textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated aviation discussion for 00Z TAFs.

KEY MESSAGES

1. There remains potential for strong to marginally severe storms to develop this evening over the western parts of the forecast area, posing a threat of damaging wind gusts. However, confidence on the severe threat remains low. 2. Scattered showers and thunderstorms could redevelop late Sunday morning through the afternoon, primarily over the Piedmont of NC and SC. These storms could organize into clusters, and if that occurs an associated risk of damaging wind could develop. 3. Following pleasant weather on Monday, another round of rain and embedded thunder arrives Monday night into early Tuesday. Can't completely rule out a severe risk at this time, but the setup doesn't currently look conducive to one. 4. Anomalously warm temperatures persist throughout the week. A strong front arrives Wednesday night, prompting rain and possible severe weather on Wednesday night or Thursday. Confidence remains limited at this timescale.

DISCUSSION

Key message 1: There remains potential for strong to marginally severe storms to develop this evening over the western parts of the forecast area, posing a threat of damaging wind gusts. However, confidence on the severe threat remains low.

Linear MCS will progress across the TN Valley and northern AL/GA through the early evening hours. The consensus of CAMs does depict the line weakening (due to the loss of daytime heating) by the time it reaches the Smoky Mtns ~01-03Z Sunday. Our western zones remain in the SPC Day 1 marginal risk area, with only isolated severe storms expected at best, with damaging wind gusts being the main hazard.

Weak convection could affect the east half of the area in the predawn hours Sunday, but will decay nocturnally as any outflow boundaries dissipate. Mountain PoPs increase as low-level flow turns more westerly/upslope along the TN border by daybreak. No significant change in airmass characteristics expected thru daybreak. Temps remain well above normal into morning.

Key message 2: Scattered showers and thunderstorms could redevelop late Sunday morning through the afternoon, primarily over the Piedmont of NC and SC. These storms could organize into clusters, and if that occurs an associated risk of damaging wind could develop.

Cold front will effectively push east of the mountains Sunday morning, encountering a still unstable airmass with dewpoints similar to those Saturday. Lapse rates may not be quite as good as those Saturday owing to deeper moisture over the area, so SBCAPE looks to top out below 1000 J/kg. Numerous, generally light showers are forecast, and isolated to scattered thunderstorms during the afternoon. Stronger flow is expected to be present aloft, with mid to upper level trough north of the area, so effective shear ends up stronger than what is seen today, around 30-40 kt. Though peak cell intensity may be less, the storms will be somewhat more capable of organization, particularly later in the afternoon as 925-850mb winds turn more westerly and low-level convergence increases. The organized cold pools could be associated with marginally severe 55-60 mph winds. SPC Day 2 outlook retains a Marginal Risk over our eastern I-85 corridor and points east for that reason. Can't disagree there is a low-end severe threat, but the risk was carried over from previous outlooks and not sure we would advocate for one were it not already there.

Key message 3: Following pleasant weather on Monday, another round of rain and embedded thunder arrives Monday night into early Tuesday. Can't completely rule out a severe risk at this time, but the setup doesn't currently look conducive to one.

By Monday morning, deterministic guidance depicts a remnant frontal boundary draped somewhere in or just southeast of the forecast area. Theta-e fields generally suggest that with increasingly zonal flow aloft and weak forcing, the boundary will become increasingly smeared...resulting in fairly little air mass change for the western Carolinas. The bulk of any residual deep synoptic forcing will be absorbed into the subtropical ridge on Monday, giving way to a partly cloudy morning and a mostly-dry afternoon.

A weak lobe of DPVA forcing will develop Monday afternoon across the lower Mississippi Valley, pivoting eastward into the Carolinas by Monday night. Upstream, decent lapse rates, instability of some 1000-1500 J/kg, and accompanying 25-30kt deep layer shear will develop...and although the CAMs have yet to extend out til then, even the global and regional models are hinting at a convective response. The question is whether any of that activity will make it to the Carolinas. By the time the core of stronger synoptic forcing makes it into the Savannah River Valley, instability will be waning...so short of a well-organized MCS making a run at us Monday night, it may be difficult to get more than some rain (perhaps a few rumbles of elevated thunder) out of this system. Confidence remains limited, however - with still some time left for timing to shift or dynamics to improve, and open the door to a better severe risk.

Key message 4: Anomalously warm temperatures persist throughout the week. A strong front arrives Wednesday night, prompting rain and possible severe weather on Wednesday night or Thursday. Confidence remains limited at this timescale.

Much of the interesting weather will take place upstream this week, as a largely-unperturbed northern stream maintains zonal flow aloft and a polar front located well to our north. A deepening upper low over Baja California will provide the impetus for our next event. Initially stagnant, it'll begin to interact with a low-amplitude shortwave dipping down into the Pacific Northwest on Tuesday...and the two will phase together and form one deeper, longwave trough by Wednesday afternoon. This feature will initially channel the majority of synoptic forcing up the Ohio Valley, and should become negatively-tilted by the time it arrives in the eastern CONUS Wednesday night.

Confidence remains quite limited on impacts associated with this system. Areas upstream over the Deep South look like they'll have a better shot at severe weather, since they'll be subject to a rich warm-sector air mass and good daytime instability. For areas from central Georgia east, however...the case is more dubious. The parent low, located over Upstate New York by this point in time, will be occluding by the time forcing peaks, and given only very weak prefrontal WAA over the Carolinas as a result, instability should struggle to stay available as the sun goes down. Rather, it appears that like the early-week system, we'll wind up too stable by the time the better forcing and shear arrive for much of a severe risk. However, confidence remains much lower in this system's risk than the early-week system's...and there's still certainly enough variability in the long-range ensembles that a faster, Wednesday-afternoon setup could materialize.

The final wrinkle is that fully 1/3 of LREF members from the latest 00z cycle depict the parent trough de-amplifying as it crosses the Ohio Valley, allowing the front to begin weakening prior to its arrival in the Carolinas. This scenario would conditionally result in far less of an air mass change following the frontal passage, and the potential for a resurgence of diurnal instability Thursday afternoon. For the I-85 corridor and points south across the SC Upstate, this would mean 2-400 J/kg sbCAPE just as the core of the z500 speed max passes overhead...providing a narrow window sometime from late morning to mid afternoon during which conditions would be ripe for severe weather. Again, though, this is just one possible scenario, and not currently the most likely.

Finally, regarding temperatures during the week, it still appears that the continued influence of the subtropical ridge will maintain anomalously high 1000-500mb thickness values and daily temperatures well above normal, at least through Wednesday. On both Tuesday and Wednesday, forecast temperatures are in the 99th percentile of the NAEFS climatology and should flirt with breaking daily high temp records, at GSP and especially CLT.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Still seeing scattered showers and thunderstorms stream northeastward across the forecast area as of 00Z. None of this activity is currently impacting the terminals. A line of showers and thunderstorms continues approaching from the west and will track across the terminals later this evening into tonight, with some lingering -SHRA possible through daybreak Sunday. Went with TEMPOs for SHRA and associated restrictions east of the mountains overnight and a TEMPO for TSRA and associated restrictions at KAVL late this evening as this terminal has the best chance to see thunder. MVFR cigs may develop overnight but confidence is low as guidance has been backing off the low stratus potential. Another round of TSRA may develop along a cold front as it tracks across the terminals Sunday afternoon (mainly east of the mountains), so went with PROB30s to account for this potential. Went with a PROB30 for SHRA at KAVL Sunday afternoon as instability will be lower for this terminal. Winds east of the mountains will remain S/SW through Sunday, with low-end gusts developing by early Sunday afternoon. Winds at KAVL will turn NW behind the front around daybreak Sunday. KCLT will see winds toggle NW behind the front ~03Z-05Z Monday.

Outlook: The spring-like weather pattern will continue for the next several days with patchy fog/low stratus possible each morning.

CLIMATE

RECORDS FOR 03-07

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 82 1974 23 1920 61 1956 15 1960 KCLT 85 1974 30 1899 63 2022 14 1899 1956 KGSP 83 1974 36 1920 62 1961 13 1901 1901 1956

RECORDS FOR 03-08

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 81 1974 23 1996 57 1921 8 1920 KCLT 84 1974 36 1989 59 1946 16 1920 KGSP 84 1974 35 1899 56 1921 14 1901 1899

RECORDS FOR 03-09

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 80 1974 22 1932 57 1964 8 1996 KCLT 83 2009 30 1960 62 1925 16 1996 1974 1921 1925 KGSP 85 2009 29 1960 60 2009 16 1996 1921

RECORDS FOR 03-10

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 76 1974 29 1932 58 1903 10 1996 1932 KCLT 82 1974 36 1924 59 1903 17 1932 KGSP 84 1974 39 1924 58 2009 17 1932 1997

RECORDS FOR 03-11

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 79 1967 29 1924 55 2016 14 1934 1925 KCLT 83 2015 37 1960 60 1986 22 1969 2009 1934 1990 KGSP 84 2009 34 1960 60 1986 17 1969

GSP WATCHES/WARNINGS/ADVISORIES

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