textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Rainfall amounts trended up slightly with minor fluctuations in temperature for the weekend.

KEY MESSAGES

1. Dry with above normal temperatures today. An isolated thunderstorm is possible over the Smokies this evening and overnight. 2. At least a A Marginal Risk of excessive rain on Saturday and Sunday, which could lead to increased potential for localized flooding. However, signals are mixed yielding low confidence. 3. Cool and dry starting Tuesday, with a warm up back to normal by the end of the week. There will be a small chance of frost in some mountain locations early Wednesday morning.

DISCUSSION

Key message 1: Dry with above normal temperatures today. An isolated thunderstorm is possible over the Smokies this evening and overnight.

One last seasonally warm and dry day is expected, courtesy of high pressure at the sfc and aloft centered off the coast of the Carolinas. The ridge should allow for temps to climb a degree or two warmer than Thursday, so highs in the 85 to 90 degree range are a reasonable expectation. This would be on the order of ten degrees above normal, but nowhere near records. Alas, the upper anticyclone should be on a weakening trend by the end of the day as it drifts to the south and flattens. A few showers or an isolated thunderstorm could develop this evening over southwest NC and the Smokies, lasting into the overnight hours, but the chance of severe weather is very low.

Key message 2: At least a A Marginal Risk of excessive rain on Saturday and Sunday, which could lead to increased potential for localized flooding. However, signals are mixed yielding low confidence.

By Friday night, a shearing short wave will have flattened the old upper ridge and should be helping to drive a sfc cold front down from the north, but still north of the fcst area at around daybreak Saturday. It is the timing and the southward progression of this boundary that has injected uncertainty into the weekend forecast. A relatively weak sfc high moving from the Great Lakes to northern New England should act to push this boundary through as a back-door cold front on Saturday, but this might not happen until late in the day. The timing on the afternoon ensembles is still all over the place, but eventually there is confidence the boundary will move south of the fcst area by daybreak Sunday, setting up a weak cold air damming wedge for Sunday that drains into Monday. Either way, eventually precip will develop across the fcst area as deep moisture moves up from the south and forcing arrives from the north, so high precip probs are not really in doubt. Neither are the cool temperatures by Sunday, with widespread precip and cloud cover keeping temps on the order of 10 to 15 degrees cooler than normal. Quite the change in 48 hours over the weekend.

The main problem is the QPF and the potential for excessive rainfall. Pattern recognition supports the idea that we should have an elevated risk for heavy rain over the weekend. A weakness aloft between the big mid/upper anticyclone centered over the Four Corners region and the weakening ridge off the Southeast Coast will support a moisture plume from the Gulf and high precipitable water above 2 inches over at least the southern half of the forecast area. However, the plume of strong Integrated Vapor Transport currently curling around to our west and north is forecast to weaken considerably by the time it moves this far east. The mid/upper forcing is also not terribly impressive either, though low level forcing should be good assuming we have a full blown wedge east of the mtns by Sunday morning. The official forecast QPF creeps upward in many spots, while the model ensemble QPF creeps downward slightly in many spots. In short...mixed signals yield low confidence in rainfall amounts. In this case, the Marginal Risk on the Excessive Rainfall Outlooks for Saturday and Sunday look like a good starting point and once more of the event is covered by the CAMs, that will better inform the trend. In the mean time, we keep an eye on it.

By Monday morning, the wedge should be draining away as the parent high is lost with another weak front coming down from the NW. There could be some shower activity over the west side of the mtns with this front moving thru. Temps will be tricky, but a rebound back to normal is the most likely outcome.

Key message 3: Cool and dry starting Tuesday, with a warm up back to normal by the end of the week. There will be a small chance of frost in some mountain locations early Wednesday morning.

The upper low moving east from Quebec to the Maritime Provinces on Tuesday should drive the broad axis of an upper trof east of the fcst area by Tuesday, allowing for cool and dry high pressure to build in from the upper Midwest and Great Lakes. This air mass will be seasonally cool, with temps generally running around ten degrees below normal for Tuesday and Wednesday. The only minor concern will be lows early Wednesday morning, with 30s common across elevations above 3500 feet. This could be cool enough to allow for some patchy frost potential in sheltered high elevation locations, but confidence and likelihood are low. We stay under a broadly cyclonic flow for the remainder of the period, but the air mass modifies enough to get temps back to normal by the end of the week.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: A relatively moist SW flow is resulting in an area of gradually expanding area of VFR stratocu along the southern escarpment of the Blue Ridge early this morning. Some of this could briefly waft over KAVL/KGMU/KGSP, but should generally remain just W/S of these terminals. Otherwise, the main concern will revolve around the potential for fog/low stratus in the mountain valleys later this morning. Signals are mixed in this regard in terms of KAVL, with uncertainty about the evolution of the higher-level stratocu being one of the limiting factors. Nevertheless, conditions are such that brief tempo LIFR conditions are warranted between 10-12Z. Otherwise, VFR is forecast through the period, with FEW/SCT cumulus in the 035-050 range expected this afternoon. Can't completely rule out a stray shower or perhaps a storm across the mountains this afternoon, but even this small chance will mainly be limited to locations west of KAVL. Calm or light/variable winds are forecast to become SW at 5-9 kts late this morning through the afternoon.

Outlook: Widespread showers and associated restrictions develop ahead of a cold front this weekend. Dry and VFR conditions return behind the front early next week.

GSP WATCHES/WARNINGS/ADVISORIES

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