textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 12z TAF issuance and clarified some details about the light showers ongoing over the area.

KEY MESSAGES

1. The severe threat remains low through Monday before increasing Monday night into Tuesday, ahead of a cold front. Some storms could linger into early Wednesday. A warming trend is expected through Tuesday, with slightly cooler highs and lower humidity Wednesday into next weekend.

DISCUSSION

Key message 1: The severe threat remains low through Monday before increasing Monday night into Tuesday, ahead of a cold front. Some storms could linger into early Wednesday. A warming trend is expected through Tuesday, with slightly cooler highs and lower humidity Wednesday into next weekend.

Surface front looks to be oriented NW-SE across the CWA early this morning, extending upstream into the lower Ohio Valley. Aloft, a full-latitude trough is in place over the Eastern Seaboard and Great Lakes, with a deep, broad upper anticyclone situated over the Rockies/Plains. This pattern will leave us in a persistent NW flow regime aloft through the first half of the week, and multiple subtle embedded shortwaves or ripples of vorticity are expected to dive through the trough and over the southern Appalachian region, potentially enhancing the development of convective complexes, some of which may be nondiurnal in nature.

Today and Monday, forcing is rather ill-defined overall. Some shower activity is ongoing as of 7 AM with one of the embedded shortwaves and the LLJ acting together. Drier air will gradually filter into the area later this morning and should push the frontal moisture southward; the morning showers should diminish in part due to the diurnal weakening of the LLJ. Ridgetop convergence, and/or a lee trough at the surface, potentially will fire a few storms this afternoon even if the shortwaves aloft aren't a factor, so we retain 20-40% PoPs over much of the area downstream of the Blue Ridge Escarpment. 0-6km shear will be high for this time of year, 30-40 kt, with 0-3km shear 15-25 kt. Though instability will be limited by weak lapse rates, any storms that do fire may form into clusters, and if that were to occur they could pose a risk of damaging wind. The 00z NAM develops convection over the area after sunset which could be partially due to one of the embedded ripples aloft, but confidence is low on where that might happen if at all. The continued drying leaves behind dry and slightly capped profiles in place for Monday, which results in only 10-20% PoPs at best. Shear parameters will be similar but the severe threat during the afternoon would appear to be lower on account of lower instability. A few models have been hinting at a possible MCV or cold pool arriving Monday night from an upstream MCS, but even if that were to occur it would appear unlikely to survive over the Appalachians with the unfavorable thermodynamic profiles. SPC does hint at the possibility of an MCS reaching the far northern or western portions of the CWA before falling apart, by including those areas in a marginal risk on Day 2. Currently can't advocate for any more of the CWA being included than those areas.

A better defined shortwave will rotate into the trough on Tuesday and will bring a cold front southward toward our area. Assuming we are not still stabilized by a cold pool or debris cloudiness from Monday night, our MCS potential still looks greatest late Tuesday into Tuesday night: WAA ahead of the front should promote warmer daytime temps and deeper mixing which should overcome the cap, so we will see instability return to values more typical of a hot July afternoon, still with favorable shear and sfc-midlevel delta-theta-e values suggesting easy cold pool formation ahead of the front. Hodographs have enough curvature to expect a few supercells could form even if the MCS were not to pan out. Timing of the front suggests convective/severe threat could be greatest after peak heating, well into Tuesday night particularly in the southern CWA. Confidence is still limited partly due to not knowing how any cold pool from Monday night come into play. Day 3 outlook now features Slight Risk for the northern two-thirds or so of the area, with Marginal for the remainder. Temps Tuesday will be the warmest of the week, mostly in the lower to mid 90s across the Piedmont; heat index is likely to exceed 100 and may reach 105 in the urban areas in the I-85 corridor or southeast.

Temps trend back to near or slightly below normal Wed-Sat in the wake of the front, but perhaps of more interest humidity should be much lower with dewpoints falling into the lower 60s each afternoon. The ridge-trough pattern across the CONUS does not break down, however. The front will stall north of the Gulf Coast and potentially will be reactivated by the next shortwave or low diving past the anticyclone. A few diurnal showers/storms appear possible by Friday with chance PoPs for all areas on Saturday with the activating front.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Widespread cigs across the terminal area at daybreak, mostly at IFR, near stalled frontal boundary. A weak disturbance higher aloft, coupled with northwest flow into the Appalachians, has produced an area of SHRA within the cloud deck, with a couple embedded TSRA. Some areas which saw brief clearing in the predawn hours developed apparent radiation fog/stratus; with that and the SHRA/TSRA mixing out clouds we have a tricky morning forecast with rapid category changes likely. All sites except KAND start off with a TEMPO for category variations. Some slight improvement should occur over the morning as the precip diminishes or advects out and as downslope flow past the mountains promotes drying/mixing. Aside from KAVL which should stay NW, winds are also going to be tricky with very weak sfc flow, mixing into the NW winds aloft, but also expected formation of lee trough this aftn. A few diurnal SHRA/TSRA are possible at the Upstate sites and KHKY today, which could be gusty; lower chance at KCLT is handled with only VCSH. Some radiation fog/stratus is likely to develop again tonight but with low confidence not yet including any restrictions for Mon AM except for KAVL, due to valley location.

Outlook: Only isolated TSRA expected Monday afternoon/evening. Strong to severe thunderstorms, as well as breezy SW winds are possible Tuesday afternoon/evening ahead of a cold front. The severe storm threat could linger into early Wednesday, but confidence is low at this time. Low stratus and/or fog will be possible each morning, mainly in the mtn valleys.

GSP WATCHES/WARNINGS/ADVISORIES

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