textproduct: Greenville-Spartanburg

SYNOPSIS

An arctic cold front will cross the region early Sunday, ushering in bitterly cold temperatures to start the week. The arctic high pressure system will quickly shift to the east and allow temperatures to quickly rebound by midweek. Another cold front may bring a return of rain chances late in the week.

NEAR TERM /THROUGH SUNDAY/

As of 1230 pm EST Saturday: A broad long wave trough over the region will sharpen tonight into Sunday, as a strong speed max digs from the northern Great Plains this afternoon to the southern and central Appalachians by late Sunday morning. Associated arctic cold front will sweep quickly across our forecast area from ~shortly after midnight through around sunrise. The strongest and deepest forcing will pass north of the CWA, but there may be enough weak moisture return ahead of the front to squeeze out a shower or two...but precip chances will be highest to our north (stronger forcing) and south (better moisture return), with mainly token 20-30 PoPs across our area. In the wake of the front, strong NW cold advection flow will become established, with moist upslope flow supporting scattered/numerous snow shower development in the vicinity of the TN/NC border from around the pre-dawn hours through much of the morning. However, thermodynamic ingredients supporting accumulating snow showers will be in place only very briefly, with the potential for a couple of inches being mainly confined to the high peaks along the state line.

Otherwise, temps will plummet behind the front, with Sunday's high temps likely occurring around midnight tonight. After the initial surge of cold advection, temps are forecast to slowly fall across the mountains, while remaining steady or slowly rising elsewhere (due to the offsetting effects of warming downslope flow.) Winds will be very gusty through the day, with gusts of 30-40 mph expected across the high elevations and in the downslope zone along the Blue Ridge escarpment. These very gusty winds will persist through sunset, when temps are likely to begin falling more sharply. As a result, wind chill values above 3500 feet or so across the northern and central NC mountains are forecast to meet or exceed Cold Wx Advisory criteria...and one will be issued for these areas from 5 pm Sunday through Sunday night.

SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/

As of 1200 PM EST Saturday:

Key Message: Arctic front brings gusty winds with bitterly cold temperatures Sunday night through Monday.

Strong 850 mb CAA and sfc pressure rises will keep gusty winds into the evening Sunday, as temps continue to plummet behind the passage of an arctic front. Winds will begin to weaken late in the evening thru the overnight, and temps will fall well into the teens across most of the Piedmont, and single digits in the mountains. These temps combined with lingering winds will bring wind chills below zero across most of the mountains, and single digits to lower teens across the Piedmont. A Cold Weather Advisory will be issued for the NC mountains above 3500 ft and all of Avery County. The Piedmont criteria are more complicated, with the Foothills and NW NC Piedmont needing 5 degrees or less, while the southern/lower Piedmont only needs 10 degrees or less. Even bumping up winds with the 75th percentile of the NBM, wind chills struggle to meet these criteria, except for portions of the I-77 corridor. Given this is still in the third period of the forecast, will hold off on issuing any Cold Advisory outside the mountains with this forecast package.

Winds should become light and skies remain clear making it look deceptively nice on Monday. But the arctic air mass will settle right over the area and keep temps mainly in the 30s to lower 40s, which is 15-20 degrees below normal for mid-December. The high will begin to slide east Monday night, allowing for decent WAA aloft. Still should see good radiational cooling and bring low temps into the 20s area-wide. With light winds, wind chills shouldn't be an issue.

LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/

As of 1215 PM EST Saturday:

Key Message: Temperatures rebound and dry conditions settle in before precip chances increase by the end of the week.

Arctic high pressure slides off the East Coast under quasi-zonal flow aloft Tuesday, allowing for a nice rebound in temps and continued dry weather. Highs return to lower to mid 50s (except 40s above 3500 ft) and lows in the upper 20s to mid 30s. There may be periods of increased mid and high clouds streaming thru westerly flow, but overall, mostly clear skies is expected. The 12z GFS tries to bring a few showers to the mountains from the west Wednesday, but this seems overdone compared to the rest of the models. Better rain chances may return late Thursday thru early Friday, as models are coming into better agreement on a cold front approaching from the west. Temps warm to slightly above normal Wednesday thru Thursday, so precip should be largely just rain. The air mass behind this front looks mild and only knocks temps down to around normal Friday into next weekend.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

At KCLT and elsewhere: VFR is forecast through much of the period. The only exception will be within a brief window from ~09Z thru ~15Z Sunday, when moisture will increase immediately ahead of an arctic cold front. Low VFR and MVFR cigs are expected to accompany this moisture...with restrictions being most likely at the NC terminals. A brief shower will also be possible...but probabilities only support Prob30s at most. Mainly light SW winds of 5-10 kts this afternoon will shift to NW and increase beginning around 09Z Sunday, with gusts increasing through the morning...to 30- 35 kts at KAVL and mostly around 25 kts elsewhere.

Outlook: Gusty N/NW winds will diminish Sunday night into daybreak Monday. Dry high pressure will keep VFR conditions in place through mid-week. The next cold front will bring shower chances and possibly restrictions by Thursday.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Cold Weather Advisory from 5 PM Sunday to 9 AM EST Monday for NCZ033-048>053-058-059. SC...None.