textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion for the 00z TAF issuance.

KEY MESSAGES

1. A few severe thunderstorms are possible today and Tuesday, mainly in the late afternoon and early evening. The primary concerns from any stronger storms will be damaging wind gusts and excessive rainfall. 2. Temperatures fall below normal Wednesday and Thursday with light precipitation likely for much of the area, perhaps with embedded heavier showers. Drier conditions are expected to develop Friday along with near-normal temperatures through the weekend.

DISCUSSION

Key message 1: A few severe thunderstorms are possible today and Tuesday, mainly in the late afternoon and early evening. The primary concerns from any stronger storms will be damaging wind gusts and excessive rainfall.

Somewhat unseasonably warm and humid conditions persist this afternoon. We lack much in the way of a distinct forcing mechanism, though a weak convergence axis (lee trough?) is evident in the lower Piedmont, south and east of Charlotte. Upward of 2000 J/kg of SBCAPE appears to be in play, along with 600-900 J/kg of DCAPE; with winds very light up through 700 mb or so, shear is accordingly also weak. Ridgetop convergence should maintain scattered showers/storms over the mountains, with a secondary maximum in coverage in the aforementioned convergence zone over our lower Piedmont. Can't rule out later development in between. SPC has most of the Piedmont in a Day 1 Marginal Risk contour; with a pulse storm environment that isn't unreasonable though severe threat is expected to remain isolated. PWATs are high for this time of year and with slow storm motion an isolated excessive rainfall or flash-flood threat may materialize.

A strong 1030mb surface high is already centered over upper Ontario. Confluent upper flow downstream of the block over the northern Plains will allow that high to strengthen further tonight into early Tuesday. An axis of vorticity associated with a weak shortwave moving thru the OH Valley could provide a boost to forcing late tonight or Tuesday morning. In advance of that feature, we should see some degree of weak upglide take shape over our CWA tonight, though the signal appears much stronger to our north, near an existing stationary front. Most guidance depicts a few hundred joules of MUCAPE lingering overnight, such that in light of the vort axis and upglide, our forecast will maintain a continuing chance for isolated to scattered showers/storms overnight. The upglide could also lead to development of a low stratus deck over the mountains and NW NC Piedmont by morning, and more diurnally driven cloud cover is expected across the remainder of the area. Temperatures should trend a bit cooler though they will remain above normal. While morning cloud cover could hinder destabilization, we will remain under broadly cyclonic flow aloft, and shear will increase a bit. SPC Day 2 outlook features a Marginal Risk over our southwest CWA for much the same reason as today; the smaller area reflects the potential stabilizing effect of the cloud cover.

Key message 2: Temperatures fall below normal Wednesday and Thursday with light precipitation likely for much of the area, perhaps with embedded heavier showers. Drier conditions are expected to develop Friday along with near-normal temperatures through the weekend.

As the Canadian high builds further into the St Lawrence Valley Tuesday night, it comes into position to foster cold-air damming. CAD will be further strengthened as the moist boundary is forced southward, and diabatic cooling occurs. With the wedge and widespread cloud cover Wednesday, temperatures will peak around 10 degrees below normal. We will remain under cyclonic flow at 500 mb, with an upper low rotating over the lower Great Lakes and OH Valley. Models depict a tongue of SBCAPE and higher MUCAPE extending into East TN and around the periphery of the wedge into SW NC. Hence those areas still have a chance at a few thunderstorms Wednesday, though thinking depictions of elevated t-storms over the wedge itself are overdone, so we only advertise showers there. A few members of the REFS develop organized clusters of storms near the Smoky Mtns which is a small possibility, given 30+ kt of 0-6km shear there; will have to monitor that trend in guidance as it could result in a small severe risk outside the wedge. While the blocked flow continues, it progresses enough for the upper low to drift east of the Appalachians Wed night into Thursday, so heavier showers could result in addition to continuing stratiform precip for the area. The additional cloud cover and precip suggests Thursday will be a few degrees cooler than Wednesday in most locations; most of our CWA is forecast to remain in the mid to upper 60s that day. The models disagree on the fine details, but there is general consensus for the upper low to enlarge and center over the Mid-Atlantic coast by late Thursday. A nor'easter also appears to develop by then. A deep dry northwest f low regime sets up; though the high remains to our north there would appear to be cold advection leading the wedge to technically erode by Friday. Precip chances decrease and skies trend clearer; under downslope flow and sunshine temps return to near normal, upper 70s to near 80 for most of the Piedmont. Humidity will be comfortably low. This pattern should last through the end of the period. Overnight lows Fri and Sat nights will reach the 40s over much of the mountains.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: Isolated convection will linger for the first couple of hours of the 00z TAFs, but confidence too low to mention at any of the sites. Overnight, some guidance suggests expanding light showers across the area, especially in the mountains. But confidence remains low for now. There is also a weak signal in the guidance for MVFR to IFR cigs forming across the Piedmont around sunrise Tuesday. But with lack of low-level flow, think it will be mainly patchy fog around areas that received rainfall earlier today. The mountain valleys should fog up again tonight, and have several hours of LIFR at KAVL, similar to last couple of nights. KHKY may have some river fog as well, with precip occurring around the site earlier. MVFR stratocu may expand across the area during the day Tuesday, but probably not enough to limit development of more widespread SHRA and TSRA. PROB30 has been added to all sites, with prevailing SHRA at KCLT Tuesday evening. A backdoor front will slide in and lower cigs from NE to SW in the 21-00z time frame (according to model consensus), ushering in cold-air damming Tuesday night.

Outlook: Development of cold-air damming Tuesday night suggests periods of vsby/cig restrictions will be likely through Thursday, with breezy winds. Drier conditions are expected to 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.