textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Lowered dewpoints on Thursday and Friday based on the last two days' verification. Heat Index forecast has come down up to 2 degrees in some locations as a result, but still expecting widespread 100-105 degree heat index values both afternoons.

The aviation forecast was updated to reflect the impending 12Z TAF issuance.

KEY MESSAGES

1. Hot and humid conditions persist through Saturday afternoon, with heat index values in the lower 100s east of the mountains. A heat advisory is in effect for portions of the I-77 corridor for Thursday afternoon. A cooling trend arrives Sunday. 2. Diurnal convection remains mostly confined to the mountains on Thursday and Friday, before a cold front drives more widespread showers and storms on Saturday.

DISCUSSION

Key message 1: Hot and humid conditions persist through Saturday afternoon, with heat index values in the lower 100s east of the mountains. A heat advisory is in effect for portions of the I-77 corridor for Thursday afternoon. A cooling trend arrives Sunday.

A broad 595dm upper anticyclone remains draped across the Southeast, extending from the Ozark Plateau, across the Tennessee Valley, and into the Carolinas. The resulting suppressed pattern has kept us mostly dry, with clear conditions tonight and no returns on radar.

The upper pattern is progged to remain mostly unchanged for the next two days, with anticyclonic flow remaining centered right over us, and resultant weak subsidence and mostly dry conditions. Weak but moist southerly surface flow looks to persist through Friday, driven by a Bermuda High-like feature over the western Atlantic. The result still looks like above-normal temperatures and unbearable humidity on Thursday, Friday, and Saturday afternoons. Heat index values are expected to climb into at least the low 100s east of the mountains each afternoon. For Thursday, a Heat Advisory is in effect from 11 AM until 8 PM for Union County, NC; surrounding zones will come close to, but not quite reach, Heat Advisory criteria.

The big question mark remains afternoon dewpoint mixing. Forecast profiles from the GFS as well as the 00Z HREF membership depict a deep mixed layer (topping out as high as 700mb, with only a weak PBL- top inversion, on Thursday and Friday) supportive of significant afternoon dewpoint drops. The last two days, this has interfered significantly with the dewpoint forecast, with afternoon dewpoints undercutting most of the guidance by 1-3 degrees and resulting in cooler (albeit still dangerously hot) heat index values for most of the forecast area. As a consequence, confidence is limited on the exact heat index forecast...and it may be more apt to say that heat index values will differ significantly from location to location due to localized variations in mixing. The thinking for Saturday is similar, but confidence is lower.

By Sunday, ensembles and global deterministic models are in excellent agreement that as the subtropical ridge becomes more and more "squished" by successive shortwaves tracking out of the Great Lakes and across the Mid-Atlantic, a surface front will be driven across the Carolinas, ushering in a somewhat cooler and drier air mass. Still think, therefore, that temperatures will return to near- normal for Monday and Tuesday. Ensembles begin to diverge wildly by the middle of the week, with some 00Z LREF members depicting continued troughing over the eastern CONUS and a building ridge over the Rockies and Great Plains...and others depicting the subtropical ridge becoming re-established over the Southeast. Confidence, therefore, is virtually nonexistent for the forecast at this point in time.

Key message 2: Diurnal convection remains mostly confined to the mountains on Thursday and Friday, before a cold front drives more widespread showers and storms on Saturday.

Still think that broad subtropical ridging will keep convection suppressed today, with GFS and 00Z HREF profiles still featuring a weak cap that should, in the absence of any synoptic forcing to speak of, be enough to keep convection at bay east of the mountains. Having said that, the 00Z suite of hi-res models is in good agreement that scattered showers, and perhaps a few thunderstorms, will initiate over the ridgetops this afternoon, within an environment devoid of any shear but with 1000-1500 J/kg sbCAPE. A few of the CAMs (most notably the NAMnest and several members of the 00Z REFS) depict convection breaking containment and riding outflows into the western Upstate through the evening hours. Generally think this is a low-probability scenario given how high LFCs will be, but should it occur, cells would have a narrow window to tap into a more unstable environment over the Upstate, featuring closer to 2000 J/kg sbCAPE and sufficient DCAPE to support a stray strong thunderstorm or two.

Flow will begin to improve a tad on Friday, with the CAMs which extend out that far again depicting a small chance of a few cells escaping the mountains, but with the same limiting factors as Thursday in the form of high LFCs, weak capping, and poor mid-level lapse rates. Nonetheless, there's a highly-conditional scenario in which a shortwave grazing the I-40 corridor Friday afternoon could contribute just enough flow aloft that if the cap breaks and permits convective initiation, and the upper flow becomes strong enough while convection is active, that an isolated strong to severe storm could produce damaging winds over the far northeastern portion of our forecast area.

Conditions won't really become more favorable for convection over the low terrain until Saturday, when upper shortaves rolling through the subtropical ridge start to erode capping and steepen lapse rates. At the same time, a cold front will settle into the lower Mid-Atlantic, possibly arriving in the Carolinas by early Saturday afternoon and providing a better triggering mechanism for showers and storms, as well as a more favorable parameter space for convective activity. Ensembles feature anywhere from 1500 to upwards of 2000 J/kg sbCAPE, a respectable 20-25kts of deep-layer shear, and DCAPE on the order of 1200-1500 J/kg with PWs > 1.8". All this could be supportive of some semi-organized segments, as well as wet microbursts. However, confidence remains limited, and although it looks like several favorable ingredients will be in place for a non-zero severe threat...there's still room for things to change.

AVIATION /12Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: Predominantly VFR conditions on tap once again for the 12Z TAF period. Valley fog and stratus have largely failed to materialize, notwithstanding isolated patches visible on satellite in the Little Tennessee Valley and northernmost reaches of the French Broad. In any case, don't think this will be able to make it to AVL before daybreak puts the damper on things. Hi-res models keep convection confined to the mountains once again Thursday, albeit with somewhat better coverage than we've had most of this week. Felt coverage was increased enough to include a VCSH at AVL for a few hours during the afternoon and early evening, but otherwise no precip mention for the 06Z TAFs. Dry and mostly VFR conditions are expected to continue into Thursday night.

Outlook: Diurnal convection, mostly confined to the mountains, returns on Friday. More widespread showers and storms are expected on Saturday as a cold front crosses the area. Suppressed weather returns Sunday and beyond.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Heat Advisory from 11 AM this morning to 8 PM EDT this evening for NCZ082. SC...None.