textproduct: Greenville-Spartanburg

SYNOPSIS

A cold front moves into the area today and to our east on Wednesday, producing widespread showers and a few thunderstorms. Much colder temperatures arrive behind the front and continue into the weekend. Forecast confidence is very low beyond Saturday as another cold front may affect the weather Sunday or Monday.

NEAR TERM /THROUGH TONIGHT/

As of 1105 PM Monday: Benign weather will give way to a more active pattern today and tonight as a series of troughs impact the area. A lead southern stream shortwave trough is currently lifting across the Lower Mississippi Valley and will slide across the Southern Appalachians this morning. Following right behind it is a sharp northern stream trough that's progged to deepen and take on a negative tilt as it drops across the Central Plains and into the Midwest late this afternoon into tonight. This will bring several rounds of weather to the region with the primary focus being on the potential for a few isolated instances of severe convection. Low- level mass response has resulted in a robust low-level jet translating across the Lower Mississippi Valley through the overnight hours which in turn is helping to draw a buoyant warm sector inland from the Gulf. Surface-based CAPE extends as far north as the I-20 corridor across Louisiana into Mississippi with a band of deep convection extending from the Louisiana coast to northern Mississippi.

With time, this activity will shift east overnight but progress will be slow owing to the positive tilt of the lead trough and boundary parallel flow. Guidance has slowed the arrival of any deeper convection across the mountains to after daybreak, which makes sense given latest upstream timing. Ahead of the convective band, warm advection in conjunction with isentropic ascent may help isolated to scattered showers blossom across the Upstate north of I-85 and the southern Blue Ridge escarpment. Eventually, what's left of the convective band is forecast to push into the mountains and northeast Georgia closer to mid morning. By this point, the convection will likely have outpaced the warm sector with surface-based CAPE confined to along and south of I-20 across Alabama and far western Georgia. Thus, this initial round of convection is expected to be elevated above a surface stable layer extending up to ~850mb per forecast soundings. This will be a saving grace as shear profiles are rather concerning with long cyclonically curved hodographs and robust SRH. However, despite convection being elevated, a few strong wind gusts could occur at the surface due to dynamic lifting and dropping of the stable layer by the convection line.

Convection will quickly weaken through the morning as the remnant line pushes across the mountains and western Upstate. A few embedded lightning strikes with some degree of elevated instability will be possible, but any isolated strong storms should have since weakened. This will make for a rather complicated temperature forecast, however, with plentiful rain cooled air along and north of I-85 where dewpoints in the 40s and low wetbulb temperatures will help to lock in a rather sharp temperature gradient. Highs across portions of the mountains, and especially the adjacent foothills, will likely remain in the mid 50s. South of I-85 on the other hand will see warmer highs again in the upper 60s to low 70s. At least scattered showers will likely continue through the afternoon before coverage starts to ramp back up late afternoon into the evening. The previously mentioned strengthening northern stream trough will bring renewed height falls across the region along with strengthening wind fields. The right entrance region of a robust upper jet is progged to translate across the Southern Appalachians through the evening and into the overnight. Mass response to a deepening surface cyclone over the Great Lakes will also help to advect upper 50s to low 60s dewpoints back into the area, especially along and south of I-85. The uptick in forcing in concert with the arrival of an established warm sector will promote the development of a second round of deep convection this evening into tonight. Several rounds of storms may lift across the area during this period with the bulk of this activity focused along the warm front in the vicinity of I-85. 100- 250 J/kg of surface-based CAPE may be realized despite nocturnal timing with around 60kts of deep-layer vertical wind shear. Low- level shear won't be as impressive by this time as flow will have veered to out of the southwest with notably less low-level curvature in the hodographs. Thus, a couple strong to marginally severe storms cannot be ruled out with mainly a threat for locally gusty/damaging winds. If a storm can favorably interact with nearby boundaries an isolated tornado cannot be completely discounted, but confidence is much lower. Flooding is not expected to be an issue owing to dry antecedent conditions and progressive nature of any storms.

SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/

As of 1210 AM EST Tuesday: Any lingering precip Wednesday morning should taper off quickly as the cold front moves east of the area and a dry and cold air mass begins moving in. That said, the cold air will be delayed until Wed nite. With clearing skies and slow air mass arrival, highs will be 10 to 15 degrees above normal with breezy conditions. The cold air arrives in earnest Wed nite. Lows will be up to 5 degrees below normal. Cooling continues on Thanksgiving day into the night. Highs will be 5 to 10 degrees below normal with lows around 10 degrees below normal. Breezy to windy conditions continue across the mountains through the period with breezy conditions elsewhere during the day. Can't rule out the need for a Wind Advisory for portions of the mountains.

LONG TERM /FRIDAY THROUGH MONDAY/

As of 1240 AM EST Tuesday: The cold and dry air mass remains over the area through at least Saturday. Breezy to windy conditions will continue Friday with winds abating Friday night into Saturday as the center of surface high pressure moves into then east of the area. Highs Friday 10 to 15 degrees below normal and around 10 below normal Saturday. Lows Friday night 10 to 15 degrees below normal. The main concern, beside the cold temps, during this period is the low relative humidity and breezy winds. Winds do drop off below Red Flag criteria Friday even though they do remain breezy and will be light on Saturday. RH drops below 25% across much of the area Friday but recovers slightly for Saturday. For now, it looks like Red Flag would not be met, but it bears monitoring.

Forecast confidence drops significantly for the rest of the period. The GFS continues to show a dry forecast until Monday when a split flow becomes active and a cold front associated with the northern stream and a Gulf low associated with the southern stream move into the area bringing rain to the area. The GFS shows a cold rain with temperatures warming above freezing but a weak cold air damming pattern setting up. The Canadian and ECMWF area faster with this system bringing precip into the area Saturday night and Sunday. Should this occur, there may be enough cold air across the mountains for a wintry mix to develop. The GFS ensemble mean is slightly faster, but still slow enough to preclude any wintry precip. Other ensembles trend toward the faster guidance. Given the uncertainty, kept precip chances Sat nite in the slight chance range with any wintry precip limited to the mountains, similar to the model blend. Precip chances increase Sunday, but temps warm enough to chance precip to all rain except for the highest elevations. The blend also keeps liquid rain chance in place on Monday, while the faster models suggest drying by then. Obviously, this will be monitored with everyone staying up to date with the latest forecasts given the busy travel day Sunday.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: A messy TAF period is upon us as a storm system impacts the area this morning through tonight. Moisture streaming into the region has brought a deck of low stratus across the Upstate with MVFR ceilings. In addition, an area of showers and thunderstorms is currently pushing across Alabama towards Georgia. What's left of this activity will move through the western Carolinas later this morning, but how well it remains intact is yet to be seen. At the least, scattered showers will be possible. Thereafter, scattered showers continue through the afternoon before a second round of more widespread showers and storms develops this evening into the overnight. Reduced visibility will be possible with any showers and storms. Precipitation will move out of the area overnight with IFR stratus developing in its wake. Winds will be light and variable through the period with several wind shifts to be expected, especially in the vicinity of any storms.

Outlook: Drier and predominantly VFR conditions should return by Wednesday morning and linger through late week.

GSP WATCHES/WARNINGS/ADVISORIES

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