textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Snow totals have trended down slightly but warning criteria snow is still expected across the forecast area.

The Winter Storm Warning remains in effect for the entire area from 4 PM this afternoon through 7 AM Sunday.

The Extreme Cold Watch was upgraded to an Extreme Cold Warning for the entire area.

KEY MESSAGES

1. A Winter Storm Warning remains in effect from 4 PM this afternoon through 7 AM Sunday as a significant winter storm will bring heavy snow to the western Carolinas and northeast Georgia. Cold temperatures leading up to and during the event will lead to significant travel impacts. 2. Gusty winds combined with very cold temperatures will lead to dangerously cold wind chills Saturday through Monday. These wind chills could result in hypothermia or frostbite if precautions are not taken. 3. Temperatures gradually warm through early next week with a return to dry conditions across the region. Precipitation chances return by mid week.

DISCUSSION

Key message 1: A Winter Storm Warning remains in effect from 4 PM this afternoon through 7 AM Sunday as a significant winter storm will bring heavy snow to the western Carolinas and northeast Georgia. Cold temperatures leading up to and during the event will lead to significant travel impacts.

A potent upper trough will dig across the Midwest/TN Valley through tonight before a closed upper low develops within the base of the trough just west of the GSP forecast area by daybreak Saturday. The upper low will swing across the Carolinas Saturday into Saturday night bringing heavy snow to much of the Carolinas before lifting northeast away from the region, allowing dry conditions to return by daybreak Sunday. Snow will develop late this afternoon to early evening across the North Carolina mountains, spreading south and east late this evening into the overnight hours across the rest of the forecast area. The highest snow rates are expected tonight into Saturday.

The 06Z ECMWF and GFS depict a mesolow developing over northeast Georgia and tracking southeastward across the South Carolina Upstate on Saturday, but the 06z NAM shows the mesolow developing farther north and east, just on the edge the GSP forecast area. Depending on where exactly this mesolow develops and tracks on Saturday is one of the main forecast challenges. This is because north of the mesolow a band of mid-level frontogenesis will develop leading to a narrow corridor of higher QPF. Depending on where this frontogenesis band sets up, we could see a band of locally heavy snow develop anywhere from the I-85 corridor to just east of the forecast area. This snow band could inflate snow accumulations if it develops over the I-85 corridor. For now, it appears that snowfall amounts should generally range from 4 to 8 inches across most of the forecast area, with higher amounts from 9-12+ inches along the NC/TN border and lower amounts west of I-26 in the western South Carolina Upstate and portions of northeast Georgia. Totals have trended down slightly compared to the previous forecast but warning criteria snow is still expected area-wide with no changes to impacts expected.

Snow-to-liquid Ratios (SLRs) of 15:1 to 20:1 (rare for our area as we are typically closer to a 10:1 ratio) will lead to a light, powdery snow for this event (not great for making snowballs but will limit power outage concerns due to less weight from the snow on trees and powerlines). With temperatures expected to fall into the teens to 20s overnight (single digits across the higher elevations in the North Carolina mountains), snow will stick rapidly to roads and elevated surfaces (such as bridges and overpasses) leading to hazardous road conditions. Gusty winds will develop tonight into late Sunday, and with fluffier snow expected, blowing snow could lead to reduced visibilities. We still cannot entirely rule out the potential for isolated blizzard conditions to develop, mainly for areas that see 3 consecutive hours of 35 mph or greater wind gusts and visiblity of 1/4 mile or less.

Slow down and use caution while traveling if you have to get out this weekend. If you must travel, keep an extra flashlight, food, and water in your vehicle in case of an emergency.

Key message 2: Gusty winds combined with very cold temperatures will lead to dangerously cold wind chills Saturday through Monday. These wind chills could result in hypothermia or frostbite if precautions are not taken.

Gusty north/northeast winds will develop Friday night into Saturday morning, gradually toggling northwest thoughout the day Saturday. Wind gusts will generally range from 20-25 mph area-wide through early Saturday afternoon, increasing to 25-30 mph east of the mountains and 30-45 mph across the mountains late Saturday afternoon into mid-afternoon Sunday. Higher elevations across the North Carolina mountains could see gusts from 45-55 mph. A Wind Advisory may be needed for portions of the North Carolina mountains Saturday evening into early Sunday afternoon but will hold off on an advisory for now. Gusts will diminish early Sunday evening east of the mountains but will linger across the mountains through late Sunday evening before gradually diminishing Sunday night.

Much colder temperatures are expected for Saturday, with highs only reaching into the 20s across most of the lower elevations and the teens across the higher elevations. Lows Saturday night will fall into the single digits to lower teens across much of the area, but elevations above 4,000 feet should see lows fall near or just below zero. Highs on Sunday will be warmer, but most locations will remain near to below freezing. The exception will be the Upper Savannah River Valley where highs could reach into the mid to upper 30s. Lows Sunday night will fall back into the single digits to lower teens.

Gusty winds combined with cold temperatures will lead to dangerously cold wind chills Saturday through Monday. Thus, the Extreme Cold Watch was upgraded to and Extreme Cold Warning for the entire GSP forecast area. Timing for the mountains is 1 PM Saturday through 1 PM Sunday as wind chills as low as 22 below zero are expected, with the highest peaks seeing wind chills as low as 25 to 30 below zero. Timing east of the mountains is 7 PM Saturday through 10 AM Sunday as wind chills as low as 5 below zero are expected.

These wind chills could result in hypothermia or frostbite if precautions are not taken. Dress in layers including a hat, face mask, and gloves if you must go outside. To prevent water pipes from freezing; wrap or drain or allow them to drip slowly. Make frequent checks on older family, friends, and neighbors. Ensure portable heaters are used correctly. Do not use generators or grills inside.

Key message 3: Temperatures gradually warm through early next week with a return to dry conditions across the region. Precipitation chances return by mid week.

By Monday, flow weakens and shifts to out of the west as the upper trough axis continues to shift offshore with broad upper northwest flow form the Ohio Valley into the Appalachians. This will bring an end to cold advection and a gradual modification of the airmass. Temperatures are expected to rise above freezing for much of the area on Monday with mid to upper 30s outside of the higher elevations. Overnight lows will continue to fall well below freezing, however, and will support refreezing of melting snow/ice each night.

Heading into Tuesday, a piece of Pacific energy will dive down the eastern flank of a tall west coast ridge and through the Rockies and into the Great Plains by Wednesday morning. Guidance varies as to how this trough evolves with some members maintaining a positive tilt while others transition the trough into a neutral to near negative tilt with an upper low trying to close off over the Tennessee Valley. Eventually, this brings a slug of moisture back into the area early Wednesday morning through at least Thursday morning. Exact timing of this system, track of the subsequent surface low, and availability of cold air will have major implications on precipitation type. As of now, ensemble means favor a mainly wet solution for much of the area with any wintry weather brief at onset Wednesday morning. However, there are some ensemble members that track the surface low farther south with a more favorable trough interaction and cold air close by. Should this scenario unfold there could be a greater potential for winter weather. There's too much forecast uncertainty at this time to lean the forecast either way, so will maintain a rain or snow precipitation type that follows the current temperature forecast.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Dry and VFR conditions linger through the late this evening before conditions gradually deteriorate tonight into Saturday morning as a Winter Storm impacts the area. Temperatures will be cold enough for snow across the terminals overnight into Saturday. Went with PROB30s for the onset of -SN then went with prevailing SN just prior to daybreak Saturday as confidence is high that SN will impact the terminals from ~10Z Saturday through the end of the TAF period. Cigs look to drop to mostly MVFR levels, although IFR cannot be entirely ruled out, especially if a heavier band of snow develops right over a terminal. Vsbys will start out MVFR at the onset of snow, trending more towards LIFR by 10Z-12Z. Winds will remain NE east of the mountains through the period. Winds at KAVL will remain SE through tonight before gradually turning more N/NNW by daybreak Saturday. Wind speeds will gradually increase Saturday morning through Saturday afternoon, becoming gusty. Wind gusts from 20-25 kts can be expected across the terminals towards the end of the 18Z TAF period.

Outlook: Snow will linger through early Saturday evening before gradually tapering off from west to east late Saturday evening into Saturday night. Dry and VFR conditions return Sunday but gusty NW winds will linger through late Sunday afternoon before gradually diminishing Sunday evening into Sunday night. Dry and VFR conditions will linger through at least Tuesday.

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

RECORDS FOR 02-02

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 76 1989 16 1908 52 1988 -2 1917 KCLT 80 1989 29 1908 61 1923 10 1917 KGSP 77 1989 28 1951 60 1923 9 1900

GSP WATCHES/WARNINGS/ADVISORIES

GA...Winter Storm Warning from 4 PM this afternoon to 7 AM EST Sunday for GAZ010-017-018-026-028-029. Extreme Cold Warning from 7 PM Saturday to 10 AM EST Sunday for GAZ010-017-018-026-028-029. NC...Winter Storm Warning from 4 PM this afternoon to 7 AM EST Sunday for NCZ033-035>037-048>053-056>059-062>065-068>072- 082-501>510. Extreme Cold Warning from 7 PM Saturday to 10 AM EST Sunday for NCZ035>037-056-057-068>072-082-502-504-506-508-510. Extreme Cold Warning from 1 PM Saturday to 1 PM EST Sunday for NCZ033-048>053-058-059-062>065-501-503-505-507-509. SC...Winter Storm Warning from 4 PM this afternoon to 7 AM EST Sunday for SCZ008>014-019-101>109. Extreme Cold Warning from 7 PM Saturday to 10 AM EST Sunday for SCZ008>014-019-101>109.