textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Made some tweaks to PoPs late tonight through late Tuesday evening to better reflect trends from the 18Z CAMs.

Updated the aviation discussion for the 00Z TAFs.

KEY MESSAGES

1. Dry through late tonight before a decaying convective line crosses the area Tuesday morning. 2. A storm system and associated cold front will bring rain chances late Tuesday night through Wednesday morning. Another round of precipitation may return over the weekend, but confidence is low at this time.

DISCUSSION

Key message 1: Dry through late tonight before a decaying convective line crosses the area Tuesday morning.

Dry through late tonight before an MCS tracks across the GSP CWA from west to east early to mid-morning Tuesday. Almost all of the high-res guidance have the MCS gradually decaying as it crosses the mountains Tuesday morning. How well this system holds together east of the mountains will be the main forecast challenge. The 18Z HRRR has come in considerably drier east of the mountains compared to the 12Z HRRR. Meanwhile, the 18Z NAMNest has come in considerably wetter east of the mountains compared to the 12Z NAMNest. Thus, confidence on PoPs east of the mountains is low due to model disagreement and run to run inconsistencies. Any rainfall that manages to hold together will be beneficial but not drought busting. Only light rainfall amounts are expected, with the highest amounts confined to the North Carolina and Georgia mountains. Rainfall amounts across the mountains will range from 0.10" to 0.30", with locally higher amounts up to 0.50" along the southern NC/TN border. East of the mountains rainfall amounts will only range from a trace to 0.10", if activity can hold together well enough. Some locations east of the mountains may not even receive accumulating rainfall if the 18Z HRRR turns out to be right. The severe threat should remain very low, if any, with activity coming through prior to peak heating. Lingering convective debris will keep conditions on the cooler side Tuesday, with afternoon highs falling a few degrees below normal for most locations. The exception will be the mountain valleys where highs should end up a few degrees above normal. Drier conditions will return briefly Tuesday afternoon into early Tuesday evening before the next system impacts the area. See Key Message 2 for more details.

Key message 2: A storm system and associated cold front will bring rain chances late Tuesday night through Wednesday morning. Another round of precipitation may return over the weekend, but confidence is low at this time.

By tomorrow night, a second in a series of southern stream shortwaves will be sliding across the Mississippi Valley and lifting into the Ohio Valley and Appalachians. Widespread deep convection will be ongoing across portions of the mid south from southern Illinois down to the I-20 corridor across Louisiana and Mississippi. This convection will reside within fairly impressive parameter space with several clusters of severe storms likely. With time, this activity will progress eastward through the evening and overnight as height falls overspread the Tennessee Valley into the Southern Appalachians. The 12z suite of CAM guidance is in rather good agreement that at least some form of QLCS will be approaching the area from the west during the early morning hours Wednesday. Deep- layer vertical wind shear will be supportive of maintenance of organized convection, but the primary limiting factor will be limited instability with generally 100-200 J/kg of MLCAPE progged during the nocturnal instability minimum. While low, this may prove to be sufficient to maintain at least some degree of linear structure into the forecast area with an associated threat locally gusty winds and perhaps a few isolated severe storms. Any severe threat will largely hinge on how upstream convection evolves. A more well established QLCS would likely survive farther into the area. The morning convection should leave a stable airmass in its wake with a good bit of uncertainty as to whether additional precipitation will be able to develop through the afternoon. A few widely scattered showers seem plausible, but confidence is low as to much beyond that.

A frontal boundary finally clears the area Wednesday night into Thursday morning as the longwave trough axis shifts east and a sprawling surface high drops across the Great Plains and into the Mississippi Valley. This should keep dry conditions in place Thursday through at least Friday morning. Forecast confidence quickly wanes by Friday into the weekend as guidance continues to vary notably as to how the synoptic pattern evolves. The frontal boundary is expected to stall to our south as active southern stream flow persists from the Southern Plains across the Lower Mississippi Valley and into the southeast states. A closed upper low ejecting out of northern Mexico and into Texas is progged to slide across the Gulf States Friday into the weekend. The exact track of this feature and placement of the boundary will determine northward extent of rainfall. The GFS has trended farther north with a nice round of rain across the region while the ECMWF solution remains suppressed with a dry forecast. The forecast will continue to follow the national model blend for now and will be adjusted as needed once guidance comes into better agreement.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: Dry and VFR through tonight before conditions gradually deteriorate around daybreak Tuesday as a decaying line of showers, and possibly isolated thunderstorms, tracks across the terminals. Maintained PROB30s for -SHRA and associated restrictions for the 00Z TAFs. However, confidence on PROB30s east of the mountains is very low as activity will weaken as it pushes off the mountains throughout the morning hours. Some of the guidance shows activity holding together east of the mountains while other guidance shows it falling apart almost entirely. Drier conditions return from west to east by the mid to late morning hours. MVFR to IFR restrictions will linger through mid to late afternoon before VFR returns towards the end of the TAF period. Winds start out SSE/SE this evening before toggling more S'ly around daybreak. Winds will gradually turn more SW Tuesday afternoon. Low-end, intermittent gusts are possible across the terminals on Tuesday but confidence is too low to mention in the TAFs at this time.

Outlook: A cold front brings rain and associated restrictions back Tuesday night into Wednesday. Dry and VFR conditions may return Wednesday night into Friday before another disturbance brings rain chances and associated restrictions over the weekend.

GSP WATCHES/WARNINGS/ADVISORIES

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