textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Snow totals have increased again in the high elevations of the NC mountains near the TN border, but remain very low below 3500 feet. Winter Weather Advisory has been expanded to areas above 3500 ft in all zones bordering TN, except Avery.

Wind Advisory and Cold Weather Advisory issued for high mountain elevations including Avery. Wind Advisory tonight through Thursday, but Cold Wx Advisory continues in the same areas thru Friday morning.

Minor changes were made to the fcst for Saturday and Sunday with up to 2 inches of snow possible over portions of the NC mtns and extreme NE Georgia. In addition, snow flurries remain possible outside of the mtns early Sunday, however confidence with regards to this portion of the fcst remains low.

KEY MESSAGES

1. Accumulating snowfall is expected to occur along the NC-TN border this afternoon and tonight. Significant travel impacts expected at elevations above 3500 ft. Winter Weather Advisories expanded to more locations along the TN border. 2. Strong winds behind the front potentially causing minor tree damage and sporadic power outages in high elevations of the mountains tonight thru Thursday, prompting Wind Advisory there. Dangerously cold wind chills also result and last in some areas thru Friday morning, so Cold Weather Advisory issued. 3. Possibly the coldest air mass of the season will settle over the region Thursday night, but low temperatures will NOT reach anywhere close to records. Dangerously cold wind chills will be possible above 3500 ft early Friday morning. 4. Light snow accumulations over the North Carolina mountains and extreme Northeast Georgia Friday night through early Saturday could result in minor travel issues. 5. Winter precipitation is possible east of the mountains early Sunday, however confidence with respect to timing and amounts remains relatively low.

DISCUSSION

Key message 1: Accumulating snowfall is expected to occur along the NC-TN border this afternoon and tonight. Significant travel impacts expected at elevations above 3500 ft. Winter Weather Advisories expanded to more locations along the TN border.

No change to the general weather pattern expected thru Thursday. Deep shortwave centered over the Great Lakes and OH Valley will swing over the Southeast coastal states. As of 1 pm, Arctic cold front now analyzed from Ohio to the Ozarks, set to push across the southern Appalachians this evening. Low level moisture profiles started out the day very dry, although light radar returns streamed over the area much of the day, associated with upper divergence from coupled jet streaks and abundant high altitude mositure. Those features are now shifting east. Per recent obs indicating accumulation finally did begin at some locations along/south of I-85, and a few GOES profiles where high clouds have scattered in our western zones, suggest the low levels may have moistened enough for precip forced just ahead of the front to reach the ground. Temps generally have been limited by cloud cover and/or diabatic cooling so far today, so previous expectation holds that high elevations will be cold enough for precip to start as snow.

On previous forecast cycles we noted that dynamic lift looked to peak before profiles saturated in low levels and while temperatures cooled as much to support snow accums. There now appears a better chance of overlap in these regards, though the upper level moisture does still dry out after the front passes, so ice nucleation zone still looks to transition from midlevels down to the PBL with rapid cooling behind the front and transition to NW flow upslope forcing. The peak snow rates still are forecast between 00-06z, and amounts have increased in that time. Average snow totals in the areas above 3500 ft along the TN border have risen to Winter Wx Advisory levels in all but Avery, where we have a policy of not doing elevation splits. The Winter Wx Advisory thus has been expanded to all the border counties except Avery. Even accounting for the cooler temps today, still did not see temps fall fast enough in lower elevations to bring the below-3500ft areas to Advisory criteria. Owing largely to a shallow convective layer depicted on prog profiles 00-03z this evening, Snow Squall parameter rises above 1 on the upslope side of the mountains; given hi-res models incorporated into model blend QPF, this also probably accounts for the higher amounts in the Smokies (isolated spots over 8 inches at the highest, unpopulated elevations).

The moisture becomes shallow by early Thu morning so rates become light again in the NW flow areas and taper to flurries farther downstream over the mountains. With strong and gusty NW winds continuing thru the day Thursday, kept slight-chance PoPs going near the TN border through mid-afternoon. There is a signal for blowing snow to reduce vsby where higher accums do occur. Winter Wx Advisory goes til noon Thu as it is, but could need to go a bit longer if the blowing snow does verify and/or if roads remain snow-covered.

Key message 2: Strong winds behind the front potentially causing minor tree damage and sporadic power outages in high elevations of the mountains tonight thru Thursday, prompting Wind Advisory there. Dangerously cold wind chills also result and last in some areas thru Friday morning, so Cold Weather Advisory issued.

Winds at the top of the mixed layer rise to around Wind Advisory criteria behind the front this evening and remain at similar values as mixing deepens diurnally Thu. NBM gusts have generally verified pretty well in recent high-elevation post-frontal events, and coverage of 46-58 mph gusts above 3500 ft was high enough from the Black Mtns northeast to warrant Wind Advisory at those elevations in Buncombe, Yancey, Mitchell, plus all of Avery. Winds will remain gusty Thu night though below advisory criteria.

Temperatures solidly fall into the teens tonight across the mountains, with lower teens and single digits in higher elevations. Lower 20s in the foothills/Piedmont. In high elevations across the mountains wind chills dip below -5F tonight. Maxes will be in the teens in those areas Thu, and with continuing wind, they remain below -5F in some spots through the day, then fall again Thu night. For simplicity, will keep Cold Wx Advisory in place until 12z Fri.

Key message 3: Possibly the coldest air mass of the season will settle over the region Thursday night, but low temperatures will NOT reach anywhere close to records. Dangerously cold wind chills will be possible above 3500 ft early Friday morning.

Looking like good old fashioned winter around some parts Thursday night as the sfc high pressure ridge moves overhead and provides for excellent radiational cooling. The low temp fcst continues to creep downward and now has plenty of single digits over the mtns and what would be the lowest min temp so far this season at AVL, but not quite so at GSP and CLT. These low temps are on the order of 15 degrees below normal, but not anywhere close to record lows. The low temps at the high elevations are so cold that we could flirt with Cold Weather Advisory criteria, but not enough of the area reaches criteria as it stands at this time. It won't take much more of a downward trend to require an Advisory. However, our forecast is already down in the colder end of the guidance envelope.

Key message 4: Light snow accumulations over the North Carolina mountains and extreme Northeast Georgia Friday night through early Saturday could result in minor travel issues.

On Friday, another upper low is expected to dive southward from Canada and move across the Great Lakes region. A broad but deep mid/upper trof will remain to our west thru the bulk of the weekend. A leading upper shortwave rounding the base of the trof may bring some amount DPVA and mid-lvl isentropic lift across the mtns Friday night and early Saturday morning, however the moisture return ahead of the trof still appears minimal overall. Low-lvl flow will remain W to WSW due to the orienta- tion of the trof axis. Some amount of light precipitation from higher- based cloud cover will probably materialize, but much of it could eva- porate before reaching the ground. Some of the model profiles suggest a weak, low-lvl warm nose could be in place during the onset of precip. However, with relatively dry low-lvl air in place, it's likely that temperatures will wet-bulb down to below freezing and erode any warm nose fairly quickly. Total QPF is still expected to be no more than 0.1 to 0.2 inches of liquid. This translates to roughly an inch of snow over the TN border zones and southwest NC mtns by daybreak Sat. Some of the more favored areas could see closer to 2 inches, but those areas will likely be isolated. A light dusting is possible over the mtns of northeast GA and the far northwestern Upstate, but any accums should remain under an inch.

Otherwise, temperatures will rebound to near-normal on Sat as sfc high pressure gets pushed offshore and the strengthening W/SW flow provides some amount of downslope warming east of the mtns.

Key message 5: Winter precipitation is possible east of the mountains early Sunday, however confidence with respect to timing and amounts remains relatively low.

The synoptic pattern will remain progressive thru the latter half of the weekend and into early next week. A deepening upper trof will push a cold front thru the region late Sat with colder air returning Sat night into early Sun. The associated digging upper shortwave could induce some robust frontogenesis and isentropic upglide back over the cold air east of the mtns Sunday morning. As such, if precip does deve- lop over our eastern zones on Sunday it would likely be mostly snow. The current fcst still only has a slight chance for precip and only a few hundreths of an inch of QPF during this time frame. If precip chances and/or amounts trend higher going forward, it's more likely that these areas could get some amount of snow. Otherwise, tempera- tures are expected to cool well-below normal for Sunday and remain below normal thru early next week.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR with breezy conditions thru sunset and gusts of 20-25 kt at times. A few sprinkles possible at the terminals. Cold front will approach the Appalachians this evening and will incur shift to NW winds overnight. Associated moisture will lead to low VFR cloud deck and high elevation -SHSN beginning by around sunset. Snow level will fall to the valleys over the subsequent few hours, but most indications are that the precip will taper off too fast to reach KAVL. More likely, -SHRA will result with temps still above freezing, so retained PROB30 for MVFR vsby/cigs. Cigs do look likely to sink later in the night, warranting prevailing MVFR; can't rule out flurries in that timeframe. Winds will remain gusty overnight, mainly 15-20 kt overnight but potentially 30-35 kt at times at KAVL. Gust potential increases again during the day Thu. Cloud bases likely to bottom out at low VFR at the non-mountain terminals, before scattering behind the front.

Outlook: Dry and VFR conditions linger through at least Friday. Precipitation and associated restrictions could return as early as Saturday, but confidence remains low at this time.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Wind Advisory from 9 PM this evening to 7 PM EST Thursday for NCZ033-049-050-053. Cold Weather Advisory from midnight tonight to 7 AM EST Friday for NCZ033-048>052-058-059-062-063. Winter Weather Advisory until noon EST Thursday for NCZ048>052. SC...None.