textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Per regional radar...light precip is already approaching the western part of the forecast area. Light snow will be possible in the vicinity of Smokies this morning, with a light dusting possible. Some sprinkles, flurries, and/or light sleet may be seen in the lower elevations of northeast GA and the Upstate as well.

Otherwise, updated for aviation discussion.

KEY MESSAGES

1. A major ice storm is expected tonight through Sunday evening across the forecast area, with an Ice Storm Warning remaining in effect for the entire CWA. Impacts will be crippling across at least a part of the forecast area, especially where ice accretion approaches or exceeds 0.75." Widespread power outages and extremely difficult to impossible travel conditions are expected to linger through at least the early part of the work week. 2. Dangerously cold wind chills may develop Monday night into Tuesday morning, which could result in hypothermia or frostbite if precautions are not taken. 3. A reinforcing cold front is expected later next week with another chance for dangerously cold wind chills Wednesday night and again Thursday night, which could result in hypothermia or frostbite if precautions are not taken.

DISCUSSION

Key message 1: A major ice storm is expected tonight through Sunday evening across the forecast area, with an Ice Storm Warning remaining in effect for the entire CWA. Impacts will be crippling across at least a part of the forecast area, especially where ice accretion approaches or exceeds 0.75." Widespread power outages and extremely difficult to impossible travel conditions are expected to linger through at least the early part of the work week.

The latest surface analysis reveals the center of sprawling arctic high pressure over the upper Miss Valley early this morning, with associated cold/dry ridge axis extending across the central Appalachians and off the Mid-Atlantic coast. Surface pressure pattern is responding with developing cold air damming east of the Blue Ridge, which will continue to steadily intensify, reaching maturity tonight as the center of the parent high settles into a most favorable position over New England. Meanwhile, the broad baroclinic zone covering much of the southeast quadrant of the country will remain activated in response to large scale central Conus height falls within initially split flow regime. Moisture and forcing will unzip along the baroclinic zone into our forecast area...beginning across the mountains by this afternoon. Actually, light precip is already approaching the southern Appalachians from east TN...and some spotty dustings of -SN are possible in the Smokies and vicinity by noon. Some sprinkles, flurries, and light sleet may be seen in the lower elevations of northeast GA and the Upstate as well.

Otherwise, more substantive precip will overspread the remainder of the CWA this evening, such that categorical PoPs are forecast by midnight. As precipitation begins to fall, the initially cold and very dry air mass will quickly cool to below freezing due to wet bulb effects, enhancing the intensity of CAD via diabatic effects. Therefore, while some locations could see a brief period of rain at the outset, a quick transition to snow (mountains) and snow/sleet (elsewhere) is likely. Low level cold air will be maintained via advection of sub-freezing wet bulb temps within the damming region...ensuring a protracted period of wintry precip across virtually the entire forecast area.

Intense isentropic lift atop the cold dome combined with anomalously moist air (PWATs of about 1"...in ~the 90th percentile of daily climatology)...followed by strong/focused ascent along a frontal boundary Sunday afternoon/evening will result in impressive storm total liquid equivalent precip...with 2-3 inches forecast across the mountains of southwest NC, SC, and northeast GA, and generally 1.5-2" elsewhere. Strong warm advection aloft will establish a warm nose that will eventually (a) support a transition from sleet to freezing rain from SW=>NE and (b) allow for the valleys of far southwest NC...which will only be weakly affected by CAD...to warm above freezing. The first main forecast challenge is timing the transition to freezing rain. Forecast soundings become quite warm (+5C or more) aloft rather quickly tonight, suggesting a quick transition to FZRA. However, the cold layer at the surface is cold and deep enough to perhaps allow for some refreezing of hydrometeors before they hit the surface. However, this is far from a given...and it lends modest confidence to our ice/sleet accum forecast going forward. Nevertheless, most of the precip is expected to fall as freezing rain such that much of the CWA is expected to see storm total ice accums of 0.5-0.75." An exception on the higher end (0.75-1.0" with locally higher amounts) is forecast along the Blue Ridge escarpment...where the greatest alignment of persistent cold air and higher liquid equivalent is forecast. These amounts are expected to extend across the foothills of SC into the Greenville metro area as well. Widespread downed trees/lines and power outages are expected to be most concentrated in this area.

On the lower end, a gradual transition to rain is expected along the southern and western periphery of the CWA during Sunday afternoon due to the synoptic pattern quickly becoming unfavorable for maintenance of cold air damming. This will occur as the upper flow pattern becomes phased...with a long wave trough being carved out over the eastern Conus and an upper ridge quickly building off the East Coast, resulting in cessation of confluent flow and allowing the weakening parent high to move far north into Quebec and become detached from the inverted ridge CAD ridge. The wind field responding to a developing coastal low will provide an additional erosion mechanism. As such, ice totals across the SC Lakelands and vicinity are expected to be in the 0.25-0.5" range, while the valleys beneath the Smokies may struggle to reach 0.25."

In summation: forecast ice totals could be too low by 0.1-0.25" if the transition from sleet->freezing rain occurs significantly earlier than currently anticipated. Forecast totals could be too high along and south of the I-85 corridor if CAD erosion is more aggressive than currently forecast. Finally, locally excessive rainfall will be possible across mountainous areas where the transition to rain occurs Sunday afternoon...with the strong frontal band (where an isolated elevated thunderstorm cannot be ruled out) likely providing the tipping point to some minor runoff issues developing late afternoon/evening.

Key message 2: Dangerously cold wind chills may develop Monday night into Tuesday morning, which could result in hypothermia or frostbite if precautions are not taken.

Arctic high pressure builds in behind the departing coastal low Monday into Tuesday. It will be windy in the high terrain Monday (with gusts of 40-50 mph in the highest elevations), then some weakening Monday night. There will be enough NW wind to produce dangerous wind chills across nearly the entire forecast area. Temps will stay below freezing across the mountains and barely get into the mid to upper 30s in the Piedmont Monday aftn before plummeting into the single digits and to lower teens Monday night. Wind chills are forecast to be in the 0 to -20 range in the mountains and mainly single digits to low teens above zero in the Piedmont. This is well within Cold Weather Advisory for most of the area, with Extreme Cold Warning criteria above 3500 ft in the Northern Mountains.

Temps are still expected to rebound slightly through Wednesday, but remain well below normal under a broad longwave trough. High temps will get into the upper 30s to maybe low 40s in spots Tuesday and Wednesday afternoon, but still very cold at night, so melting of any lingering sleet and ice will be slow. We can expect a black ice problem each night/morning probably thru the end of the week.

The cold temps thru the week will exacerbate threats of hypothermia for anyone traveling or still without power. Make sure to check up on vulnerable loved ones and neighbors, pets, and have plenty of blankets and warm clothing. Remember to avoid burning fuels like propane or kerosene indoors as this increases fire risk and carbon monoxide poisoning.

Key message 3: A reinforcing cold front is expected later next week with another chance for dangerously cold wind chills Wednesday night and again Thursday night, which could result in hypothermia or frostbite if precautions are not taken.

Another strong shortwave will dive out of central Canada on Wednesday and cross the central and southern Appalachians Thursday. This will bring a refreezing arctic cold front thru the area, dropping temps down further below normal for Thursday and Friday. Gusty NW winds, especially across the mountains will likely produce dangerous wind chills again Wednesday night thru Thursday morning, with Cold Weather Advisory criteria possible across the mountains. Wind chills east of the mountains may dig into the single digits to lower teens in spots, which may flirt with Cold Weather Advisory as well. The latest NBM has even colder wind chills Thursday night into Friday morning for the entire forecast area. The models show relatively little moisture with the front, but a brief shot of NW flow snow showers may occur Wednesday night. Minor accums in the usual upslope areas may occur.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Cigs will gradually fill and lower through the day as moisture increases in advance of a dynamic winter storm system. Light precip is already approaching the southern Appalachians, and -SN could develop at KAVL by afternoon...warranting a Prob30 for -SN and MVFR conditions there from 19-24Z. Precip chances will ramp up across the remainder of the area by early evening, with Prob30s starting out as -SNPL at KHKY and -RA or -FZRA and PL for the remaining TAF sites. A transition to categorical FZRAPL is forecast at most sites by late evening, accompanied by lowering cigs (to MVFR) and increasing potential for visby restrictions. Precip rates are expected to intensify during the overnight, with continued deterioration in cigs (to IFR) and visby (MVFR forecast, but periods of IFR also likely). Further transition to mostly FZRA is expected from around sunrise through late morning Sunday. NE winds will increase to (or remain) 9-12 kts at most sites by late morning, with some gusts of around 20 kts expected at the upstate SC terminals and KCLT. Meanwhile, winds will transition to SE at around 10 kts at KAVL by this afternoon.

Outlook: A major winter storm is expected to continue impacting the entire terminal forecast area through Sunday, and possibly into the early part of Sunday night. Significant accumulations of freezing rain and sleet are likely at all the TAF sites, as well as smaller terminals. Extended flight restrictions should be expected into Monday, even after precipitation ends, with VFR unlikely to return until Tuesday.

CLIMATE

RECORDS FOR 01-27

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 80 1999 17 1940 54 1916 2 1986 KCLT 75 1890 24 1940 58 1890 6 1940 KGSP 73 1954 29 1940 57 1949 8 1940

RECORDS FOR 01-28

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 75 1914 7 1897 52 1916 -2 1986 1897 KCLT 79 1944 20 1897 59 1916 5 1986 KGSP 77 1944 29 1986 55 1957 5 1986

RECORDS FOR 01-29

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 76 2002 25 1977 52 1917 -1 1897 1897 KCLT 79 2002 29 2014 59 1957 8 1897 KGSP 78 2002 27 1897 57 1957 6 1897 1916

RECORDS FOR 01-30

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 76 2002 11 1966 55 1950 -7 1966 1914 KCLT 78 2002 19 1966 59 1914 4 1966 KGSP 76 1975 20 1966 53 2002 -6 1966

GSP WATCHES/WARNINGS/ADVISORIES

GA...Ice Storm Warning from 1 PM this afternoon to 1 PM EST Monday for GAZ010-017-018-026-028-029. NC...Ice Storm Warning from 1 PM this afternoon to 1 PM EST Monday for NCZ033-035>037-048>053-056>059-062>065-068>072-082- 501>510. SC...Ice Storm Warning from 1 PM this afternoon to 1 PM EST Monday for SCZ008>014-019-101>109.