia by mid-evening, and triggering a round of convection across the NC mountains. The 00Z suite of hi- res guidance isn't especially consistent in how it handles coverage, HRRR a with the late-evening timing limiting convective initiation; the ARW and FV3 depict better coverage (over at least the NC zones), while the HRRR, NAMnest, and NSSL-WRF feature scattered coverage at best, and quickly kill it off east of the mountains. The RRFS is the lone outlier with impressive response all the way into the Upstate and I-77 corridor. Whatever convection does develop will do so in a fairly impressive environment for this time of year - featuring some 1000-1500 J/kg lingering MLCAPE, 20-25kts of deep layer shear, and a notable mid-level dry slot evident in forecast profiles. This should support some degree of organization, and a threat of damaging straight-line wind gusts, with the extent of the threat mainly dependent on how much convection actually initiates. The severe threat looks to diminish steadily as the system moves east, with a progressively less-conducive-to-severe environment farther and farther southeast.

The rest of the week looks more typical for August. Both Tuesday and Wednesday should be a tad more suppressed than usual, owing to the suppressed postfrontal environment left in the wake of Monday night's FROPA and the lack of any good upper-air features this far east; coverage may increase again by Thursday and Friday, as the air mass steadily becomes more favorable for deep convection. By Friday or Saturday, most ensembles still feature a cold front arriving in the Carolinas. This could increase both convective verage and associated severe potential, but confidence in the details is murky at this point.

AVIATION /06Z MONDAY THROUGH FRIDAY/

At KCLT and elsewhere: Another benign summer night is in progress, with mostly clear skies across the region, and light 1-4kt winds in most locations. A few garden-variety thunderstorms have redeveloped along the I-40 corridor, bound for the Triad, but aren't expected to impact any of the TAF sites. Convective debris should be swept eastward over the next 1-2 hours, leaving most of the terminal forecast area clear. Some mountain valley fog/stratus may develop again tonight, but in general, a steadier wind and drier air should limit potential as compared to last night.

Quiet conditions are expected for the first half of the day Monday. A shallow cu field should develop, but remain unperturbed the first part of the afternoon. An approaching cold front will incite late- afternoon initiation over the NC mountains, potentially driving convection east into the Piedmont and Upstate past sunset. Though showers and storms are a better bet at KAVL and KHKY, confidence remains lower on whether any convection will make it to KCLT or the Upstate sites.

Outlook: Scattered showers and thunderstorms, with associated flight restrictions, will be possible each afternoon thru Tuesday. Low stratus and fog will be possible each morning, especially in the mountain valleys.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from noon today to 8 PM EDT this evening for GAZ018-026-028-029. NC...Heat Advisory from noon today to 8 PM EDT this evening for NCZ070>072-082. SC...Heat Advisory from noon today to 8 PM EDT this evening for SCZ008>014-019-104>109.