306 FXUS62 KGSP 300007 AFDGSP Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 807 PM EDT Thu Jun 29 2023 .SYNOPSIS... Increasing heat and humidity is expected Friday into the weekend as high pressure dominates the Deep South. Shower and thunderstorm chances return to the area Friday and persist into Sunday as several waves of energy move from the Ohio Valley into the southern Appalachians. Chances for showers and thunderstorms will continue into early next week as a cold front approaches the region, bringing some relief from above-normal temperatures. && .NEAR TERM /THROUGH FRIDAY/... As of 6:05 PM Thursday: We remain mostly dry across the CWA early this evening with a fairly broad area of showers just to our west moving southward over southeast TN and northern GA. The RADAR is beginning to show some light returns over the French Broad Valley, but most of this precip is likely not reaching the ground. We've also got several sites that continue to report reduced visby with haze (HZ) from the Canadian wildfires burning well to our north. Otherwise, a robust upper ridge remains centered to our west with an upper low centered over southern Ontario. Stuck in the middle is a conveyer belt-like feature setting the southeastern portion of the CONUS for multiple rounds of thunderstorms and convective complexes firing off from the active jet north of the high pressure. Thunderstorm activity is already becoming more active across Indiana/ Tennessee as two MCSs are ongoing with the more eastern one diving S/SE. As for how these systems could affect the CWA still remains uncertain and predictability low. Guidance from recent CAMs is inconsistent with convection and movement into the area this evening and overnight hours. The latest runs indicate the first MCS quickly dissipating in TN, while the second, more stout system reaches the NC mtns and falls apart. As of now, a Marginal Risk for thunderstorms is in place from SPC across the far western NC mtns, which accounts for the more southern progression of the first MCS track. For now there are no mentionable PoPs through this evening, with an uptick to slight chance across the NC mountains after midnight. However, with the disagreements in guidance and given the continuing haze in the boundary layer, confidence remains low as to whether or not the CWA receives any convection tonight. Additionally, temperatures will likely struggle to reach the 90 degree mark as high cirrus from the first MCSs' anvil moves into the fcst area, along with the high amounts of haze near the surface. As for tomorrow, the SPC has a Slight risk for thunderstorms across the NC mountains and a Marginal Risk for the remainder of the CWA. Parameters are more favorable for strong to severe thunderstorms approaching the area by Friday night and spilling over into the short term. As of now, convective parameters ahead of a possible MCS include stout mid-level lapse rates approaching 8.0C/km, 2000-2500 J/kg of SBCAPE, and 35 to 40kts of SFC to 6km shear. Conditions, as previously mentioned, will be more favorable for strong to severe thunderstorms toward the end of the forecast period. Temperatures Friday will likely reach the 90s for most of the area and set the stage for warmer temps and potentially triple digit heat indices into the holiday weekend. && .SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/... As of 230 PM EDT Thursday: At the beginning of the period Friday night, the region will remain underneath northwest flow aloft as the upper-level heat ridge remains anchored near the central Gulf Coast. The overall pattern and mean flow, combined with ample instability and forcing over the Midwest and Ohio Valley, continues to support the development and propagation of upstream convection into or near the southern Appalachians. Depending on the various and ever-changing CAM solutions, once such system may be entering or impacting our area around the beginning of the period. As such, PoPs reflect the possibility of showers and storms from Friday evening into the overnight hours. It is important to stress, however, that at this time we cannot predict with any degree of certainty the existence, timing, or intensity of organized convection and it will be important to stay weather aware as our understanding of the threat evolves. Nevertheless, the environment supports robust convection upstream and if this convection can sustain itself in a somewhat less unstable airmass over the mountains and especially the Piedmont, the potential for strong to severe storms is notable. As such, SPC has a Slight Risk for severe thunderstorms in place through Saturday morning for the western NC mountains as any organized thunderstorm clusters could produce damaging wind gusts. Saturday's weather will be dependent on the presence and evolution of nocturnal convection. The presence of ongoing convection, a residual cold pool, or convective cloud cover debris may impact diurnal heating and the recovery of the airmass for afternoon shower and thunderstorm development. However, absent of these complications, the overall airmass supports slightly warmer high temperatures and higher dewpoints for the daytime hours, and we continue to advertise heat index values into the lower 100s across much of the SC Piedmont and the Charlotte metro. As has been discussed previously, the mixing out of dewpoints in the afternoon tends to be underdone in guidance blends, which means we may see lower dewpoints to suppress heat indices somewhat. In the end it won't matter much because it will still be oppressively uncomfortable, but we may be able avoid widespread Heat Advisory criteria (>104). The heat and humidity continues to support significant SBCAPE values and above-climo PoPs over the mountains, with PoPs decreasing to slight-chance over the lower Piedmont, where mid-level warm air is still expected to suppress activity. Saturday evening into Sunday, the upper-level anticyclone begins to weaken slightly in response to a potent shortwave trough over the central Great Plains. The guidance is not in agreement about the timing of this shortwave energy which may determine whether or not we will experience at least one more night of northwest flow and MCS potential Saturday night into Sunday morning. The same threats and uncertainties mentioned previously apply here as well. Sunday still appears to be the hottest day of the 7-day forecast period, with highs in the mid 90s east of the mountains. There will be not much change in heat index expectations on Sunday and confidence in reaching Heat Advisory criteria remains low. Similar atmospheric profiles compared to Saturday maintain above-climo PoPs over the mountains with decreasing PoPs east. Bulk-shear values are somewhat lower for Sunday afternoon, which may reduce the chances for organized convection. && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... As of 300 PM EDT Thursday: A potent shortwave trough will be swinging through the lower Great Lakes region at the beginning of the period which will suppress the infamous upper-level anticyclone to the Gulf Coast. With the attendant ridge axis to the east of the region, the upper-level mean flow will therefore be southwesterly, as a cold front enters the Tennessee Valley. Therefore, while this flow will end the threat of upstream MCS propagation for the overnight period into Monday morning, the pattern will remain unsettled as we begin the work week. The height falls associated with the approaching trough will improve mid-level lapse rates across the area such that instability will improve over the Piedmont beginning Monday afternoon, with likely PoPs over the mountains. The deep-layer southwesterly flow will sustain the heat and low-level flow around the anticyclone to our south will support continued moisture advection into the region. Therefore, despite the potential for increased cloud cover from shower and thunderstorm activity, high temperatures and heat indices only decrease a few degrees or less. Thereafter, the average upper-level pattern becomes fairly stagnant over the Southeast with a trough axis roughly extending from the central Gulf Coast to the Northeast as a heat ridge over the Desert Southwest and subtropical ridging over Florida persist through the end of the period. There will be shortwaves moving through this mean flow throughout the period, while the aforementioned cold front slowly approaches the area. The cold front will enter the area later Tuesday into Wednesday, at which point it may slowly dissipate near or over the western Carolinas. Therefore, each day we are advertising above-climo, diurnal PoPs area wide, but highest over the climatologically favorable mountain zones. Bulk shear remains below 20-25kts generally each day; therefore, while coverage may be robust, storm severity is not expected to be at this time. Given the proximity of a weak front and somewhat lower thicknesses due to the upper-level pattern, high temperatures gradually revert to near-normal by the end of the period. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... At KCLT and elsewhere: Expect VFR conditions to prevail thru most of the 00z taf period. KCLT and KHKY continue to observe MVFR visby with HZ from the Canadian wildfires to the north. It's difficult to predict when the haze/smoke will dissipate and for exactly how long, but I suspect that visby will return to VFR by 06z or so. Areas of patchy fog could develop again over the mtn valleys overnight and into the morning, but my confidence is fairly low that KAVL will see any restrictions from fog. Confidence also remains low on the likelihood of any widespread showers overnight and into the morning, but some of the near-term guidance still generates sct showers across the area between roughly 10z to 14z tomorrow morning and another round of showers and thunderstorms tomorrow aftn/evening. Winds will remain light and vrb thru the overnight and well into the morning at most sites. They will come back up from the S to SW tomorrow aftn. Outlook: Chances for showers and thunderstorms in addition to early morning fog and low stratus will increase on Friday and continue thru the weekend. && .GSP WATCHES/WARNINGS/ADVISORIES... GA...None. NC...None. SC...None. && $$ SYNOPSIS...JMP NEAR TERM...CP/JPT SHORT TERM...JMP LONG TERM...JMP AVIATION...JPT