textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

A Cold Weather Advisory was issued for Avery County and the elevations above 3500 ft for Mitchell and Yancey counties due to low wind chills tonight into Thursday morning.

The chance of snow continues to increase Fri night/Saturday, with measurable snowfall now likely across most of the forecast area.

KEY MESSAGES

1. Below-normal temperatures continue through tonight. A Cold Weather Advisory has been issued for Avery County and Yancey and Mitchell counties above 3500 ft for dangerous wind chills. Also, patchy black ice this morning will result in slippery spots that could cause hazardous travel. 2. Extended cold wave continues, as temperatures will remain well below normal through early next week. The coldest weather is expected over the weekend, when a return of temperatures supporting Cold Weather Advisories is forecast. Otherwise, dry weather continues through Thursday night. 3. Confidence is increasing that a significant winter weather event will impact at least a portion of the forecast area from Friday night through Saturday, with snow expected to be the primary precipitation type. Due to cold temperatures leading up to the event, impacts to travel could be significant.

DISCUSSION

Key message 1: Below-normal temperatures continue through tonight. A Cold Weather Advisory has been issued for Avery County and Yancey and Mitchell counties above 3500 ft for dangerous wind chills. Also, patchy black ice this morning will result in slippery spots that could cause hazardous travel.

A passing mid-level shortwave will drive a dry reinforcing cold front thru the area early this morning. Temps will still manage to be similar, if not a deg or two warmer than yesterday east of the mountains, thanks to downslope flow. But the higher elevations, especially above 3500 ft near the TN border will likely see colder highs today with strong CAA. The passing frong will bring increased winds across the mountains thru the day and into tonight, with peak gusts expected this evening. Not expecting advisory-level gusts except at the highest elevations. But the winds will combine with colder temps to produce sub-zero wind chills again tonight in the elevations above 3500 ft. A Cold Weather Advisory will be issued for Avery and above 3500 ft in Mitchell and Yancey counties, where -5 or lower wind chills will be possible into Thursday morning.

Also, there are some lingering black ice concerns from refrozen melting sleet. A Special Weather Statement for patchy black ice across parts of the region continue thru early morning. There could be slippery spots in shady places and roads that still have melting snow or sleet piles on the shoulders.

Key message 2: Extended cold wave continues, as temperatures will remain well below normal through early next week. The coldest weather is expected over the weekend, when a return of temperatures supporting Cold Weather Advisories is forecast. Otherwise, dry weather continues through Thursday night.

A deep longwave trough will remain entrenched across much of the eastern Conus through the end of the week. Temperatures remain 10-15 degrees below climo Thursday through Friday, but with winds being a non-issue, headlines for Cold Weather Advisories are not anticipated. Nevertheless, the potential for damage to infrastructure from cold weather will gradually increase due to the prolonged nature of the cold wave (Friday will be the 7th consecutive day of 10+ degree below normal temperatures.) Even colder weather develops for the weekend as temperatures plummet on the west side of a developing coastal storm system, with widespread Cold Weather Advisory criteria looking like a good bet Saturday night.

Otherwise, dry conditions are anticipated through Thursday night, a although a weak disturbance passing across the TN Valley could bring some flurries or light snow showers to the high peaks and ridgetops on Thursday.

Key message 3: Confidence is increasing that a significant winter weather event will impact at least a portion of the forecast area from Friday night through Saturday, with snow expected to be the primary precipitation type. Due to cold temperatures leading up to the event, impacts to travel could be significant.

An intense short wave trough/evolving upper low is forecast to dig from the Corn Belt at the start of the period, to the Tenn Valley by Sat afternoon before the center of the upper low passes over, or just south of our CWA late Saturday...becoming increasingly negatively tilted w/ strong downstream diffluence as time passes. The timing of the development of this diffluence will be key in determining the placement and timing of cyclogenesis near the Southeast Coast Fri night/Saturday and associated sensible weather impacts for our forecast area. What is becoming clear is that surface development will occur close enough to the coast to allow frontogenetically-forced moisture and lift to impact at least the eastern half of the CWA Fri night into Saturday. PoPs have been increased to likely in these areas by Saturday morning...with at least solid chances across the western half. In terms of the precip type, temperatures may borderline at the RA/SN boundary for some areas at the beginning of the event, but all locations should quickly transition to snow as cold advection intensifies on the west side of the developing cyclone. In terms of QPF, the general consensus among deterministic and ensemble guidance is in the 0.25-0.5" range for much of the area, with the higher end of that range being more probable along/east of the I-77 corridor. However, with forecast profiles quickly cooling to well-below freezing through a deep layer, and surface temps that are expected to linger in the 20s, snow ratios look to be higher than the textbook 10:1 rule of thumb with this event, with snow ratio guidance from the National Blend of Models suggesting ratios of 15-20:1 are very possible. Based upon the latest NBM probabilistic guidance...which is largely supported by the individual ensemble systems of the ECMWF/Canadian/GFS, much of the forecast area will likely see at least Advisory-level snowfall, with solid chances for Warning-level snow of 3" or over areas roughly east of I-26. Much of this event is beyond the short term...and so some degree of deterministic run-to-run model shenanigans are to be expected over the next 24 hours or so, with single model runs depicting jogs to the east (less snow) or to the west (more snow) to be expected, but the key point is that at least Winter Weather Advisory-level snowfall now appears likely across all but the western quarter of the CWA by the end of Saturday.

Temperatures are expected to remain well below normal through Sunday to the tune of 15-20 colder than climo...likely even colder than that in areas where snow covers the ground. Nevertheless, sunny skies anticipated on Sunday should provide some degree of melting. Temps are forecast to moderate by Tuesday, albeit remaining below normal.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Not much change for the 12z TAFs. VFR conditions expected thru the period. Light WNW wind or VRB at the Piedmont sites thru late-morning, then toggling to SW and increasing to 5-10 kt with some low-end gusts possible at the Upstate sites late aftn into early evening. Winds will stay NNW at KAVL and look to remain gusty until late tonight. An area of cirrus will exit east of the area by early aftn, leaving basically clear skies.

Outlook: VFR conditions are expected to continue thru at least Thursday night under a cold and dry air mass. Confidence continues to increase in a coastal low possibly producing an area of snow across the area Friday night thru Saturday. The low pulls away from the area Sunday, but gusty N/NW winds will linger.

CLIMATE

RECORDS FOR 01-31

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 78 1975 13 1909 57 1950 1 1966 KCLT 77 1975 27 1909 59 1950 4 1966 KGSP 79 1975 31 1934 60 1950 3 1966 1909

RECORDS FOR 02-01

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 74 1950 22 1898 55 1950 4 1909 KCLT 76 1950 27 1900 55 1896 10 1900 KGSP 73 1957 30 1980 56 1969 9 1900 1936 1950 1916

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Cold Weather Advisory from 7 PM this evening to 10 AM EST Thursday for NCZ033-049-050. SC...None.