textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated details on possible thunderstorms tonight into early Tue. Otherwise no major changes to forecast thinking.

KEY MESSAGES

1. Scattered showers and isolated thunderstorms possible tonight and Tuesday. Near-record warmth returns Tuesday. 2. A strong cold front arrives Wednesday night into Thursday morning, producing widespread showers and embedded thunder, with a very limited severe weather threat. Cooler and drier conditions develop behind the front on Thursday and Friday. 3. Dry conditions persist the first half of the weekend before another cold front arrives Sunday into Monday.

DISCUSSION

Key message 1: Scattered showers and isolated thunderstorms possible tonight and Tuesday. Near-record warmth returns Tuesday.

Split-flow regime will remain in place thru Tuesday; ahead of the deep cutoff low near Baja California, heights will rise as southwesterly flow occurs out of the western Gulf, promoting an increasingly warm and humid airmass across the lower MS Valley. The front which effectively reached our CWA and stalled late Sunday will be reactivated across the TN and lower OH valleys, leaving a NW-SE oriented gradient of moisture/instability from TN to SC this afternoon and tonight.

Thunderstorms now near Memphis as of 130 PM are beginning to organize with the aid of a weak shortwave over the Ozarks. This activity is slightly farther north than CAMs depicted, and also has taken shape slightly earlier. CAMs had been in decent agreement on the incipient MCS tracking ESE across AL/GA tonight along the aforementioned moisture gradient. It had appeared that it would be steered just past our area to the SW, but with it forming a bit further north, it now bears a greater chance of impacting our NE GA zones and adjacent parts of NC/SC, if nothing else bringing light rain and a few stray lightning strikes via the precip shield, with the most likely time being 10 PM to 2 AM. SPC brought the MCS-inspired Marginal Risk closer to us on the midday update, and in light of the northerly trend could see it being expanded into our western CWA on the late afternoon update. Regardless of the MCS possibility, the midlevel capping inversion weakens via the shortwave while moist SW flow continues at 850mb. A few hundred joules of MUCAPE thus develop over the CWA in the early morning hours Tuesday, so elevated convective initiation could occur in our SW areas via that process also. Accordingly chance range (30-50%) PoPs will expand across the area at that time. Prog soundings remain marginally favorable for convection through the end of the day Tuesday, though with the shortwave moving east of the CWA by early afternoon, the better chance is probably during the morning. Can't rule out isolated redevelopment over the mountain ridges which then could propagate eastward.

Temps this afternoon will warm several degrees compared to Sunday but are expected to peak below the daily records. Cloud cover from the MCS or morning convection currently look to advect out Tue morning, but that does lower confidence slightly in max temps Tue aftn, which otherwise should be warmer than Monday. The current forecast high at CLT and AVL does break the daily record, with GSP falling a couple degrees short--perhaps reflecting guidance that hangs onto the cloud cover longer. Record warm min temps are also likely to be broken Tue night.

Key message 2: A strong cold front arrives Wednesday night into Thursday morning, producing widespread showers and embedded thunder, with a very limited severe weather threat. Cooler and drier conditions develop behind the front on Thursday and Friday.

By Wednesday morning a remnant cutoff low over western Texas will be in the process of interacting with a low-amplitude trough extending along the Rocky Mountain front range...and should begin opening into the mean flow by late in the day, resulting in an elongated trough axis spanning the entire Mississippi Valley and progged to arrive in the Appalachians by late Wednesday night. At the surface, a low pressure center will follow the trough axis northeastward from the lower Midwest into New England through late Wednesday night. Recent operational runs depict slower phasing of the upper low and trough and weaker steering overall. As a result, progression of this frontal boundary occurs more slowly than it did in previous forecast cycles, and much better dewpoint pooling is depicted ahead of the front for Wednesday night and Thursday morning. Long-range ensembles seem to favor this idea as well, with over half of LREF membership trending toward a slower solution.

The thinking remains, therefore, that the severe risk will be nonzero but limited. It still appears as though instability will be severely constrained by the late night / early morning timing when deep synoptic forcing is expected to peak. Whatever narrow ribbon of DPVA and associated frontal uplift push across the area ahead of the trough axis will do so in the middle of the night, absent much surface-based instability. After 06z Thu, models depict a narrow tongue of 50-100 J/kg sbCAPE dipping out of the mountains and into the NC Piedmont/SC Upstate...paired with some 50-65kts of deep layer shear...enough to organize whatever storms develop. Given such scant instability, it's hard to imagine more than a a few isolated storms will develop, but those that do will have the capacity to become strong to severe.

Strong postfrontal CAA will set up on the leading edge of a cooler continental air mass on Thursday, as the front departs to our east. This will provide a narrow window on Thursday and Thursday evening for gusty winds across the mountains as well as light NW flow snow showers, and perhaps even some accumulation at higher elevations. By Thursday night, however, ensembles are in good agreement that a developing low over the Midwest will become the dominant feature, allowing the Carolinas to quickly toggle back to a southerly moist return flow...so cold, near- or even below-normal conditions on Thursday night will already be trending warmer by the end of the week.

Key message 3: Dry conditions persist the first half of the weekend before another cold front arrives Sunday into Monday.

A weak reinforcing cold front may cross the area with little fanfare on Friday, but by the time of its arrival most guidance suggests it'll be fading to obscurity. Thus, we can expect to more or less remain within a weak return flow through the weekend. This will result in the return of at least low-end PoP by Sunday afternoon...confined mainly to the NC mountains...and a warming trend that will persist through the end of the seven-day forecast.

Rapid cyclogenesis will take place in the lee of the Rockies on Sunday as well. Operational models are in remarkably good agreement that this feature will track into the lower Midwest and eventually the Allegheneys by Sunday night, driving another cold front across the area into Monday. The potential for convection associated with this front will depend heavily on its timing and the availability of decent instability and shear; right now, there's too much uncertainty to make much of a solid assessment.

AVIATION /18Z MONDAY THROUGH SATURDAY/

At KCLT and elsewhere: VFR this afternoon though a few cumulus below FL030 will be seen in the first couple hours near KCLT and potentially KHKY. Winds also will be somewhat VRB at KCLT/KHKY owing to SW winds aloft mixing with W to WSW sfc winds, but in general SW winds will prevail this aftn, aside from KAVL seeing NW winds when not calm. SHRA and a few embedded TSRA may develop in the early morning hours Tue over the SC sites and KAVL, either as light rain on the periphery of a thunderstorm complex moving thru GA, or with elevated convection developing ahead of a weak upper level disturbance. PROB30s included at those sites for SHRA and associated MVFR, except KAND where greater instability warrants TSRA. Otherwise just BKN cloud deck at FL100-150. Can't rule out development of isolated SHRA/TSRA after daybreak, so VCSH included except at KHKY where chance too low to mention. Predominantly SW winds tonight/Wed except for KAVL where winds likely light/VRB.

Outlook: The spring-like weather pattern will linger for the next few days with patchy fog/low stratus possible each morning. Isolated TSRA may return Tue aftn near KAVL/KHKY. Better TSRA chances expected for all sites Wed night into early Thu along with gusty winds. Drier conditions return late Thu into Fri.

CLIMATE

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.