textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes for this forecast update.

KEY MESSAGES

1. Another frontal system is expected to arrive from the west this weekend, perhaps briefly stalling across our area early next week, bringing back high rain chances, below-normal temperatures, and some potential for locally heavy rainfall. 2. A return to typical mid-summer weather is expected Tuesday, continuing through the remainder of next week.

DISCUSSION

Key message 1: Another frontal system is expected to arrive from the west this weekend, perhaps briefly stalling across our area early next week, bringing back high rain chances, below-normal temperatures, and some potential for locally heavy rainfall.

One more afternoon/evening of convection-free weather is ahead, as below-normal surface dewpoints are resulting in virtually no instability across the forecast area this afternoon. However, this will begin to change tonight, as deep layer ridging move off the East Coast, and a significant trough/upper low approaches the Ohio and TN Valleys...resulting in gradually increasing southerly flow and increasing low level moisture across our area. Convection is expected to develop to our west within warm conveyor belt regime ahead of frontal system attending the height falls, likely making a run toward our western areas by the end of the day. Increasing dewpoints should allow for at least modest destabilization in these areas...and this should allow for convective activity to make some inroads into the area, and generally 50-70 PoPs are advertised across the western third or so of the area.

Warm conveyor belt/deep moisture plume characterized by precipitable water values over 2" will translate into the area Saturday night, with SSW low level jet expected to accelerate to 20-30 kts. Generally confluent low level flow underneath diffluent flow aloft within the very moist air mass will allow convective coverage to blossom Saturday night into early Sunday. The potential for training cells...especially around terrain features with a SSW aspect will increase during this time frame, increasing the potential for excessive rainfall. Having said that, there's not a strong signal in QPF output from convection-allowing models that this threat will be anything more than very localized...at least through Saturday night.

While early day convection/widespread thick cloud cover will limit destabilization during the daylight hours Sunday, modest instability should be realized during the afternoon, allowing coverage of showers and storms to increase...with 70-80 PoPs advertised across much of the area. The locally excessive rainfall potential will continue as the frontal boundary moves into the area by afternoon. With shear parameters also increasing above seasonal normals, can't rule out an organized storm or two producing damaging wind gusts...especially across the eastern half of the area. Shower and storm coverage will likely continue into the early part of Sunday night, but the global models are in general agreement that the deep moisture plume will begin to shift east of the CWA Sunday night. This doesn't preclude another round of solid, mainly diurnal convective coverage on Monday, as upper troughing remains west of the area, and low-level moisture/diurnal destabilization are forecast to be more than adequate for initiation.

One more day of near-normal max temps is expected Saturday, but this will be followed by a couple of days with maxes around 5 degrees below climo under plenty of cloud cover, not to mention anomalously low heights. Min temps will be right around climo in light of the increasingly high humidity.

Key message 2: A return to typical mid-summer weather is expected Tuesday, continuing through the remainder of next week.

An upper low is expected to steadily to steadily spin down across the TN/OH Valleys early next week, although a relative weakness in the subtropical ridge is likely to linger over our region through the end of the week. A return to seasonal levels of heat and humidity is expected by mid-week, supporting near-normal temperatures and typical scattered (mountain) to isolated (elsewhere) coverage of diurnal convection each day from Tuesday through Friday.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: The air mass over the Terminal Forecast Area remains relatively dry this afternoon, and another afternoon/evening that will be free of convection is anticipated. Generally light SW winds are expected this afternoon, becoming more S or SE tonight. Low level moisture will therefore begin to steadily climb such that mountain valley fog/low stratus may be a bit more widespread late tonight/early Saturday, but this is far from certain...especially for KAVL...so a VFR forecast will be advertised for now. The SSE winds are expected to begin increasing after sunrise Sat, likely becoming gusty after 18Z. Increasing moisture and instability is expected to result in better chances for diurnal convection Saturday, but this will be most likely after 18Z at most sites...and actually after 00Z Sunday at KCLT, so there's no need for any Prob30s at this time.

Outlook: Shower and thunderstorm chances and associated restrictions return Saturday afternoon/evening and continue into early next week. While convection will be most active during the afternoon/evening, some degree of overnight and early morning coverage is anticipated Sat/early Sunday. Widespread low clouds and fog are possible Sunday morning and again Monday morning. A return to mostly diurnal convection and early morning mountain valley fog/stratus is expected during the new work week.

GSP WATCHES/WARNINGS/ADVISORIES

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