textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated aviation discussion for 12z TAF issuance.

KEY MESSAGES

1. Temperatures will trend warmer through the weekend. Isolated or widely scattered showers and non-severe thunderstorms are possible in some of the area Saturday. 2. Anomalously warm temperatures develop through the end of the weekend and into the beginning of the new workweek. 3. Unsettled, but generally low-impact, weather continues through the end of next week.

DISCUSSION

Key message 1: Temperatures will trend warmer through the weekend. Isolated or widely scattered showers and non-severe thunderstorms are possible in some of the area Saturday.

Anomalously deep western CONUS upper ridge will persist through the weekend, though slowly weakening. Upper trough initially aligned with the East Coast this morning will drift into the Atlantic, with heights gently rising over the CWA accordingly, and temperatures remaining on an upward trend. Tonight into early Saturday, a shortwave diving past the upper ridge will carry a weak sfc low and cold front to the central/southern Appalachians. The shortwave will phase with the coastal trough, with the combination moving further away from the area and promoting more substantial height rises by Sunday morning.

Winds will remain SW'ly this afternoon with occasional gusts as gradient tightens ahead of the cold front, with dewpoints rebounding. However, with max temps in the lower 70s in the Piedmont and valleys, RH still looks to dip to near 30 percent in some spots; winds still should be too low to expect elevated wildfire danger. Lapse rates weak and profiles too dry aloft for any shower activity. Breezy conditions persist tonight as gradient continues to increase, maintaining elevated overnight temps. As flow turns westerly over the mountains near the front, highly isolated showers could result in the early Saturday morning hours. Ridgetops may see gusts of 20-30 mph. It still appears the cold front will stall just as it reaches the CWA Saturday morning; upper support exits as the trough moves off to the east. Some drying is noted aloft over the northern CWA for a time late Fri night or Sat morning. However, another more ill-defined shortwave does round the ridge on a more southerly track, and the major models depict this feature moving over the CWA during the day. The W to NW flow above the PBL backs again, potentially negating the drying effect of the stalled/remnant front. This makes low-level profiles more supportive of surface-based convection in the afternoon, particularly south of the old boundary. HREF probs of 500 J/kg of uninhibited SBCAPE are greater than 50% over the western mountains and most Piedmont areas south of I-40. The main limiting factor is probably the lack of a good trigger, but the available CAMs depict at least a couple of loose clusters of storms forming late in the day, perhaps via the shortwave. Accordingly PoPs and thunder chances have ticked upward Saturday afternoon/evening, mainly slight-chance (15-25%) for the Piedmont and chance (25-50%) in the mountains/foothills. Shear is enough to believe the cluster mode but insufficient to expect severe weather at this time.

Key message 2: Anomalously warm temperatures develop through the end of the weekend and into the beginning of the new workweek.

On Sunday and Monday, steady expansion of a deep subtropical ridge centered over the Chihuahuan Desert will result in rising heights across the Carolinas and increasing temperatures. Despite very minimal temperature advection in the low levels, the region's consistent exposure to a moist subtropical air mass combined with building thicknesses aloft will push highs into the mid-80s on Sunday and Monday...some 20 degrees above normal.

On Monday afternoon, a low-amplitude shortwave axis will dip into the Ohio Valley before being deflected to the east by the southern- CONUS ridge. Some amount of mid-level forcing will nonetheless skirt the Carolinas, but by this time, the same mid-level subsidence responsible for our heat wave will also strongly cap any prospective convection; indeed, while some afternoon showers look like a possibility, their potential to tap into any deeper instability and generate even garden-variety thunder looks limited. Rather, as the shortwave's surface reflection slides south across the area Monday afternoon, most operational guidance depicts scattered ridgetop showers (perhaps even isolated showers over the low terrain) followed by a punch of dry, cooler air by Monday night.

In the wake of the front, a dry enough air mass looks to settle into the region to promote critical RHs over at least part of the forecast area on Tuesday afternoon. The need for a Fire Danger Statement cannot be ruled out...but by the time the RH drops, winds should be light enough to preclude any Red Flag potential.

Key message 3: Unsettled, but generally low-impact, weather continues through the end of next week.

Following a brief period of quiet conditions on Tuesday, another, more robust z500 trough should dive out of the Midwest and cross the Carolinas by early Wednesday. By this time, surface high pressure will be centered somewhere along a corridor extending from coastal North Carolina to Cape Cod. Depending on the location of the high, surface flow may be favorable for a weak hybrid cold-air damming event as a brief but intense period of low-level isentropic ascent develops ahead of the shortwave axis. Ensembles are...not consistent...in their handling of the high, so confidence isn't great on whether CAD will develop. Even if it does, mid-level moisture will be limited enough that very little QPF would be expected.

Ensembles really begin to diverge past this point. Some ensemble members - primarily those from the 00z ENS - depict a compact shortwave crossing the upper Ohio Valley through the end of the week, driving another cold front across the area by the end of the period. The bulk of GEFS and GEPS membership, comparatively, feature a more a zonal upper pattern and allowing temperatures to rebound into the 70s within a warmer air mass by the end of the week.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR through Saturday morning at all TAF terminals. Largely calm winds with only FEW250 if not SKC this morning; winds should pick up from the SW by around noon, becoming breezy during the afternoon with a few gusts developing at 15-20 kt. A few clouds above FL180 will also drift in this afternoon, possibly becoming BKN at times overnight. Light SW winds continue tonight with cloud cover increasing ahead of approaching frontal boundary. As winds increase aloft LLWS could result at KAVL, 05-12z Sat. Lower VFR likely at that time over the western slopes of the Appalachians, including KAVL; MVFR possible over higher mountain elevations. Light SW to WSW winds continue Saturday with isolated SHRA or TSRA possible after 18z Sat.

Outlook: VFR expected through the weekend aside from the possibility of a TS hit and brief MVFR to IFR vsby. A largely dry cold front will cross the area Monday, which may bring brief restrictions with isolated showers.

CLIMATE

RECORDS FOR 03-21

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 82 1948 34 1960 58 1948 14 1965 1958 KCLT 87 1935 35 1914 65 1912 18 1965 KGSP 88 1907 43 1958 60 1948 18 1965 1906 1927 1906 1921

RECORDS FOR 03-22

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 86 1907 32 1915 58 1991 15 1965 KCLT 89 1907 38 1883 64 1948 24 1965 KGSP 85 1935 42 1985 62 1948 20 1906 1914

GSP WATCHES/WARNINGS/ADVISORIES

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