textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Revised information about isolated severe weather this afternoon and evening.

Confidence is increasing for near record high temperatures for Sunday, especially locations east of the mountains.

We are monitoring forecast trends for possible elevated wildfire danger on Monday.

KEY MESSAGES

1. A few showers and thunderstorms are possible this afternoon and evening. An isolated severe storm is possible, posing mainly a threat of large hail. 2. Anomalously warm temperatures develop on Sunday and Monday. A cold front crosses the area Monday afternoon, resulting in some showers, perhaps a few thunderstorms, and low-end fire weather concerns. 3. Unsettled, but generally low-impact, weather continues through the end of the week.

DISCUSSION

Key message 1: A few showers and thunderstorms are possible this afternoon and evening. An isolated severe storm is possible, posing mainly a threat of large hail.

A weak frontal zone is expected to settle NW-SE from KY to central NC by this afternoon before the associated trough moves offshore and the front stalls. Not seeing any meaningful potential for airmass change in our CWA, but some convergence should occur as a result, again bisecting the area NW to SE. The position will be important for locating the peak potential for deep convection. A weakly capping warm layer is present above the PBL on prog soundings, even when modified for the expected sfc temps in the low 80s outside the mountains. Isolated convective initiation appears possible near peak heating with the aid of the convergence zone, particularly over the Upstate, SW NC mountains, and NE GA, these areas being where capping is smallest. Furthermore, a weak shortwave is shown to move out of the Ozarks by midday and cross the southern Appalachians around 00-03z Sunday. CAM response has been fairly consistent in depicting storms firing near the Great Smoky Mtns near the wave, and tracking SE on the warm side of the boundary; some activity may bubble further north near the TN/NC border later in the evening though that is a less consistent signal. These findings are sufficient to go a bit higher than NBM and advertise a band of slight-chance to low chance range (20-30%) PoPs beginning in SW NC at peak heating and expanding downstream into the NC/SC Piedmont by 00z, with chances tapering off in the 04-06z timeframe.

LCLs/CCLs are on the high side owing to early-spring moisture but temperatures more typical of May, though lapse rates will be rather strong through the 0-3km and 700-500mb layers. SBCAPE appears to peak at 500-1000 J/kg this afternoon. Fast flow will be present aloft and effective shear of 30-35 kt could be realized. The main factor limiting confidence in any severe risk--and which keeps PoPs low--is the lack of a strong and clear triggering mechanism but also the potential for slight capping. SPC retains a Marginal Risk for the Day 1 outlook in virtually the same orientation as the old Day 2. We agree with them on the most likely threat being hail up to around 1" in diameter. DCAPE of 700 J/kg also suggests a nonzero threat of damaging wind, particularly if storms cluster along cold pools as depicted in some of the CAM output.

Key message 2: Anomalously warm temperatures develop on Sunday and Monday. A cold front crosses the area Monday afternoon, resulting in some showers, perhaps a few thunderstorms, and low-end fire weather concerns.

Broad and deep subtropical ridging will persist over the Desert Southwest and northern Mexico on Sunday and Monday, with steady eastward expansion expected through the period. The result still looks like an extended period of moist subtropical air remaining entrenched across the Carolinas...and promoting exceptionally warm temperatures each day. To wit, on Sunday highs could climb into the mid- or even upper-80s across the low terrain...within a category of daily records...and on Monday, they'll reach the lower-80s, not quite as close to record warmth but still nearly 20 degrees above normal.

To some extent, Monday's modestly cooler temp forecast can be explained by the influence of a weak, but amplifying shortwave dipping across the Ohio Valley and into the lower Mid-Atlantic through mid-afternoon, lowering heights and ushering in a weak backdoor cold front by evening. Guidance is in decent agreement on an upstream line of showers and embedded thunder tracking south across the Mid-Atlantic early Monday morning as the front approaches...but losing steam upon arrival in the Carolinas, partly due to disruption of the system's circulation by the mountains, and partly due to the diurnal minimum in instability. By afternoon, however, operational models depict destabilization across the southern half of the CWA, and resultant redevelopment of showers and storms...mainly across the SC Upstate. At this time, deep layer shear looks too limited for much severe potential...although a layer of dry air extending through roughly the 700-500mb layer may briefly promote potential for isolated strong winds, should the advancing convective line become reorganized south of I-85.

Finally...most of the forecast area...or at least the northern 2/3...looks to transition to a postfrontal air mass by peak heating, resulting in some locations reaching critical RH even as gusty CAA winds peak. It would currently appear that true Red Flag conditions are unlikely for more than isolated areas in the Piedmont...but fire weather conditions will nonetheless need to be monitored closely, especially since as described above, those regions where low RH and gusty winds will be overlap will also see the least QPF response from faltering convection early in the day.

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

A much-needed cooldown is still expected on Tuesday as cool and dry high pressure filters into the area on the periphery of a cP air mass crossing the upper Ohio Valley into New England. Dry (albeit partly cloudy) conditions are expected during this time, with largely unperturbed flow aloft.

Compared to previous forecasts, the potential for weak hybrid cold- air damming seems to have decreased for Tuesday night and Wednesday. All the operational guidance now favors the center of the surface high drifting southeastward off the Long Island coast, and winding up centered east of the Chesapeake Bay or even the NC Outer Banks by Wednesday morning...resulting in too much of a southerly component to low-level winds for CAD to develop. We nonetheless can expect weak but steady moisture advection on Wednesday and Thursday. Synoptic forcing looks quite nebulous during this time, however - confidence on rainfall potential remains limited, therefore, with only 20-30% PoP currently advertised. Thunder potential looks quite low.

By the end of the week, a series of low-amplitude shortwaves should drive another cold front across the Carolinas from the north; this should help return above-normal temperatures to within a category of normal, possibly by Friday, and very likely by Saturday, depending on the timing of the front.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR likely to last through the TAF period, aside from the risk associated with TSRA. Breezy mainly SW'ly winds this morning; weak frontal zone arriving from the north will veer winds to NW during the early day at KAVL, KHKY and KCLT, but at KCLT these should mix back to near due W in the afternoon and then go SW again for the evening. Only alto/cirrus clouds seen most of the day, with cigs above FL120 near the front. Isolated SHRA/TSRA may develop across the area, with the best overall chance being after peak heating as weak shortwave trough interacts with the stalled frontal boundary. PROB30 TSRA retained at all sites except KHKY and KAND where chance too low. TSRA likely will be high-based and potential for restrictions somewhat mitigated, though MVFR vsby/cigs cannot be entirely ruled out. SHRA/TSRA coverage will diminish by 06z at all sites; no restrictions expected at KCLT at daybreak Sunday.

Outlook: Brief restrictions could return with isolated showers Monday or Monday night.

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.