textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Rainfall chances were increased slightly for Monday afternoon.

The aviation discussion was updated for the recent 18Z TAF issuance.

KEY MESSAGES

1. Scattered showers and some embedded thunderstorms will continue this afternoon and evening ahead of an advancing cold front. The front will cross the area tonight, resulting in quieter conditions on Sunday and early Monday. 2. A backdoor cold front should arrive on Monday afternoon, but confidence is limited on what, if any, impacts will develop as a result.

DISCUSSION

Key message 1: Scattered showers and some embedded thunderstorms will continue this afternoon and evening ahead of an advancing cold front. The front will cross the area tonight, resulting in quieter conditions on Sunday and early Monday.

It's now coming up on 2 PM, and as expected a lobe of better moisture has pushed into the area beneath a sharp z500 shortwave trough aloft. Weak low-level upglide has developed in response, and scattered showers with rain rates of 1-2" per hour have developed, mainly west of I-26 but continuing a northeastward push.

The 12Z suite of model guidance is in good agreement that the shortwave will translate eastward through this evening and tonight, crossing the GSP forecast area and moving east of I-77 through early morning Sunday. Most of the hi-res models feature a weak LLJ developing after sunset, ahead of a weak advancing sfc front. Confidence is limited, but with PWs already 2-2.2" across the SC zones and parts of the I-77 corridor, this could result in increasing rain rates as the system pushes across I-77 and the Charlotte Metro area. This area isn't especially primed yet for hydro issues, but can't entirely rule out isolated nuisance flooding in some of the flashier spots across those zones (and in fact, we do already have an advisory in effect farther upstream, across the western Upstate, for that same axis of moisture!). Severe prospects appear much smaller...with limited instability, poor lapse rates, and very moist profiles keeping stronger convection suppressed. Nonetheless, there will be a meager ~20kts of deep layer shear, particularly across the I-77 corridor this evening, so a few semi-organized segments, perhaps even an isolated strong to severe wind gust, isn't entirely out of the question.

Hi-res ensembles are in good agreement that a surface front will slide across the area overnight, ushering in drier air on the backside as depicted by low-level theta-e fields. As a consequence, a potentially murky night tonight will give way to much nicer conditions on Sunday, as a weakly-capped air mass filters in and moisture retreats to our east. Generally think the Sunday/Sunday night period should be mostly dry and quiet, notwithstanding a few ridgetop showers across the Blue Ridge on Sunday afternoon.

Key message 2: A backdoor cold front should arrive on Monday afternoon, but confidence is limited on what, if any, impacts will develop as a result.

By Monday, a building upper anticyclone over the southern Great Plains and Gulf Coast will begin to interact with longwave troughing over eastern Canada and New England. A number of longer-range ensembles features a 500mb shortwave pivoting out of the Ohio Valley between these two features, and at least clipping our northern and eastern zones Monday afternoon and evening. There's decent confidence we'll see some synoptic-scale lift, enough to expect an increase in cloud cover, and low-level theta-e fields again suggest that a weak backdoor cold front will push across the area Monday evening, trailing behind the trough. However, the antecedent air mass will have had little to recover moisture following tonight's FROPA, and as a result most ensemble members don't feature much rain with this backdoor front.

The convective environment will once again be constrained by poor lapse rates, and limited instability (by some models' account, barely hitting 1000 J/kg). That being said, ensemble profiles do at least feature some dry air aloft, and marginal deep-layer shear values ahead of the front, to the tune of 20-25kts, so a nonzero threat of strong to severe winds could be in order, conditional upon any convection actually developing...which the recent round of guidance isn't especially jazzed about.

Behind the front, there's a signal for possible cold-air damming, but the upper pattern is devoid of much moisture, so this could have more of an effect on mid-week temperatures than anything else, with limited potential for rainfall through mid- to late-week.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: Generally VFR conditions in place this afternoon, with some locations, mainly in the Upstate for the time being, seeing intermittent flight restrictions owing to ongoing showers across the area. Expect an axis of rain, at times with thunder embedded, to continue marching across the Upstate over the coming hours, and cross the NC Piedmont and I-77 corridor through this evening. A few lingering showers may persist past sunset and into the first part of tonight. Intermittent MVFR to IFR restrictions are likely in association with this rainfall. Winds will generally be out of the SW, but could be erratic near stronger showers and storms.

Tonight, lingering low-level moisture should contribute to some areas of patchy fog, as well as MVFR to IFR ceilings, mainly before daybreak Sunday. However, an advancing cold front will scour out most of this moisture quickly through perhaps 14Z Sunday, and VFR conditions are expected to prevail with a light 3-6kt NW breeze through the day Sunday.

Outlook: Quiet conditions are expected to persist through Sunday night and early Monday. A second round of showers may develop on Monday afternoon and evening, but confidence is limited, and coverage may only be isolated.

GSP WATCHES/WARNINGS/ADVISORIES

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