textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Sunday's PoP and dewpoint forecasts were updated based on recent real-time data on the position of a diffuse frontal boundary.

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

KEY MESSAGES

1. Diurnal showers develop again on Sunday and Monday across the NC mountains, but coverage should be isolated to nonexistent the rest of the week. 2. Hot and humid weather returns by Tuesday, with dangerous heat index values expected over the low terrain each afternoon this week.

DISCUSSION

Key message 1: Diurnal showers develop again on Sunday and Monday across the NC mountains, but coverage should be isolated to nonexistent the rest of the week.

Not much change to the large-scale setup tonight. Recent water vapor imagery depicts a shortwave axis located southeast of I-85, with synoptic-scale lift maximized east of the forecast area. At the surface, a diffuse quasi-stationary boundary remains draped across the southeastern tier of the forecast area...and most stations are reporting some flavor of weak easterly flow, resulting in weak isentropic ascent over the boundary. Radar is mostly clear, but a few isolated, elevated showers continue to pop up all over the forecast area in response to the upglide.

Convection appears likely to remain suppressed and isolated the next few days. Can't rule out some continuing elevated showers the first part of tonight; a rumble of thunder can't be entirely ruled out, but elevated instability looks anemic at best, with a nearly isothermal layer above 700mb where weak subsidence develops behind the shortwave. Don't expect conditions to get too much better during the daylight hours Sunday; as the last of any lingering cyclonic flow in the mid-levels retreats northward, upper flow will slacken. So despite moist southerly flow in the low-levels, and surface-based instability to the tune of 1000-1500 J/kg during the afternoon...the lack of large-scale ascent or any discernible triggering mechanism will keep showers and thunderstorms confined to the ridgetops of the NC Blue Ridge, and likely non-severe in nature.

Subtropical ridging is expected to intensify through most of the new workweek. Thus, still think that the environment will become increasingly hostile to convection. Another round of isolated mountain showers may take place Monday afternoon...beyond which it's looking unlikely that ANY convection will be able to develop the rest of the week.

Key message 2: Hot and humid weather returns by Tuesday, with dangerous heat index values expected over the low terrain each afternoon this week.

Subtropical ridging is still expected to become re-established across the Southeast U.S. by Monday, with deepening anticyclonic flow aloft depicted in virtually all global ensembles by Monday or Tuesday...centered over the Ozarks but extending well into the Tennessee and Mississippi Valleys. As a result, forecast profiles depict stout subsidence aloft and dramatically increasing heights over our area, with little evolution of the synoptic pattern through the end of the seven-day forecast.

Triple digit heat index values are thus likely to return to the low terrain each afternoon for most of the week. The current temp and dewpoint forecast supports isolate 100+ degree heat index, mainly south of I-85, on Monday, with more widespread triple digits on Tuesday and beyond. By the second half of the week, still think some zones could be under the gun for Heat Advisories...but confidence of course remains limited at this time.

AVIATION /09Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: More or less as expected, widespread low stratus, mostly in the IFR range but as low as (V)LIFR in a few spots, has developed across most of the NC Piedmont, and some portions of the SC Upstate along I-77. Mountain valley fog/stratus have thus far been less prominent than expected, but there remains potential for brief restrictions to develop at KAVL around daybreak.

Restrictions may lift more slowly this morning than they have the last two days...with cloud bases gradually lifting from IFR to MVFR and finally back to low-end VFR through mid- to late-morning. Convection should be extremely isolated Sunday afternoon - likely confined to the mountains, with too little coverage to justify so much as VC precipitation in the KAVL TAF. Another round of MVFR to IFR stratus may develop again on Sunday night.

Outlook: Isolated mountain showers and thunderstorms may develop again on Monday afternoon. Drier weather is likely to return the rest of the week. Fog and low stratus will be possible each morning, especially in the mountain valleys.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.