textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Details were added for the convective and heavy rain forecast on Thursday and Friday. QPF has increased in many locations.

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

KEY MESSAGES

1. Showers and thunderstorms return this evening, with some potential for isolated strong to severe storms. At least scattered convection persists each afternoon through Saturday, before dry weather returns Sunday and beyond. 2. Hot and muggy weather persists through Friday. Over the weekend, a cold front will usher in a cooler air mass, and more seasonable temperatures will persist into next week.

DISCUSSION

Key message 1: Showers and thunderstorms return this evening, with some potential for isolated strong to severe storms. At least scattered convection persists each afternoon through Saturday, before dry weather returns Sunday and beyond.

As of around 1AM, radar imagery depicted a mature MCS located over western Kentucky and gradually moving eastward following the environmental flow, which was nearly due westerly across the lower Mid-Atlantic and Southeast on the periphery of deep upper troughing extending from Quebec down into the Ohio Valley. A weak cold front was stalled across the Ohio Valley, extending from central Indiana eastward into the Allegheneys. A bank of stratus was located over the southern Blue Ridge, and down into the upper Savannah River Valley, but was actively shrinking.

Hi-res guidance continues to feature the MCS settling into the Ohio Valley through dawn, and passing mostly north of the GSP CWA even as it deteriorates. This is in line with shear profiles and the environmental wind; no members of the 00Z suite of CAMs depict much, if any impact in the western Carolinas. By day, however, lingering cirrus debris from the MCS may hinder destabilization, with some recent runs of the HRRR and RRFS featuring barely 1000-1200 J/kg sbCAPE over the area for much of the day. By mid- to late-afternon, though, several guidance sources feature an embedded shortwave diving across the Tennessee Valley and pushing the cold front closer to our area, resulting in improving instability through the late afternoon and evening hours, and inciting a round of later-than- typical convective initiation. CAMs at this point do not have a great handle on how, exactly, convection may evolve, but there's good agreement that at least part of the area will be under the gun for a round of semi-organized thunderstorms, developing in an environment with 20-30kts of deep layer shear, respectable lapse rates, and some 2000-2500 J/kg of instability. This is enough to expect perhaps a few organized linear segments, capable of producing isolated damaging winds through late Thursday evening. SPC's Marginal Risk for Severe Weather looks reasonable, if perhaps conditional upon successful initiation so late in the day.

Daily shower and thunderstorm chances will persist through Friday and Saturday. By Friday, the Ohio Valley cold front may enter the forecast area and provide a better focus for convective initiation that afternoon, again within a fairly unstable environment with decent deep-layer shear. That shear, however, will be mostly parallel to the boundary, so upscale growth and storm training may ultimately pose a greater threat than severe. A few deterministic models even depict a mesolow-like feature developing along the frontal boundary, further enhancing the heavy rainfall threat. Indeed, the 00Z HREF has come in with 3- and 6-hour PMMs of 2-3" of QPF...and this doesn't yet account for rainfall received after 00Z Saturday. The area of higher concern would appear to be along or east of I-77, and confidence is low whether the highest totals will even be within our forecast area. Will need to wait for the 12Z CAMs to fully capture the event. An additional round of rainfall may take place on Saturday afternoon, though models remain uncertain where the frontal boundary will be by this point in time (those which develop a mesolow also tend to punt the boundary farther southeast, faster, resulting in a drier forecast for our area Saturday...while those which do not produce a mesolow allow the front to linger longer for another round of convection in our area).

Key message 2: Hot and muggy weather persists through Friday. Over the weekend, a cold front will usher in a cooler air mass, and more seasonable temperatures will persist into next week.

The core of the subtropical ridge is now located well to our west, with a 597dm upper anticyclone centered over the desert Southwest, and weak cyclonic flow actually in place over the Deep South and lower Mid-Atlantic today. Nonetheless, we remain within a fairly warm and moist air mass, with dewpoints currently in the low- to mid- 70s. Whatever benefit we reaped from Monday night's cold front have largely been eroded, and we are again under the gun for triple digit heat index this afternoon, and again on Friday.

Guidance is in good agreement that the next two days will feature weakly warm-advective SW flow in the low levels the next two days, which will help maintain soupy dewpoints and hot afternoon temps. To wit, today's heat index values are expected to climb into the lower 100s across the Upstate and I-77 corridor, and the upper 90s or around 100 across the remainder of the Piedmont and Foothills. On Friday, afternoon heat index values will again surge into the lower 100s, with some locations along and south of I-85 potentially climbing to 105 or greater. Confidence is limited, but can't completely rule out the need for a Heat Advisory on Friday afternoon. This will depend heavily on the placement of a frontal boundary which may stall across the area, producing widespread showers and storms the second half of the day; the exact placement of the front and coverage of convection will determine whether the above heat index forecast works out.

By Saturday, the arrival of a cooler postfrontal air mass should help keep afternoon temperatures in check. At least a couple days of cooler, more suppressed weather appear likely for the start of the new workweek, but by Tuesday long-range ensembles begin to diverge significantly. Despite little clarity on what the large- scale pattern will look like (and thus what the convective/rainfall forecast will look like Tuesday and beyond) it at least appears that most solutions keep us more seasonably cool, with highs in the upper 80s or lower 90s through mid-week.

AVIATION /06Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: A deck of mid- to low-level cloud cover located over the southern Blue Ridge and upper Savannah River Valley remains in place tonight, but continues to shrink and shows no signs it'll impact any of the TAF sites tonight. Expect generally VFR conditions with light and variable winds overnight, with only limited chances for valley fog/stratus this morning, and then a rebound in S/SW winds during the daylight hours Thursday for all sites (except KAVL, which will see its winds channeled out of the W/NW as is typical). While some afternoon convection can't be ruled out today, mainly over ridgetops...think that the best TS chances will be late in the afternoon or into the evening, as an upper-level disturbance approaches the area. With little consistency in hi-res models at this stage, opted for PROB30s at a few sites in the Piedmont and mountains; confidence is lower for the Upstate sites.

Outlook: Daily shower and thunderstorm chances will continue on Friday and possibly Saturday. By Sunday, a drier pattern should arrive. Mountain valley fog and low stratus will be possible each morning.

GSP WATCHES/WARNINGS/ADVISORIES

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