textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The aviation forecast was updated to reflect the recent 00Z TAF issuance.

KEY MESSAGES

1. Following one more dry day Wednesday, rain chances increase for Thursday and Thursday night as remnants of Tropical Storm Arthur cross the area. Widespread rainfall and embedded thunder are expected; instances of locally heavy rainfall and damaging wind cannot be ruled out. A cold front will bring an end to the rain by late Friday. 2. Drier conditions settle in over the weekend, but the unsettled pattern continues into next week.

DISCUSSION

Key message 1: Following one more dry day Wednesday, rain chances increase for Thursday and Thursday night as remnants of Tropical Storm Arthur cross the area. Widespread rainfall and embedded thunder are expected; instances of locally heavy rainfall and damaging wind cannot be ruled out. A cold front will bring an end to the rain by late Friday.

All quiet across the region this evening as weak quasi-zonal flow aloft continues, and a nose of high pressure across the area generally keeps the atmosphere suppressed. Though observed profiles do not exhibit a true inversion, lapse rates are paltry, and sans any good upper forcing, mostly clear conditions will persist through the overnight into early Thursday.

The bigger story remains the influence of Tropical Storm Arthur on our forecast the next two days. Water vapor imagery places Arthur off the coast of southern Texas, and moving slowly northeastward. Over the next 24 hours, operational guidance is in good agreement that it'll lift across the Deep South, arriving in the western Carolinas by late Thursday. On its leading edge, most guidance still depicts a strong 35-45kt LLJ on the 850mb charts...resulting in windy conditions across most of the area for much of Thursday. The probability of reaching true wind advisory criteria remains low, but forecast gusts of 30kts or so are still very unusual for mid June.

By 00Z Friday, ensemble consensus places Arthur's remnants either over the GSP CWA or just to its west. While prior forecasts suggested possible interaction between Arthur and an approach shortwave trough dipping out of the Midwest, guidance increasingly suggests the more potent interaction will be between Arthur and a remnant frontal boundary located to our south. Most of the available hi-res models depict a decaying MCS tracking across central Georgia on Thursday afternoon, arriving in the southern reaches of the Upstate that evening...and providing the first impetus for severe weather. Whatever convection makes a run at the area will indeed find itself in a fairly limited environment...featuring modest deep-layer shear and very limited low- level shear. This would primarily manifest as an isolated damaging wind threat. The exception could be in the immediate vicinity the frontal boundary, where low-level shear profiles may be amplified and, combined with the focusing effect of the boundary, contribute to a somewhat higher severe weather threat. However, ensembles broadly indicate the front will remain to our south through the duration of the event...so this should have limited effect on us in the Upstate and Piedmont

Otherwise, expect widespread rainfall as the wave passes overhead. It remains a little unclear where the greatest rainfall totals will be focused. Several operational members depict the formation of a compact mesolow over the Upstate, perhaps in association with the decaying MCS referenced earlier. Operational guidance generally agrees on a corridor of 2-3.5" of rainfall dropping somewhere south of I-85...but continues to disagree on its placement. Given the increased likelihood for Arthur to interact with the pre-existing frontal boundary, and not the Upstream shortwave, it looks more likely that the heaviest rainfall won't occur over the NC mountains, but rather somewhere in the Upstate. A number of model sources even bring the axis of heaviest QPF south of the CWA entirely. Confidence in the bigger picture (that there will be an axis of heavier rain somewhere in SC) is improving, but confidence on the details (exactly where that axis will set up) remains low.

By mid-day Friday, what remains of Arthur's mid-level trough will pivot to our east. The approaching shortwave will finally push a reinforcing cold front into the area by early afternoon Friday, and guidance is in decent agreement that increasingly dry conditions will persist the rest of the day as weak CAA and dry advection set in.

Key message 2: Drier conditions settle in over the weekend, but the unsettled pattern continues into next week.

High pressure will settle into the area on Saturday, bringing an end to rain chances and ushering in a period of quieter weather that should persist through Sunday. Temperatures will be near-normal through the weekend...meaning highs in the upper 80s to lower 90s in most locations. No convection is forecast.

By Sunday night...with the remnant front and associated baroclinic zone still to our south...we'll come under the influence of another upper disturbance, a trough tracking out of the central CONUS. It's unclear exactly when the best forcing will intersect the baroclinic zone at the surface. Further, as per some LREF members this trough could wind up being just the first in a series of waves rotating around broader troughing over New England and eastern Canada. Thus, expect most of next week to be active, if perhaps more likely to just be diurnal summertime convection than any more organized severe or heavy rain threat.

AVIATION /00Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR conditions and SW winds around 10 kts are expected through the evening across the Terminal Forecast Area. Winds are forecast to increase a bit and possibly become gusty late tonight at the upstate SC terminals. A brief period of low cloud development is again expected from the pre-dawn hours until shortly after sunrise, mainly across the southern half of the area. MVFR cigs are forecast at all sites except KAVL and KHKY at some point during that time frame. IFR is possible, although it doesn't appear likely at this time, but FEW/SCT layers in the 007-009 range are prudent in order to address that potential.

SW winds become gusty at all sites by late Thu morning, with gusts to around 25 kts expected at most sites during the afternoon. More unstable conditions are expected to result in diurnal convective development Thu afternoon and evening, with Prob30s for TSRA warranted at all sites between 18-24Z.

Outlook: A brief lull in convection is expected during the late evening and early part of the overnight Thu night. Additional convection is expected late Thu night into early Fri, as the remnants of Tropical Storm Arthur pass close to the area. Dry/convectively inactive weather will return by Fri night and continue through at least Sat night. Diurnal convection is expected to return early next week.

GSP WATCHES/WARNINGS/ADVISORIES

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