textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated information on thunderstorm threats this evening and potential unsettled weather in the second half of this week.

KEY MESSAGES

1. Bands of showers and thunderstorms will progress across the area this afternoon and evening. A few storms embedded in the bands will pose a risk of damaging wind gusts. Today will be the last hot and muggy day before a cold front pushes through the area tonight. 2. Drier and slightly below-normal temperatures Monday trough Wednesday, with moisture and rain chances increasing in the second half of the week.

DISCUSSION

Key message 1: Bands of showers and thunderstorms will progress across the area this afternoon and evening. A few storms embedded in the bands will pose a risk of damaging wind gusts. Today will be the last hot and muggy day before a cold front pushes through the area tonight.

A weak vort max initiated clusters of showers and storms which have now pushed through the western half of the CWA as of 2 PM. Extrapolated motion has proven to be a decent predictor of timing with this activity so far, due to the support of the vort max. These initial clusters will probably push east of the eastern CWA border by 5 PM. Pockets of 40-50 mph gusts have occurred with the stronger cells and some scattered power outages (not yet confirmed to be due to wind). LCLs appear to be fairly high out ahead of the current activity, per GOES soundings, and that may explain why cumulus development has been limited so far, which suggests coverage and intensity of storms may be kept in check despite diurnal destabilization, though DCAPE is higher in that area than what the initial storms were exposed to, so at least an isolated damaging wind gust is possible. Whether the airmass recovers to support a second round of storms later remains to be seen; clear skies remain in the cold pools, but forcing does increase later this evening ahead of the main prefrontal trough and cold front. Instability gradient evident on SPC Mesoanalysis in NW-SE orientation, such that the best instability this afternoon would be in the Savannah Valley. New convection forming in the mid Tennessee Valley near those features would appear to track thru N GA and potentially into the Lakelands, NE GA and the western Upstate, if the earlier round did not overturn the airmass there. For now we advertise a 20-40% chance of a second round after sunset in those areas. A few showers also could redevelop tonight near the TN border along the front itself.

Heat index is expected to top out near 100 this afternoon in our southeastern zones and the CLT metro before the arrival of storm outflows. Temps will trend cooler in the mountains tonight but Mon morning mins likely will be similar to those the previous morning elsewhere.

Key message 2: Drier and slightly below-normal temperatures Monday trough Wednesday, with moisture and rain chances increasing in the second half of the week.

Temperatures trend cooler Monday, back to around normal in the Piedmont and slightly below normal in the mountain valleys. Drier air will however advect in on northwest winds behind the front, and heat index will be substantially lower, remaining below 90 in most locations. The drier air also should result in suppressed diurnal convection such that precip chances are less than 10 percent through evening. Min temps return to near or a little below normal Monday night. No significant change in that the front still looks to remain stalled to our south through Wednesday. Tuesday trends still cooler, below normal for all, and also likely free of precip. There remains the possibility that the broad shortwave moving thru the Ohio Valley Tuesday night or Wednesday could initiate upglide over the stalled front and result in a few showers over the area, so a few blips of 20% PoP return. Temps also trend back at or above normal Wednesday with the airmass modifying quickly under June sun, though humidity remains low.

Deterministic models still depict a somewhat unseasonably strong sfc low developing near the Great Lakes by Thursday, bringing another cold front southward thru the lower MS Valley Thu night or Friday along with another broad trough. They generally also depict a remnant of the tropical disturbance interacting with the front as it nears the Gulf Coast, though it becomes absorbed into the trough more quickly than on earlier depictions, looking more like a frontal wave as it reaches our area--aside from the EC which still shows a compact but distinct low riding along the boundary. Details are murky, and thus confidence low, as to how far south the front settles before stalling, but it still bears watching whether this might set us up for a period of heavy rain and/or severe weather. Gusty winds remain in the forecast Thursday and Friday. Temperatures remain above normal Thursday but currently look to trend below normal Fri-Sat if the front manages to ooze its way south.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

At KCLT and elsewhere: Fairly confident a round of SHRA/TSRA will push through KHKY and KCLT by 20-21z, potentially bringing brief restrictions and gusty winds. Elsewhere, convection passed prior to 18z and the sites begin the period in a lull. There remain other weak boundaries entering from upstream in TN/GA, and also a prefrontal trough set to pass the area this evening, ahead of a cold front early Monday. At KAVL and the SC sites, there appears time for the atmosphere to destabilize again before sunset ahead of those features, so PROB30 or TEMPO included during the late afternoon and early evening. Confidence is low as to whether the airmass will recover at KCLT or KHKY, in order to support a second round of convection. Those sites thus have only a period of VCSH later in the evening. Gusty SW'ly winds this afternoon will give way to W or NW winds this evening, turning NW in time for ops Monday. KAVL may experience low VFR or intermittently MVFR cigs with upslope flow developing behind the front tonight. Less humid Monday with that and subsidence suppressing SHRA and probably diurnal cu altogether.

Outlook: Quiet conditions expected thru midweek. Valley fog and low stratus is possible each morning, primarily in the mountain valleys. Diurnal convection and gusty winds may return to the region Thursday and Friday.

GSP WATCHES/WARNINGS/ADVISORIES

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