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FXUS62 KGSP 071139 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 639 AM EST Thu Dec 7 2023

SYNOPSIS
Dry high pressure will continue the remainder of the week, leading to warmer temperatures through the early weekend. Rain will begin Saturday night and Sunday as a cold front crosses our region from the west. Thunderstorm appear likely on Sunday, then the rain may change over to light snow in the North Carolina Mountains Sunday night as the precipitation ends.

NEAR TERM /THROUGH TONIGHT/
As of 631 AM EST Thursday: A patch of orographic cirrus downstream of the Blue Ridge has been slowly migrating northeast up the mountain chain for the last several hours and should continue to move away from the fcst area. This has allowed temps to drop about 5-8 degrees below normal courtesy of a sfc high parked overhead at daybreak.

We should begin the day with a sfc high sitting right over the top of the fcst area. Fair and cool weather will continue for the next 24 hours. A slowly progressive upper ridge will take the sfc high pressure center and push it off the Southeast Coast by this evening. Weakening flow aloft as the ridge axis approaches should gradually shift the mtn wave cirrus farther north up the chain through the middle part of the day, so high temps shouldn't be greatly affected. Still going with a small rebound this afternoon, maybe a category or so warmer compared to yesterday. The upper ridge axis should move overhead late tonight and carry the sfc high even farther away to the east, allowing for some return flow and air mass modification, but not much in the way of moisture return. Low temps tonight should be on the order of ten degrees warmer than this morning.

SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/
As of 104 AM EST Thursday: No major changes to the forecast in the short term. A broad z500 ridge will translate eastward across the Atlantic coast Friday, maintaining dry profiles and a wide diurnal temperature range...with highs 1-2 categories above normal and lows around or even a little. By Saturday, upper energy sliding out of the Great Basin will rotate around and phase with a trough over the northern Great Plains...such that by Saturday evening a deep trough extending from the lower Plains all the way to the Great Lakes and southern Ontario will be sliding east paired with excellent channeled DPVA and an impressive 140+ kt upper jet.
With surface high pressure well off to the east at this point and the Carolinas' dynamics increasingly influenced by the advancing Plains system...strengthening WAA and some associated sprinkles will be possible through the day, though the vast majority of guidance keeps any measurable QPF to no more than 0.01-0.02" and confined to the upper Savannah River Valley and Great Smoky Mountains until at least Saturday night. Otherwise, Saturday will be another warm day as WAA boosts already above-normal temps...and highs surge into the mid- to upper-60s.

LONG TERM /SUNDAY THROUGH WEDNESDAY/
As of 255 AM EST Thursday: As deep upper troughing slides into the eastern CONUS, rapid height falls will be accompanied by impressive channeled DPVA and enhanced upper-level divergence in the right entrance region of an upper jet. This potent synoptic forcing will intersect an advancing surface front which, based on all the deterministic guidance at this point, will cross the forecast area through the afternoon and evening, an opportune time for a severe weather threat. The GFS remains the fastest among the GFS/ECMWF/GDPS trio, but all three depict a plume of 75-300 J/kg sbCAPE developing east of I-26 during the early afternoon Sunday. With a 500mb speed max crossing the area at this time, the environment will be ripe with speed shear.
Sample forecast soundings depict some 55-70kts of deep layer shear and excellent 0- 1km shear of 30kts or more. As noted by the previous forecaster, hodographs are straight, but being that they're oriented out of the WSW and solely by virtue of the shear magnitude, the environment should surely be able to support embedded mesovortices (and in fact, the kind of "ripples" in mid-level moisture associated with rear inflow development as a precursor to mesovortex development are hinted at in both the 18z and 00z GFS)
As a further sanity check
a quick cluster analysis of global ensembles was done...revealing that of three forecast modes - one supportive of early/mid afternoon convection like what's depicted in most of the deterministic guidance, one supportive of later afternoon convection, and one supportive of no convection at all - only 10% of all ensemble members now favor a forecast solution with no severe threat.

Separately...concern remains for the possibility of isolated flash flooding. All the deterministic guidance supports PWs of 1.5-1.7" in the moist air mass ahead of the advancing convective line, with expected QPF of 2-2.5" across much of the forecast area, and localized higher totals at or above 3" across the southern Blue Ridge. The limiting meteorological factor, though, will be lapse rates...which are currently forecast to be steep enough to keep the warm cloud depth below the critical 10kft threshold.
Moreover, given very dry antecedent conditions, it'll be difficult to get a flash flood going. Nonetheless, the threat can't be entirely discounted.

Sunday night high pressure will build in behind the departing system. A few hours of NW flow snow may be in store for the NC-TN border areas...but only 1-2" of accumulation are currently forecast even at the higher elevations (which is pretty consistent with earlier forecasts). More efficient drying and postfrontal CAA will occur through early Monday, permitting temps dewpoints to drop off sharply...and allowing for below-normal temperatures to persist right through the end of the period.

AVIATION /12Z THURSDAY THROUGH MONDAY/
At KCLT and elsewhere: VFR expected at all terminals through the period. Sky should be clear through the period, now that a patch of orographic cirrus has moved away to the northeast, leaving us with mainly a wind forecast. Wind begins light/variable at issuance time, but when the wind returns in the late morning, expect the direction to come around to SW outside of the mtns as a lee trof develops. Wind should go light SSW or variable at sunset.

Outlook: VFR should prevail through Friday and Friday night. Moisture will return from the SW late Friday night, and a ceiling restriction could move in as early as daybreak Saturday. Additional restrictions and precip will move in Saturday night ahead of an approaching cold front, which low pressure passing NW of the region will push through on Sunday. VFR conditions return for Monday.

GSP WATCHES/WARNINGS/ADVISORIES
GA...None.
NC...None.
SC...None.

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