textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The potential for strong to severe convection this morning has increased somewhat.

A brief period of strong gradient wind gusts of 40 to 50 mph is possible in the immediate wake of the front this morning, mainly in the vicinity of the Blue Ridge shortly after sunrise.

The approach of the next cold front over the weekend has slowed somewhat. It is now forecast to move thru our area during the day on Monday.

KEY MESSAGES

1. A strong cold front will bring widespread showers and isolated thunderstorms posing a marginal risk of severe weather this morning. A brief period of very strong gradient wind gusts are possible in the immediate wake of the front, followed by a brief transition to a wintry mix across the mountains, producing light accumulations across the higher elevations. 2. Cooler and drier conditions will linger over the area on Friday and Saturday. Rain chances return Sunday and Monday, with snow showers possible near the NC/TN border on Monday. 3. Much colder air returns behind a strong cold front Monday night and lingers through at least Wednesday. Most areas will likely see temperatures at or below freezing both Tuesday and Wednesday morning.

DISCUSSION

Key message 1: A strong cold front will bring widespread showers and isolated thunderstorms posing a marginal risk of severe weather this morning. A brief period of very strong gradient wind gusts are possible in the immediate wake of the front, followed by a brief transition to a wintry mix across the mountains, producing light accumulations across the higher elevations.

The latest water vapor imagery depicts a split flow regime atop the Conus, with strong short wave troughs over the Miss Valley...one in the northern stream, and more importantly for our area, a southern stream wave over Louisiana. These two features will progress east through the morning, coincident with a process of phasing of the two streams...which will be completed east of the Appalachians later today. Associated with these height falls...a strong cold front currently extends roughly along the Cumberland Plateau, attendant with a broad frontal band of precip...the bulk of which is to be found on the cold side of the boundary. This activity will continue to move steadily east through daybreak...likely accelerating a bit later this morning as the flow aloft strengthens with the phasing of the pattern.

Looking upstream...strong-to-severe convection is ongoing across southern Alabama...where strong forcing associated with the southern stream wave is interacting with a moderately unstable and strongly sheared environment. Closer to home...there is little instability currently to be found across our forecast area, but lower 60s dewpoints are not too far upstream, and these are expected to advect into at least the southern half of the area later this morning...which should destabilize these areas to the tune of 200- 400 J/kg of sbCAPE. One somewhat recent development in short term guidance is suggestion of meso-low development along the cold front as it moves across northeast GA and the Upstate between 5 AM and 9 AM...which could be responsible for local enhancement of 0-1km shear values in excess of 35 kts while also producing very focused low level ascent along the leading edge of the front. This is creating some concern that the threat for severe storms will be a little higher than previously anticipated...with the potential for isolated damaging wind gusts as well as a brief, weak tornado or two being in play.

Winds will shift abruptly to the NNW within 1-2 hours of precip onset, with strong cold advection overspreading the area through the morning. Short term guidance indicates a very strong gradient on the cold side of the boundary later this morning, which could yield a period of 40-50 mph gusts for an hour or two after fropa...especially along and just downwind of the Blue Ridge. Since this period of strong winds is expected to be brief (with mainly 25-35 mph gusts expected to continue through the afternoon), we're not planning on on wind headlines at the moment. With the surge of strong CAA, snow levels will plummet across the mountains from around sunrise onward...with cold air likely interacting with the back edge of the forcing/moisture for 2-3 hours. This is expected to result in a period of mostly snow or a rain/snow mix across at least the higher elevations. Anywhere from a dusting to an inch of accumulation is expected across the TN border counties...mainly above 3000.' Upslope enhancement could yield as much as 3 inches in the high elevations along the immediate state line.

Skies will quickly clear from the west from late morning through the afternoon, with afternoon temps expected to be 25-30 degrees cooler than the past couple of days. Widespread freezing temps are expected across the mountains tonight for the first time in about two weeks, with mins in the lower-to-mid 30s expected across the remainder of the area.

Key message 2: Cooler and drier conditions will linger over the area on Friday and Saturday. Rain chances return Sunday and Monday, with snow showers possible near the NC/TN border on Monday.

Cool and dry sfc high pressure briefly settles over our area later tonight into Friday, lowering temperatures to a few degrees below normal for the first time in a couple weeks. Temperatures roughly 20 degrees cooler than the last few days will certainly be noticeable. Temperatures rebound somewhat Friday afternoon, with highs near normal. In addition, surface low pressure passing well to our north on Friday will tighten the pressure gradient over the Carolinas and produce some gusty winds across the area. Gusts of 20 to 35 mph are expected over the higher elevations, with gusts in the 15 to 25 mph range across our lower terrain. On Saturday, temperatures will warm to roughly 5 to 10 degrees above normal, as high pressure moves well offshore and SLY return-flow sets up over the area.

The next storm system will eject out of the Plains on Sunday, lifting a warm front over our area. This front may generate isolated to sct showers and a few thunderstorms, as some amount of sfc-based CAPE will likely be present over our southern zones during the afternoon/evening. A cold front will then push into our fcst area Monday morning and exit to the east by Monday evening. Some amount instability and shear will likely support a non-zero severe thunderstorm threat, especially over our eastern zones, but if the timing of the fropa changes, so will any severe threat. QPF is not expected to be excessive at this time. We can expect gusty winds across the NC mtns ahead of the front on Sunday and stronger gusts on Monday across our entire fcst area. Gusty winds are likely to persist over the mtns well into Tuesday. It remains unclear how much moisture will linger in the NWLY flow behind the front, but an inch or two of snow is possible above 3500 ft along the TN border before the precip tapers off Monday evening.

Key message 3: Much colder air returns behind a strong cold front Monday night and lingers through at least Wednesday. Most areas will likely see temperatures at or below freezing both Tuesday and Wednesday morning.

Broad and deep upper trofing will dig across the central CONUS on Monday, settle over the eastern CONUS on Tuesday, and linger thru at least the middle of the week. This will allow broad Canadian sfc high pressure to settle over our region and linger thru mid- week. Temperatures will fall 5 to 15 degrees below normal by Tues morning, with highs 10 to 20 degrees below normal Tues afternoon. While the growing season usually does not begin this early, the recent warm weather is likely resulting in some tender vegetation blooming, which may be vulnerable to this cold snap. Temperatures will gradually warm Wed and Thurs, but remain below normal for mid- March.

AVIATION /06Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: SW winds of around 10 kts with occasional gusts up to 20 kts will continue through daybreak in advance of an approaching strong cold front. Widespread showers, reduced visby, and lowering cigs will begin overspreading the Terminal Forecast Area between 09-12Z. Categorical SHRA along with MVFR conditions w/ tempo IFR conditions is expected mostly between 10-16Z. An abrupt wind shift to NNW is expected within 1-2 hours after precip begins, with gusts increasing to around 25 kts. A brief winds of a couple of hours of 30-40 kts gusts are possible around sunrise, especially at KAVL. Isolated TSRA are also expected to impact the area. For the time being, coverage is expected to be too limited for a TAF inclusion, but this will need to be closely evaluated, especially for the upstate SC terminals.

Skies will clear from the west from late morning into the afternoon, with gusty conditions continuing into the evening hours. Winds will diminish this evening, but are expected to remain gusty at KAVL through the end of the period.

Outlook: VFR from Thursday night through Saturday. Gusty winds are again expected Friday. Another cold front may bring showers and associated restrictions Sunday.

GSP WATCHES/WARNINGS/ADVISORIES

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