textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Axis of heaviest rainfall has begun to shift southward, out of the NC mountains and into the SC Upstate. However, confidence in any one scenario remains low.

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

KEY MESSAGES

1. Mostly-dry conditions continue Tuesday and Wednesday, with widespread cloud cover driven by a stationary front to the south. 2. Rain chances increase again Thursday into Friday as an upper wave approaches the area from the Gulf. Locally heavy rainfall and damaging winds cannot be ruled out. Drier conditions return over the weekend behind a cold front.

DISCUSSION

Key message 1: Mostly-dry conditions continue Tuesday and Wednesday, with widespread cloud cover driven by a stationary front to the south.

Quasi-zonal flow continues aloft this afternoon with a weakly-confluent zone visible on WV imagery extending from southern Mississippi across the Deep South and into the southern reaches of our forecast area. Based on recent surface obs, there would appear to be a stagnant stationary front located near or just south of our forecast area's southern border...across the lower Upstate or the SC Midlands. Lots of light radar returns are still evident across the Upstate, but given reported dewpoint depressions of 10-20 degrees...think the dry air behind the front is evaporating most of that precipitation, so it's presenting as virga and never reaching the ground.

Weak confluence over the Deep South and into the Carolinas will continue through tonight, channeling ripples of z500 vorticity across the area, but generally failing to induce any strong upglide in the low levels as per most operational guidance. As a result...think rain will remain to our south for most of today and tonight. A few models depict a brief surge in WAA early Wednesday morning as a potent shortwave crosses the upper Ohio Valley...with stronger forcing staying to our north. This may be enough to briefly set up a round of rainfall south of I-85 across the Upstate, but dry conditions are set to return by mid-morning, as the shortwave moves east and any low-level dynamics go with it. Despite sustained SW flow during the daylight hours, the front itself should remain just to our south, and profiles should remain stable, at least with respect to surface-based parcels. Consequently, little to no afternoon rainfall is expected on Wednesday.

Temperatures today should top out a solid two categories below normal owing mainly to widespread cloud cover associated with the front to our south, as well as some degree of postfrontal CAA. As SW flow continues tomorrow, highs should return to within a degree or two of normal in most locations.

Key message 2: Rain chances increase again Thursday into Friday as an upper wave approaches the area from the Gulf. Locally heavy rainfall and damaging winds cannot be ruled out. Drier conditions return over the weekend behind a cold front.

The pattern should become more active late this week. Guidance remains in decent agreement that by Wednesday evening, a tropical wave (or perhaps even a closed low) will be located somewhere along the northwestern Gulf coast. That feature is still progged to lift northeastward through Thursday morning, interacting with the pre-existing stationary front across the area. Meanwhile, a 500mb shortwave should track out of the lower Midwest on Thursday, trailing the tropical wave as it arrives in the Carolinas. The 12Z suite of operational guidance has come in slower, with less prominent phasing of the shorwave and tropical wave, in favor of more interaction with the surface boundary and an overall more southward storm track. The biggest change to the forecast that results from this is that the axis of heaviest QPF gets shifted south...out of the NC mountains and into the Upstate (or even, in some ensemble members, south of our area entirely). Still, would like to see a more consistent trend before expressing much confidence in this scenario; QPF spread remains enormous even among the subset of ensembles that feature this scenario, so a lot is still, unfortunately, up in the air as to what, if any, heavy rain / flash flood threat may materialize.

A secondary issue will be the potential for severe weather as the system arrives. There are seemingly two opportunities for severe weather to develop; first, along the remnant frontal boundary over the Upstate on Thursday evening or overnight...where decent shear will develop within the stronger flow aloft, and low-level SRH may be further enhanced near the front itself...and second, on Friday afternoon, as the trailing cold front associated with the 500mb shortwave arrives in the area. For at least the former of the two, the threat will be moderated by the ultimate path of the disturbance...which as described above, remains in question. A somewhat more reliable severe risk looks warranted on Friday ahead of the secondary cold front, but even then, much will depend on what storm environment exists in the wake of the wave.

The weekend forecast has not changed much. Behind the cold front, conditions should dry out and begin to warm back up for Saturday and Sunday as high pressure settles into the Mid-Atlantic. However, the upper pattern will remain progressive, with indications that another cold front will approach the region by Monday.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Widespread BKN to OVC mid- and high-level clouds in place across the area this afternoon, with VFR ceilings everywhere. Expect ESE winds currentyly in place across the area to toggle around to the SW through mid-afternoon, with occasional low-end gusts possible. Rain has mostly remained south of the terminal forecast area today, but a couple showers could clip the southern tier of the forecast area this evening...with mentionable chances only at KAND. Elsewhere, still expect ceilings to fall overnight, with MVFR possible at the Upstate sites (perhaps, based on recent statistical guidance, even brief IFR at KAND). Light and variable winds expected overnight. A steady return to VFR conditions is expected at all the TAF sites after daybreak Wednesday.

Outlook: Conditions Wednesday night still look similar to tonight, with mainly VFR conditions possible, but brief MVFR to IFR ceilings possible before daybreak Thursday. More widespread convection is expected across the area Thursday afternoon through Friday. Dry weather returns for the weekend.

GSP WATCHES/WARNINGS/ADVISORIES

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