textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Temperatures have trended slightly downward for today and thunderstorm chances increased. The overall rainfall amounts for the event have trended downward.

Updated the aviation discussion for the 06Z TAF issuance.

KEY MESSAGES

1. Showers and thunderstorms will become numerous this afternoon and evening along and ahead of a front dropping down from the north, followed by widespread rain overnight and again Sunday afternoon and evening. The combination of events will raise the threat for excessive rainfall that could result in some localized flooding through Sunday night. 2. Dry from Monday afternoon onward. Cool temperatures return for the middle of next week, followed by a steady warmup late in the week.

DISCUSSION

Key message 1: Showers and thunderstorms will become numerous this afternoon and evening along and ahead of a front dropping down from the north, followed by widespread rain overnight and again Sunday afternoon and evening. The combination of events will raise the threat for excessive rainfall that could result in some localized flooding through Sunday night.

A complex rainfall scenario is expected to develop across the forecast area through the weekend with several factors to consider, the combination of which could become favorable for excessive rain and increased flooding risk. Fortunately, we begin the event on a recent dry streak.

The initial, more convective, phase should develop this afternoon as a cold front currently laid out across the TN Valley/Mid-Atlantic gets driven southward in back-door fashion by a sfc high moving east from the Great Lakes to New England today. The front will probably not reach the fcst area until the afternoon, such that at least the southern half of the forecast area should be able to warm up into the lower 80s and destabilize with up to 1500 J/kg of sfc-based CAPE in a warm advection flow. By afternoon, the back door front will impinge upon this light SW low level flow and should trigger numerous showers and thunderstorms. Heavy rain looks like the biggest threat from this initial round of activity as the warm advection flow lifts a remnant plume of deeper sub-tropical moisture northward into the Upstate/Piedmont. Precipitable water should rise above 2 inches in this plume, up near the climatological maximum. The concern is that the opposing NE flow behind the backdoor front and the SW flow at 850 mb could result in an effective slow storm movement at some point this evening in some location, probably east of the mtns. The 00Z HREF shows some 6-hr rainfall potential above 2 inches. This might not be enough to cause flooding outside of urban areas, but it could set the stage for flooding in the second round. However, if the frontal boundary should stall, flood problems will ramp up overnight. The threat for severe storms through Saturday night appears relatively low owing to poor lapse rates.

Behind the front, a cold air damming wedge will set up east of the Blue Ridge and will persist through Sunday with temperatures remaining in the upper 60s to lower 70s. Most of the CAMs suggest something of a lull in the action Sunday morning before deeper and better forcing arrive ahead of a shearing short wave moving in from the west in the afternoon. This second phase will be more dynamically forced and will have the potential to produce another inch or two of rain in some spots. Locations that get heavy rain in both phases could see flooding issues. That being said, there is too much uncertainty regarding what locations have the best potential, and the overall QPF has trended downward on this forecast cycle. Thus, a Flood Watch was not entertained.

Key message 2: Dry from Monday afternoon onward. Cool temperatures return for the middle of next week, followed by a steady warmup late in the week.

The main short wave axis should move east across the forecast area early in the day on Monday, after which the support for any lingering precip is quickly swept eastward and the remaining wedge should drain away. The idea is that we would have a window of opportunity before the cooler air associated with the sfc high over the upper Midwest/Great Lakes spills this far south, allowing temps to rebound nicely back to normal with sky slowly clearing. The model guidance that has some precip response into the afternoon over the fcst area appears to be overdone.

From Monday night onward, high pressure will stay in control of our weather with the upper pattern slowly transitioning to quasi-zonal flow. The first high pressure air mass will bring seasonally cool temperatures on the order of 5-10 degrees below normal with mostly clear sky for Tuesday and Wednesday as it moves past to our north. Temps have crept up ever so slightly again in this model cycle, meaning that only elevations above 6000 feet stand much of a chance of seeing temperatures getting down into 30s on Tuesday night. A chunk of that first high will break off and linger over the southern Appalachians Thursday and Friday, with the air mass modifying back to normal temperatures. Should be more spectacular autumn weather. A dry cold front may bring in a reinforcing high for next weekend.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Increasing moisture ahead of approaching frontal boundary is expected to enhance the probability for restrictions in low clouds and/or fog later this morning across the Terminal Forecast Area. Most sites have at least a tempo for restrictions...mostly IFR or low MVFR cigs...although LIFR conditions are tempo/d at KAVL and KHKY. Can't rule out LIFR at the other terminals as well, but this appears to be unlikely. Showers and thunderstorms are expected to develop this afternoon as the atmosphere destabilizes ahead of the front...which is forecast to sag gradually south across the area throughout the afternoon and evening. Prob30s for TSRA are carried at all sites at some point this afternoon, followed by categorical -SHRA/VCTS from mid/late afternoon into the evening.

General light S/SW winds or light/variable winds are expected this morning. Winds will turn to the NE at 5-10 kts behind the front, but timing this transition will be tricky due to uncertainty on the speed of the front. Shower and TS coverage will diminish by the overnight hours...although some degree of SHRA coverage is likely to continue. The larger concern tonight into early Sunday will be the expectation of low clouds developing in association with cold air damming developing north of the frontal boundary...and IFR/LIFR cigs are likely at most sites by 06Z Sunday.

Outlook: Widespread SHRA, cold air damming, and associated widespread, persistent flight restrictions are expected across the area on Sunday. Improvement to VFR is expected by Monday, and will persist well into next week.

GSP WATCHES/WARNINGS/ADVISORIES

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