textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated for Aviation Discussion. Made some minor tweaks to precip probs thru Saturday afternoon.

KEY MESSAGES

1. A few thunderstorms will be possible through this evening, and again Saturday afternoon. More numerous showers and thunderstorms are expected Saturday evening and Sunday. A small risk of damaging wind may transpire Saturday evening. 2. Warm and well above normal with mainly diurnal convection each day through the middle of next week. Cooler temperatures and better coverage of convection returns Thursday into Friday in association with a cold front.

DISCUSSION

Key message 1: A few thunderstorms will be possible through this evening, and again Saturday afternoon. More numerous showers and thunderstorms are expected Saturday evening and Sunday. A small risk of damaging wind may transpire Saturday evening.

Isolated thunderstorms were on the downward trend as of 2330Z, though the environment retains most-unstable CAPE of 1000-1500 J/kg. Cannot rule out additional convection into the mid/late evening mainly closer to the Blue Ridge Escarpment, so a small chance will be kept there. The risk of severe storms appears to be low.

Otherwise, a warm and humid, weakly forced regime reminiscent of late spring will remain over the Southeast through Saturday; key features will be the Bermuda anticyclone and a deep trough centered over the Rockies. That trough will shear apart Saturday, leaving a northern stream shortwave which progresses toward the Great Lakes and a cutoff low near Baja California. The advance of the northern wave will bring a cold front into our vicinity by early Sunday, which then stalls. Prior to the front, 700mb and 500mb heights over NC/SC/GA remain above the 95th percentile of climatology, reflected in unusually warm temperatures. Southwest flow around the ridge looks to maintain dewpoints in the upper 50s to around 60 in the Piedmont and mountain valleys through early Sunday.

Radiation stratus and some fog look to develop tonight, and/or low stratus of Atlantic origin may creep as far north as the I-85 corridor. With that moisture being trapped under the aforementioned inversion, most guidance that develops the clouds keeps them through the morning, delaying warming such that max temps end up a few degrees cooler than Friday. This would delay diurnal destabilization as well, and CAMs generally produce less convection near peak heating than what they develop for today (Friday). Lapse rates not having changed much and upper forcing still lacking, if diurnal initiation is able to occur isolated showers/storms could again result. Thus we feature PoPs at 20-30% through late afternoon. Somewhat stronger 850 to 700mb flow will be present in advance of the trough, so shear will be slightly more capable of organizing any convection that develops. A convective line is expected to move thru the lower OH and TN valleys Saturday afternoon; its outflow could reach the mountains late in the afternoon or early evening, and could initiate storms after the diurnal peak. Accordingly PoPs increase to 60-80% over the western half of the CWA Saturday evening, with 30-50% chances east of there. Effective shear of 30-40 kt will be available so storms could organize along the outflow, which would be the most likely way severe weather (damaging wind) could materialize. This is the reason the SPC Day 2 Marginal Risk extends into our CWA. The front settles east of the mountains on Sunday, providing a continued focus for precip and isolated to widely scattered thunderstorms. Instability will be slightly lower, although 0-6km bulk shear is slightly higher, so a low severe risk cannot be ruled out.

Key message 2: Warm and well above normal with mainly diurnal convection each day through the middle of next week. Cooler temperatures and better coverage of convection returns Thursday into Friday in association with a cold front.

Weak upper troughing lingers over the forecast area through Wednesday night before upper ridging builds over the Southeast Tuesday into Wednesday. Meanwhile, at the surface, the southwestern periphery of a surface ridge will remain extended over the Southeast through Wednesday. This will keep warm, well above normal temperatures around as well as allow for the return of mainly diurnal convection through Wednesday. Global models generally show 30-40 kts of deep layer shear returning each afternoon (mainly on Monday and Tuesday), with at least a a few hundred J/kg of SBCAPE developing during peak heating each day, with values generally generally remain below 1,000 J/kg. These SBCAPE values appear to line up well with the LREF and the NBM, with the NBM only showing low probabilities (20%-30% or less) of exceeding 1,000 J/kg of SBCAPE each afternoon (mainly along and south of I-85).

Upper ridging gradually breaks down late Wednesday into Thursday as weak upper troughing tracks across the northern CONUS while an upper low gradually weakens and opens into an upper trough as it pushes out of Texas and tracks eastward across the Gulf Coast. At the surface, a cold front will approach out of the west on Wednesday before tracking across the forecast area wednesday night into daybreak Thursday, allowing the southwestern periphery of the surface ridge to gradually retreat eastward. This will allow for better coverage of convection late Wednesday night into at least Thursday night, possibly lingering through Friday (despite the front pushing east of the GSP forecast area) as global models depict a surface low developing along the frontal boundary. For now, it appears that severe weather associated with the front/area of low pressure will be low as guidance generally shows limited surface instability developing, especially on Friday. Confidence of course will remain low on any severe threat as trends still have time to change with this being right at the end of the forecast period. Cooler highs return Thursday, but will still end up several degrees above normal. Below normal highs should return Friday, especially if extensive cloud cover and widespread rain chances linger.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Convective trends are downward at issuance time, so the only location that might have a chance in the immediate term would be KAVL where a dying storm was moving past to the NW. Otherwise, we will have widespread convective debris clouds and wind mostly from the S to SW, but modified locally by storm outflows. Late tonight, once again we look to the south for development of a low cloud deck and/or fog that would move in with the continuing light southerly flow. The boundary layer is moist enough that the development of a ceiling restriction during mid-morning is a decent bet as convection starts to bubble up with some heating. By mid-afternoon the boundary layer should deepen enough to lift the ceiling to VFR, if it hasn't already scattered out. The S to SW flow will continue with some gusts possible in the afternoon. Guidance suggests a better chance of more shower/thunderstorm activity in the late afternoon and early evening as a front approaches from the west. However, only KAVL and KAND warrant inclusion of a PROB30 thru 00Z Sunday, and KCLT after that by virtue of the later end time. Wind will stay SW.

Outlook: The spring-like weather pattern will persist over the next several days with patchy fog/low stratus expected each morning. A stalling front will likely bring showers and associated flight restrictions Saturday night into Sunday.

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.