textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

More details to differentiate the possible morning convective activity from the main, more likely severe round this afternoon.

KEY MESSAGES

1. Any thunderstorms developing ahead of a cold front today and this evening will pose a risk of severe weather, though confidence remains low on how much of the area will see thunderstorms develop. The main hazards with any severe storms are damaging winds and large hail. Make sure you have multiple ways to receive warnings. 2. Hot and humid today. A Heat Advisory remains in effect for northeast Georgia from noon to 8 PM. Stay hydrated, limit time outdoors and spend as much time in air conditioning as possible. Never leave people or pets alone in a closed car. Look before you lock! 3. Less humid, drier, and cooler behind the front Wednesday through Friday. Even cooler conditions expected over the weekend with shower and thunderstorm chances returning.

DISCUSSION

Key message 1: Any thunderstorms developing ahead of a cold front today and this evening will pose a risk of severe weather, though confidence remains low on how much of the area will see thunderstorms develop. The main hazards with any severe storms are damaging winds and large hail. Make sure you have multiple ways to receive warnings.

We'll have more day in the active, northwest-flow "ring of fire" pattern we have been locked into since late last week. An MCS decayed late Mon evening over KY and some of its cirrus shield is advecting over the area now; aside from a few remnant showers over the NC mountains, it appears unlikely precip will develop in our area before daybreak.

A well defined shortwave and surface low now between Lakes Michigan and Erie is associated with a cold front which will track through the Ohio Valley early this morning. Near the shortwave, a couple clusters of storms are already ongoing, and that activity could congeal into an MCS which would primarily track to our north. However, preceding the cold front, a strong LLJ is promoting WAA near the Ohio River, and as the front approaches conditions appear favorable for a new round of storms to fire on the southwest flank of the MCS, propagating southward ahead of the actual front. There is a chance those storms could survive into the NC mountains during the morning, though CAMs appear to be trending away from the idea of them growing into a line as some earlier runs (as recently as the 00z NAMNest and HRRR) suggested. Even if the cells are weak or discrete the NBM PoPs appeared too low to reflect their possibility. Thus, PoPs have been raised over western NC and adjacent areas of SC/GA through midday. Anomalously strong flow in the 700-500mb layer leads to effective shear perhaps exceeding 40 kt along/west of the Appalachians this morning, and 2000 J/kg or more of MUCAPE may be present. Thus there would appear to be some risk of organized severe weather with this morning activity, though the northerly shear vectors do suggest updrafts may remain discrete and could develop into supercells.

Cloud cover associated with the prefrontal WAA, and/or cirrus debris from the morning storms, still could inhibit convective development later in the day, though without a strong cold pool from an MCS it's hard to see that completely inhibiting enough diurnal destabilization to preclude a second round of activity nearer the actual cold front, this afternoon and evening. PoPs are lower in the afternoon for the southwest quadrant of the CWA, where the morning activity (and arguably strong downslope flow) has the most potential to keep things quiet. PoPs are highest in the afternoon along the TN border for upslope, but also in the eastern Piedmont south of I-85, where morning activity is likely to have the least effect. However we will have to monitor convective trends and surface obs throughout the morning to evaluate the likelihood of the second round taking shape. By late afternoon or early evening, assuming no inhibition, 2000-3000 J/kg of SBCAPE still may overlap with 40+ kt of northwesterly 0-6km bulk shear (and strongly curving hodographs). As the cold front looks to advance SE'wd through the NC Piedmont accompanied by the shortwave, discrete supercells remain the favored mode there. Damaging winds and large hail perhaps approaching 2" in diameter are certainly on the table this afternoon, but 0-1km helicity exceeds 100 m2/s2 in some areas and a tornado cannot be ruled out. Farther west, the frontal orientation is more E-W and a more linear mode may result, though still with wind/hail risk from embedded supercell structures. SPC Day 1 outlook remains Slight Risk for virtually the whole CWA, but with the probabilistic contours more suggestive of the greatest threat being in our eastern zones.

Key message 2: Hot and humid today. A Heat Advisory remains in effect for northeast Georgia from noon to 8 PM. Stay hydrated, limit time outdoors and spend as much time in air conditioning as possible. Never leave people or pets alone in a closed car. Look before you lock!

Still expecting today to be the hottest and most humid day this week. Heat index will flirt with 105 in a few scattered locations in the lower Piedmont; the contiguous area of 105+ heat indices in a portion of NE GA warranted issuance of a Heat Advisory there. Considered expansion into some counties in the SC Lakelands, but there is too much uncertainty about how morning convection may affect temperatures there and elsewhere in the Piedmont for that matter.

Key message 3: Less humid, drier, and cooler behind the front Wednesday through Friday. Even cooler conditions expected over the weekend with shower and thunderstorm chances returning.

Behind the front, temperatures trend back to around climo on Wednesday, with lower humidity that should keep indices below 100, and much more stable conditions such that precip chances remain near zero, except for the TN/NC border area where upsloping could kick off a shower or two. Appreciably drier air filters in for Thursday that brings temps slightly lower but heat indices more noticeably lower. Similar conditions last through Friday. The Upstate and NE GA, being warmer, actually see RH fall to around 35% Thursday and Friday, which is exceptionally low for this time of year. A few spots with enhanced mixing could go even lower.

The Western ridge / Eastern trough pattern lingers into next weekend, though not as unstable upstream and MCS threats do not look to recur. Precip/storms may return Saturday, when a reinforcing shortwave or upper low rotates into the Eastern trough and reactivates a warm front over the Southeast; PoPs are now likely for Sunday. The 00z GFS and ECMWF actually seem to occlude the low to our west by Sunday, whereas the Canadian GDPS maintains an open wave, which is more similar to earlier solutions. Thus confidence and PoPs decrease Monday given the spread in the speed of the low pressure system. Temperatures do remain mild, likely at least a little below normal.

AVIATION /07Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: Cirrus debris from decaying convection upstream should inhibit radiation and keep fog at bay at the TAF terminals this morning; some rural/valley locations have already experienced brief restrictions due to fog but that should become more isolated as the higher clouds move in. Some low VFR stratus also is forming in some locations in the present warm advection regime and that should have the same effect. Winds will be light/VRB thru daybreak but will pick up from the SW 13-16z with diurnal mixing (exc. NW at KAVL) and gusts of 20 kt are expected to be common around the area by midday. A cold front will approach from the north through the morning and conditions are favorable for storms to form to our north and west during the morning. It is possible another round of cirrus debris from said activity will advect over the terminals late morning to midday, inhibiting the development of storms in the afternoon or evening. Included VCSH and an uptick in cloud cover during the morning for the debris effects during the morning. In turn, confidence remains relatively low on the potential for afternoon and early evening storms, so PROB30s are still included at that time; may not have confidence to go TEMPO until 12z or later AMDs. If the morning clouds don't inhibit activity, it appears storms may fire earlier than usual, so the PROB30s have been shifted a bit earlier than on the previous set of TAFs. Environmental factors suggest if those TSRA do form, they will easily produce strong outflow winds and torrential downpours, and pose a threat of localized damaging winds and greater than normal hail potential than most storms in this area this time of year. The PROB30s now feature VRB gusts to 30 kt and IFR vsby. Activity should shift out of the region this evening as the front passes, and winds shift to NW at all sites behind the front. A few SHRA could develop near KAVL overnight due to upslope flow into the mtns. Post-frontal drying could inhibit fog formation tonight but can't rule out some shallow fog after 06z especially where heavy rain falls today.

Outlook: Dry and mostly VFR conditions return Wed thru Fri, with shower and thunderstorm chances and their associated restrictions returning over the weekend. Low stratus and/or fog will be possible each morning, mainly in the mtn valleys.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Heat Advisory from noon today to 8 PM EDT this evening for GAZ018-026-028-029. NC...None. SC...None.