textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Revised PoPs and temperatures this afternoon and evening per latest hi-res model guidance. Revised details about heavy rain and severe threats through tonight.

KEY MESSAGES

1. Slow-moving thunderstorms may produce locally excessive rainfall this afternoon and evening, posing a risk of isolated flash flooding. Cannot rule out an isolated severe storm producing damaging wind gusts. 2. Much cooler temperatures and cloudy skies Wednesday and Thursday, with occasional light rainfall or drizzle especially in the Piedmont. A few pockets of heavy rain or thunder could occur Wednesday in the far southwest North Carolina mountains. Drier and warmer but still mild conditions return Friday into early next week.

DISCUSSION

Key message 1: Slow-moving thunderstorms may produce locally excessive rainfall this afternoon and evening, posing a risk of isolated flash flooding. Cannot rule out an isolated severe storm producing damaging wind gusts.

1034+ mb high pressure now centered over Ontario/Quebec. With the aid of confluent flow over the NE CONUS downstream of a Rex block, the high will build south along the Eastern Seaboard, and support development of cold-air damming which will persist through at least Thursday. Per satellite imagery, a boundary is evident near the southern and eastern borders of our CWA, probably a convergence zone between maritime air and that associated with the high. However, models largely depict a backdoor cold front settling southward through VA into NC during the evening, north of which isentropic upglide will develop along with clouds and light stratiform precip.

The upper low component of the Midwest Rex block is surrounded by a broad trough, and per water vapor imagery a shortwave will eject out of the lower OH/TN valleys and over our area tonight. NAM and SPC mesoanalysis depictions of CAPE currently appear overdone compared to GOES soundings, but at least a few hundred joules of SBCAPE are present across the area this afternoon, and some MUCAPE is expected to remain for parcels lifted above the wedge front tonight. Between that and the shortwave, showers/storms should increase in coverage from late afternoon through the evening. CAMs have trended toward more activity firing near the wedge boundary and less of a signal for activity progressing west to east as if forced by the shortwave. Assuming skinny CAPE profiles as seen on most guidance this evening, PWATs being over the 90th percentile, and particularly the slow steering flow, a locally heavy to excessive rainfall threat will persist through late evening. CAM consensus suggests the main threat is in our southeastern CWA, particularly the Charlotte metro and locations south and east. HREF and REFS PMM output features the greatest likelihood of extreme values (3"/3hr) in that portion of the CWA.

Temps have been limited by morning clouds and precip over the western Upstate, NC mountains and far NW Piedmont, and accordingly the area with the most potential to destabilize this afternoon is smaller than previously expected. SPC shrank the Day 1 Marginal Risk (level 1/5) with the midday update, omitting our northern couple tiers of Piedmont counties. Cannot rule out an isolated severe storm in the remainder of the area due to weak shear suggesting pulse mode and wet microbursts, but as noted above instability overall appears limited. More concerned about the heavy rain threat at this point.

Key message 2: Much cooler temperatures and cloudy skies Wednesday and Thursday, with occasional light rainfall or drizzle especially in the Piedmont. A few pockets of heavy rain or thunder could occur Wednesday in the far southwest North Carolina mountains. Drier and warmer but still mild conditions return Friday into early next week.

The Rex block configuration persists over the Great Lakes and lower OH Valley Wednesday though it does progress slightly south and east. By Wed morning CAD should basically be entrenched across our CWA with low clouds and fog, plus occasional drizzle, continuing through the day. Max temps will be sharply cooler, mainly in the 60s except low to mid 70s on the southwest periphery of the CWA. A small amount of MUCAPE is shown to linger over the wedge throughout the area, increasing toward the south and west. A portion of the NC mountains west of the French Broad, as usual, may remain outside the wedge with warmer temps and potential for surface based instability to develop. A vort lobe surrounding the blocking upper low is shown to rotate over East TN and far western NC in the late afternoon. In the western mountains, the resulting dynamic lift may realize SBCAPE of 500-1200 J/kg. Effective shear will be near 30 kt, so some clusters of storms may organize into line segments and strong or even damaging wind gusts could result, perhaps with some embedded dime to nickel size hail. SPC Day 2 mentions this possibility in text but the chance was not sufficient to include a risk area, only general thunder. That should provide some degree of lift to expect some of the MUCAPE to be realized over the wedge, so a few embedded heavier showers may result in the Piedmont, though precip is expected to be mainly stratiform. For the Piedmont under the wedge, a few embedded heavier showers may result.

The upper low should increase winds atop the wedge so it should continue to be reinforced Wed night. Went a little warmer than NBM that night and a little cooler than NBM on Thu given wedge should still be strengthening. Max temps will be a category or so cooler--lower 60s for the NW NC Piedmont and Escarpment areas, and mid to upper 60s in most other areas. The already breezy winds should turn a bit gustier on Thursday with the better flow aloft. As the low drifts still further east toward the coast Thu night, northwesterly CAA is shown to occur aloft, which would promote wedge erosion with clouds thinning and scattering from top down, though overnight wedge erosion is generally hard to do. This CAA regime continues into Friday when upper levels are shown to continue to dry, so any remaining clouds should erode. Sunshine should offset CAA, but we advertise only a modest rebound in temps Fri since the wedge could hang on longer than models suggest, as usual. The warming trend should continue Saturday through Monday, at which time afternoon temps rise back to around normal.

AVIATION /20Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Axis of SHRA/TSRA persists southeast of KCLT as of 1830z. Convective chances increase later this aftn and evening as backdoor cold front settles south through the Piedmont, and/or with influence of shortwave west of the Appalachians. Best overall chance for a direct hit appears to be during the late afternoon and evening. Retained PROB30s for TSRA at KCLT/KAVL/KGSP/KGMU, and for only SHRA at KHKY. KAND was worked over earlier today and only VC mention is made there. Any cigs will mainly be low VFR except for possibility of MVFR during TSRA. As backdoor front settles southwestward through the sites this evening, cold-air damming and IFR will develop, possibly overlapping with the TSRA chance if front initiates convection as some models suggest. Periods of -DZ likely in the morning with MVFR to IFR vsby under IFR to LIFR cigs; though cigs may lift somewhat, IFR or worse is likely to continue through the end of the respective periods. While winds are largely VRB or light northerly this afternoon, gusty NE winds also develop with the CAD.

Outlook: Restrictions likely through Thursday, with mainly light precip and breezy winds. Drier conditions return Friday into the weekend, though mountain/river valley fog/stratus could still develop near daybreak each day.

GSP WATCHES/WARNINGS/ADVISORIES

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