textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated temps and dewpoints this morning per latest obs, and accounted for more valley fog than originally expected. Aviation discussion revised for 12z TAFs.

KEY MESSAGES

1. Near-normal temperatures with relatively low humidity through Friday, and a near-zero chance of rain. 2. Another frontal system is expected to arrive from the west this weekend, possibly stalling across our area early next week, bringing back high rain chances but also below-normal temperatures.

DISCUSSION

Key message 1: Near-normal temperatures with relatively low humidity through Friday, and a near-zero chance of rain.

Sfc high pressure south of the Great Lakes will maintain light northerly breezes over most of the area through today; front will remain stalled over the SC Lowcountry and south GA. Under clear skies some fog formed in the mountain valleys and a couple of rural riverside locations in the Piedmont, but these should dissipate quickly this morning with full sunshine. Temperatures today will run a couple degrees below normal, and very dry air should mix down from the top of the PBL such that dewpoints fall well into the 50s at many locations during mid to late afternoon. RH will reach the mid to upper 30% range, with a few localized areas closer to 30% likely. As the previous shift said, "Aaahhh." The high will shift over the Appalachians tonight and winds should trend lighter; under mostly clear skies and with dewpoints not really rebounding much, it appears reasonable to favor the cooler end of the guidance range for Fri morning mins, so our forecast ends up 5-8 degrees below normal.

Weak return flow should develop in low levels during the day Friday as the high shifts to our east, promoting some degree of moisture return, but that will be offset by good mixing into still dry air above the PBL, and with temperatures trending slightly warmer we end up with similar min RH in the afternoon. A shallow layer of instability could develop below the subsidence inversion, so we should see an uptick in diurnal cumulus though not enough to suggest a return of more than single-digit PoPs.

Key message 2: Another frontal system is expected to arrive from the west this weekend, possibly stalling across our area early next week, bringing back high rain chances but also below-normal temperatures.

High pressure will be offshore by late Friday. Southwesterly low level flow will continue Friday evening, by which time a maturing low pressure system will be present in the upper Mississippi Valley. This system will remain on a southeastward track as it begins to occlude early Saturday morning. The near-sfc flow appears to back slightly in advance of the system, which should increase moisture transport from the Atlantic and develop cloud cover during the morning. The forward progress of the system slows as it continues to occlude but WAA strengthens aloft and PWATs march upward through Saturday. Vertical profiles again appear at least marginally favorable for deep convection and PoPs increase steadily from west to east, along with isolated to scattered thunderstorms. Assuming cloud cover forms as expected temperatures should remain similar to Friday's although RH values will be back to around normal, so it will be on the muggy side.

The shortwave appears to phase with a broader wave moving across the Great Lakes late Saturday or Sunday, and the combined trough promotes moist flow further up the Eastern Seaboard, washing out the last vestiges of the dry airmass and bringing back subtropical dewpoints and PWATs Sunday, and the best overall chance of precip appears then. Clouds and precip hinder warming such that temperatures end up 6-9 below normal. The high PWAT, deep warm cloud layer, and small MBE vectors imply a threat of at least locally heavy rainfall. Models still mostly show an upper low being left behind as the northern trough remains progressive, and a convergence zone stretching out between the lower MS Valley and the Northeast states. There remains spread as to the position and how long this lingers over our region but it appears likely to last through Tuesday if not Wednesday. Hence PoPs remain above climatology each day and overnight precip cannot be ruled out. Temperatures remain below normal but trend gradually upward each day Mon-Wed.

AVIATION /12Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: Dry and VFR thru the period aside from some mountain valley fog/stratus in the first hr or two of the period, though the fog is not expected to spread to KAVL. Given the dry conditions diurnal cumulus today will be rather sparse. Winds mainly NNW to NE thru the period, possibly backing to SW at KAND during the afternoon. Enough gusts are expected to occur at KAVL to warrant a mention; a brief gust or two is possible elsewhere. Mountain valley fog likely again tonight near the TN border but confidence too low to include at KAVL. Winds will come up from the SW late Fri morning at KCLT, and diurnal cu potentially will be a little lower (still VFR) and more numerous as moisture begins to the area.

Outlook: Dry and VFR through Friday, with shower and thunderstorm chances and associated restrictions returning this weekend into early next week. Low stratus and/or fog will be possible each morning starting Friday, mainly in the mtn valleys.

GSP WATCHES/WARNINGS/ADVISORIES

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