textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Some adjustments to the morning forecast due to gravity wave having caused showers to develop over SW NC and NE GA.

KEY MESSAGES

1. A few light showers possible over the North Carolina mountains, northeast Georgia, and the western Upstate this morning. Thunderstorms return across the area 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: A few light showers possible over the North Carolina mountains, northeast Georgia, and the western Upstate this morning. Thunderstorms return across the area 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.

A remnant of an MCS persists over TN. A gravity wave ejected from that feature overnight and appears to be the main driver for weak convection near the intersections of TN/NC/GA/SC, within the existing stratus layer. Behind the wave front stratus have mostly cleared out over AL and NW GA, so any shower activity this morning would appear to be short-lived either in the wake of the wave or as modest diurnal warming erodes the cloud deck. There probably will be a window of clear skies in the late morning to midday period during which we should destabilize further. It had appeared the MCS debris might ride over the area from the west and keep us stable through most of the day, so some revision to PoP trends may be warranted as the day goes on; eager to see the 12z cycle CAMs once they come in.

The expectation had been that by mid- to late-afternoon, an shortwave embedded in the longer wave trough centered over the upper MS Valley should push a 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 /13Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR on tap again today. Still expect 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. Confidence is improving on scattered to numerous showers and storms developing over the NC sites this evening, continuing well into tonight, and producing gusty winds and some near-storm flight restrictions.

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.