textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes were made to the forecast this morning.

The aviation forecast was updated to reflect the impending 12Z TAF issuance.

KEY MESSAGES

1. Hot and muggy conditions continue for most of the week, but subside just enough that no heat advisory is necessary for Tuesday. A pattern change may be in store for this weekend. 2. Following a few days of suppressed convection, diurnal showers and thunderstorms return by Thursday and persist through the weekend. A cold front may further enhance storm activity over the weekend.

DISCUSSION

Key message 1: Hot and muggy conditions continue for most of the week, but subside just enough that no heat advisory is necessary for Tuesday. A pattern change may be in store for this weekend.

Heights have fallen significantly over the forecast area following the passage of a weak cold front and associated h50 vort lobe; water vapor imagery and recent RAP analysis now suggest that the subtropical ridge is more of an elongated anticyclone, centered more or less over the Four Corners region of the US Southwest, and extending east toward the Gulf Coast.

Don't expect terribly much change in this configuration through at least Wednesday. The subtropical ridge will become re-established over the western Atlantic over the next 24 hours or so, but the general weakness over the Southeast will persist as a massive bowling ball upper low meanders from northern Manitoba into the Hudson Bay, ejecting a series of shortwaves across the Midwest and upper Mid-Atlantic through the middle of the week. These disturbances won't impact the western Carolinas directly, but will serve to maintain weak troughiness over the area, keeping heights constrained and more generally resulting in cooler temperatures the next two or three days than we've had since late last week. No heat advisory will be issued for Tuesday afternoon; deterministic guidance suggests significant daytime mixing of dewpoints, resulting in dry enough conditions in the postfrontal air mass that confidence simply remains too low on any locations reaching the 105-degree heat index threshold today. The same is true for Wednesday; by Thursday, a resurgence in moisture could allow a few spots south of I-85 to flirt with advisory criteria once again.

By the weekend, long-range ensembles are still in good agreement on a weak cold frontal passage further eroding the warm, moist air over our area. Saturday and beyond, high temperatures should trend back toward seasonal normals and provide some relief from the late-summer heat.

Key message 2: Following a few days of suppressed convection, diurnal showers and thunderstorms return by Thursday and persist through the weekend. A cold front may further enhance storm activity over the weekend.

A cold front (which was largely responsible for Monday evening's severe weather) is now located to southeast, placing the entire forecast area in a postfrontal air mass and keeping conditions suppressed the next few days. Weak capping atop the PBL should be enough to prevent convective initiation today and Wednesday afternoon...and indeed none of the 00Z suite of hi-res guidance features any showers or thunderstorms the next two days.

By Thursday, another weak boundary associated with an h50 shortwave crossing the Mid-Atlantic will settle across the Tennessee Valley. Though guidance doesn't depict an actual frontal passage through the end of the week, the environment across the western Carolinas should become moist and unstable enough to support a return to diurnal convection on Thursday and Friday. It's not even entirely out of the question that a decaying MCS originating on the frontal boundary could slide across the area, though the upper pattern isn't as favorable for such a system to move into our area versus locations farther north. Regardless, even if we only see seasonally-typical diurnal convection, profiles depict enough DCAPE to support a couple of strong to severe storms producing wet downbursts.

By Friday, and certainly by Saturday, the approaching front and strengthening upper flow will allow deep layer shear to increase, and we'll begin to see the potential for more organized convection to develop, until the front crosses the area over the weekend.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: Conditions are expected to remain dry and VFR tomorrow, with little to no convective activity in the wake of tonight's frontal passage. Winds should favor a N/NW direction behind the front, becoming increasingly N through the day. Winds will become lighter overnight tonight, then turn NE at 2-4kts during the daylight hours Wednesday. Isolated ridgetop showers may develop on Wednesday, but in general, expect conditions to remain dry, with far too little coverage for a mention in the TAFs.

Outlook: Diurnal convection is likely to return Thursday, developing each day through this weekend. Low stratus and fog will be possible each morning, especially in the mountain valleys.

GSP WATCHES/WARNINGS/ADVISORIES

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