textproduct: Greenville-Spartanburg

SYNOPSIS

Temperatures remain unseasonably warm today. Widespread rainfall ongoing will continue through today and into tonight, until a cold front pushes through the area and dries us out. Temperatures return to near- or even below-normal Sunday into early next week under building high pressure.

NEAR TERM /THROUGH TONIGHT/

As of 134 AM Saturday:

Key Message 1: Widespread showers and few embedded thunderstorms through late afternoon, with a non-zero threat for a severe thunderstorm across northeast GA and Upstate SC this afternoon.

Deep moisture and forcing will continue to work together to produce a wide band of shower activity through daybreak, streaming up from the SW and riding along/ahead of a SW-to-NE oriented boundary to our west. There could be a few embedded isolated thunderstorms within this broad area, reference the recent lightning flash in western Elbert County GA. There are two minor concerns as this activity slowly translates eastward with the boundary today.

First is the threat for excessive rain. Note how the pertinent Excessive Rainfall Outlook marginal risk area has been gradually shrinking as evidence of the low chances of getting enough rain to cause problems. Rainfall rates thus far have remained eminently manageable and our QPF remains in the 1-1.5 inch range through tonight. With as dry as it has been it is hard to imagine this rain being anything other than beneficial and the amounts are unlikely to produce flooding, but elevated stream flows can be expected, particularly in the SW-facing slopes. We will continue to monitor.

Second is the threat for an isolated severe thunderstorm later today. Buoyancy will remain minimal and elevated through late morning, so anything more than an isolated storm is not expected. However, there will be a window of opportunity from late morning through late afternoon when some weak sfc-based CAPE could develop along/S of the I-85 corridor, perhaps in the 100-300 J/kg range. Meanwhile, as a 40-50kt low level jet moves overhead, hodographs will elongate and upwards of 40 kt of effective bulk shear could develop. This puts us in the realm of the "non-zero threat" mainly for a brief weak tornado. Note there is much more working against this than for it at this time, and of note is the 00Z HREF which fails to develop any rotating updrafts or interesting reflectivity structures in any of the CAM output. Either the unfavorable alignment of the shear being parallel with the boundary or the lack of buoyancy will be the limiting factor, or both. This will also be monitored for signs of model guidance picking up a favorable trend.

Key Message 2: Breezy conditions expected with a cold front passage east of the mountains this afternoon and in the wake of the front tonight across the mountains, though it should fall short of Advisory criteria.

Not much change to earlier thinking with the wind speeds with our frontal passage. Unless we get more convection to bring the stronger winds to the sfc this afternoon, the gusts will fall well short of Advisory criteria. Tonight, with the cold advection behind the front, winds will pick up across the mtns but again should stay below criteria at all but the highest peaks.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/

As of 1136 PM EST Friday...

Key Message #1: Breezy conditions on Sunday behind a departing cold front, resulting in low-end fire weather concerns.

By Sunday morning, the cold front should be located solidly to our east, with low-level CAA underway across the forecast area. This will contribute to widespread gusts in the 25-30kt range through at least Sunday afternoon, when the core of postfrontal CAA slides through the region. Across the high terrain, where little to no mixing will be required to tap into strong flow in the 925-850mb layer, gusts could surge as high as 40kts, toying with Advisory Criteria but not quite getting there at most locations. These gusty conditions will intersect rapidly-drying dewpoints as postfrontal air digs into the area...and RH is expected to fall below 35% Sunday afternoon. RH is known to tank in localized areas in such regimes, so lower values certainly can't be ruled out; however, given Saturday's rainfall (see near term discussion above) it appears unlikely that fuels will reach critical moisture...so fire danger appears limited at this time. In any case, as CAA tapers off Sunday evening, so too will gusts.

A secondary, related issue will be the potential for NW flow snow flurries the first part of Sunday, mainly across the NC-TN border. Deterministic guidance indicates that only a fairly shallow layer of moisture will remain in place past 12z Sunday...mainly north of the French Broad Valley. Given the strength of low-level CAA, one would expect this moisture to get scoured out fairly quickly...with flurries over by late afternoon. Little to no accumulation is expected.

Key Message #2: Dry and cold conditions persist Sunday night and Monday.

Lingering, weakening CAA will continue into Sunday night as progressive surface high pressure expands across the U.S. Southeast. Lows should drop to 1-2 categories below climo Sunday night (in the mid-20s for the low terrain, and into the upper teens at higher elevations) before only back into the upper 40s across most of the area on Monday. Excellent daytime mixing should develop Monday afternoon, tapping into a deep dry layer aloft; however, tame winds across the entire area should limit fire danger issues.

By Monday evening, into the overnight, the center of...increasingly ill-defined...high pressure will shift to our east; as a result, we'll find ourselves back in a SW flow regime with subtle low-level WAA developing overnight and prompting an increase in low-level thickness. As a result, lows Monday night should be up to a category higher than on Sunday night...but still below normal.

LONG TERM /TUESDAY THROUGH FRIDAY/

As of 1229 AM EST Saturday...

Key Message #1: Quiet conditions expected through mid-week.

One more day of borderline/critical RH is expected Tuesday, but again sans much of a breeze fire weather issues should be limited. In general, a strengthening SW flow / WAA regime is expected Tuesday and Wednesday, as the upper pattern again becomes amplified across the eastern CONUS...with a series of z500 waves developing within the northern stream and digging across the Midwest into the region through Tuesday night. In general, moisture and cloud cover should increase during this time, but little if any rainfall can be expected through mid-day Wednesday.

Key Message #2: Active weather returns the second half of the week, with unusually low predictability.

The forecast gets increasingly difficult-to-parse by Wednesday night. Guidance depicts the presence of a pre-existing coastal front just off the SC coastline, even while multiple upper vort lobes phase together and prompt the development of a deep, high- amplitude trough axis over the central Ohio Valley. As this feature migrates eastward, it should interact with the surface boundary; virtually all the long-range ensembles depict some flavor of cyclogenesis on Thursday. The consensus, however, is that this will take place a tad too far east to get much good moisture return into the area (which is to say that we'll be on the dry side of this system)...severely limiting winter precip potential, except in the mountains, where colder temperatures will lend themselves to accumulating snowfall. Generally think accums will stay below advisory criteria, though the usual spots in the Smokies could get close. Depending on exactly how far east the coastal low sets up, and how quickly the upper trough axis crosses the area, there's potential for another round of flurries across the northern third of the CWA, mainly along and north of I-40, on Thursday, as depicted in a few GEFS and CMC-GEPS members.

An important takeaway here is that there's still quite a bit of room for change. With such an active, unsettled synoptic pattern, and still some 5 days to go until we get there, it's very difficult to pin down important details at this stage.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Variable vis and ceiling will be the rule through the remainder of the night and the better part of the morning hours as moisture lifts over a weak boundary located inconveniently close to I-85. That puts several terminals in a position where vis and ceiling could come in-and-out from LIFR to the upper end of MVFR depending on heavier showers. There will also be brief VFR holes, but VLIFR seems unlikely. The TAFs will attempt to account for the prevailing condition being IFR or LIFR one way or the other by daybreak or so. Sfc winds will be light/variable, so improving flow above the sfc could introduce some LLWS for the start of operations. Eventually this morning, the main front will cross the mountains and move east across the fcst area, so the variable restrictions are likely to continue into the early afternoon. It is not clear that we will get much in the way of thunder, so that was included only at KCLT which has the best shot of seeing the necessary instability. The front will bring stronger SW sfc winds with occasional gusts. The recent guidance is more optimistic with lifting the ceiling to MVFR this afternoon once the front moves in, so this improvement was indicated in the new fcst, but it is possible that IFR could linger through the afternoon at some locations, particularly KHKY which is typically the last place to lose the lower clouds. Note that the passage of the front will bring the wind around to more W to WNW this evening at some point, but some details were left out.

Outlook: Flight restrictions should end Saturday night but gusty winds continue through Sunday night. Dry high pressure will bring VFR for Monday and Tuesday. A cold front could bring precipitation and restrictions Wednesday.

GSP WATCHES/WARNINGS/ADVISORIES

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