textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No significant changes to the timing and expected rainfall amounts associated with the cold front this afternoon through Sunday.

Updated for the the 18Z aviation discussion.

KEY MESSAGES

1. Well above normal and humid with isolated to scattered diurnal showers and thunderstorms possible this afternoon and evening. 2. The passage of a cold front Sunday will bring the best rain chances over the next week. The risk of flooding rain or severe weather appears to be low. 3. Frost and/or freeze concerns are still in place for the mountains starting Sunday night, and will continue to be an issue each night through Wednesday. The coldest night regionwide is Tuesday when frost chances spread into the North Carolina piedmont. Protection of sensitive vegetation may be required.

DISCUSSION

Key message 1: Well above normal and humid with isolated to scattered diurnal showers and thunderstorms possible this afternoon and evening.

Bermuda High will continue to filter in deep low-level WAA with the deep layer southwesterly flow helping to aid in a warm and humid airmass with afternoon highs 8-12 degrees above normal and dewpoints in the low to mid 60s. Subtle height falls in response to a closed upper low over the Northern Great Plains and the Southeast ridge propagating further offshore will set the stage for diurnal convection this afternoon and evening. Earlier showers hampered the environment slightly, but latest mesoscale analysis places 1000-1500 J/kg of SBCAPE across the area. Mid-level lapse rates have steepened slightly, which will erode any cap that was in place. Could see a few strong to possibly severe storms, but confidence is low as to whether or not storms get that strong. Likely trigger for convection will be in the form of a differential heating boundary and orographic enhancement in the southwestern mountains. Bulk shear will be <=20 kts and will limit organized convection, which will lower the threat of severe storms. DCAPE values are higher than originally forecasted with modest inverted-v profiles, so a wet microburst can't be ruled out. This activity may linger into the later evening hours if outflow boundaries develop and potentially collide with each other. Highest PoPs will be in the mountains and lower the further east.

Key message 2: The passage of a cold front Sunday will bring the best rain chances over the next week. The risk of flooding rain or severe weather appears to be low.

The aforementioned stacked low over the Northern Great Plains today will quickly shift into the Upper Great Lakes into Ontario and Quebec tonight and Sunday as an attendant cold front moves in from the west late tonight. Timing for rainfall will likely be late overnight through Sunday afternoon before the front completes a full fropa by Sunday evening. Guidance show a QLCS setup as the front encroaches the region, but loses its organization and weakens as it moves across the NC/TN border. Expect a broad band of organized showers likely with embedded thunder overnight across the mountains and shifting into the Piedmont around or shortly after daybreak. There is a very limited severe threat Sunday afternoon as increasing instability develops ahead of the front and the main line crosses into locations along and east of I-77, but will have to see how that shapes out closer in time with the main threat being isolated damaging wind gusts, if any.

With the antecedent dry conditions, especially ongoing severe to extreme drought conditions, any rainfall we receive will be beneficial as a hydro threat would basically be zero. Some favorable southwesterly slopes could receive 1-2", but the rest of the area likely receives 0.50"-1" and some locations receiving less that miss out on convection from Saturday and gets the short end of the stick with the rainfall associated with the incoming cold front.

Much drier air filters in behind the front Sunday night as CAA helps to quickly drop dewpoints and temperatures, while skies clear. Expected overnight lows to drop into the 30s across the mountains and 40s elsewhere. Winds will pick up ahead, along, and behind the front as the direction turns the dial from southwesterly ahead of the front to northwesterly and eventually northerly behind the front by daybreak Monday. Any gust will be low-end with the exception of the higher elevations where gusts could get up to 45 mph behind the front and remains less than 30-35 mph elsewhere.

Key message 3: Frost and/or freeze concerns are still in place for the mountains starting Sunday night, and will continue to be an issue each night through Wednesday. The coldest night regionwide is Tuesday when frost chances spread into the North Carolina piedmont. Protection of sensitive vegetation may be required.

In the wake of Sunday's cold frontal passage, the leading edge of a colder airmass will make inroads into the Southern Appalachians giving rise to light advective freeze possibilities for the high elevations of the NC mountains. Llvl ridging builds in starting on Monday with maximum temperatures returning to the early April climo.

After the initial continental sfc high builds in, a reinforcing shot of colder air arrives Tuesday night thanks to stronger hipres, the center of which translates eastward within the quicker flow off to our north. Frosty temperatures are still forecast to develop in the NC piedmont during the pre-dawn hours on Wednesday but with the tightening sfc pressure gradient and easterly flow it is doubtful much in the way of frost will develop thanks for the wind remaining up all night.

The GSP frost/freeze program for the concern area in NC, (roughly along and south of I-40), commences tomorrow, 5 April. With the initial shot of chilly air Sunday night, light freeze conditions should be limited to just the highest elevations and much in the way of any frost development will be hampered by lingering wind. Tuesday night still looks like the best chance to have frost/freeze issues but again, wind should limit frost development save for select sheltered mountain valleys. So looking ahead, a frost advisory for the NC mountains may be in the cards for pre-dawn hours Wednesday.

Fire weather concerns for next week will concentrate on the likelihood that minimum RH values will dip below 25-30 percent each afternoon from Monday through Wednesday, though accompanied by generally light winds. After what we hope will be a wetting rainfall on Sunday, fuel moisture will dry back out by Tuesday and fire danger will likely increase by then.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: Very low-end VFR stratocu will stream across the terminals this afternoon with showers and thunderstorms possible through this evening. Placed either a TEMPO or PROB30 and associated restrictions with prevailing VCSH through ~00Z. South to southwesterly winds at 6-12 kts are expected through this afternoon with low-end gusts. Winds should lower to 4-8 kts overnight as the coverage of showers and thunderstorms lower, ahead of a cold front. The cold front will move in from the west overnight and bring a round of widespread rainfall across the area, with the onset likely just before or around daybreak as the front moves in from west to east. Can't rule out some embedded thunder, but confidence is not high for more than a PROB30 mention for TSRA. IFR cigs and MVFR/IFR vsbys are likely to accompany the line of showers and embedded thunder overnight through early Sunday afternoon. Any rainfall should be out of the region by 00Z Monday. Winds ahead of the front and after daybreak Sunday will continue out the south and southwest at 5-10 kts with low-end gusts picking back up by the afternoon. Drier air filters in behind the front Sunday evening as winds toggle out of the northwest with low-end gusts continuing through the evening hours on Sunday. Clouds will lift and scatter to MVFR/VFR Sunday afternoon and will be completely VFR by Sunday night at all terminals.

Outlook: High pressure builds into the region behind the front Sunday night into the middle of next week allowing dry and VFR conditions to return.

GSP WATCHES/WARNINGS/ADVISORIES

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