textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

A Winter Storm Watch has been issued for our entire forecast area from late Friday afternoon through Saturday night.

Snowfall totals have decreased slightly, but Warning-criteria totals of 2-4" are still forecast across much of the western half of the area, with 4-7" across the eastern half, and across much of the area along the NC/TN border.

KEY MESSAGES

1. Anomalously cold temperatures continue through this morning. A Cold Weather Advisory remains in effect for Mitchell, Yancey, and Avery counties through 10 AM. 2. A significant winter weather event is expected to impact the region from Friday night through Saturday night, with snow expected to be the primary precipitation type. Due to cold temperatures leading up to the event, impacts to travel could be significant. 3. Dangerously cold temperatures and wind chills will impact the region Saturday night through Monday morning. This air mass will pose potential threat to life and property if precautions are not taken. 4. Temperatures gradually warm through early next week with a return to dry conditions across the region.

DISCUSSION

Key message 1: Anomalously cold temperatures continue through this morning. A Cold Weather Advisory remains in effect for Mitchell, Yancey, and Avery counties through 10 AM.

Elevated winds within strong cold advection will continue thru early this morning. As temperatures drop, sub-zero wind chills will develop at some higher peaks, especially across the northern Blue Ridge. A Cold Weather Advisory remains in effect for Avery County and elevations above 3500 feet in Yancey and Mitchell counties through 10 AM Thursday. Temps will generally be a category or so cooler than yesterday under periods of thicker high and mid clouds.

Patches of black ice can be expected around the area this morning as temperatures start out below freezing, though the extent will not be as great of extent as the past two nights. Further melting will occur again today with highs in the upper 30s to lower 40s. Increasing clouds and lighter winds should keep wind chills out of the sub-zero range tonight. Lows ranging mainly between 15 to 25.

Key message 2: A significant winter weather event is expected to impact the region from Friday night through Saturday night, with snow expected to be the primary precipitation type. Due to cold temperatures leading up to the event, impacts to travel could be significant.

A developing upper low will dig from the Corn Belt Friday afternoon to the TN Valley by Saturday morning, before pivoting across South Carolina as the associated trough becomes increasingly negatively tilted. In response, low level frontogenesis will intensify across our region throughout Friday into Fri night, with precipitation expected to gradually blossom across the southern and central Appalachians...possibly overspreading western NC as early as Fri afternoon. Depending upon timing, some rain could initially fall in the valleys, but a quick transition to snow is expected as surface temps cool due to wet bulb effect, and cold advection intensifies. The precip will steadily develop southeast...east of the upper low position Friday night into early Saturday, with the entire forecast area likely seeing at least some precip during this time frame. With strong cyclogenesis expected to be underway off the Southeast Coast, cold advection will steadily strengthen across our forecast area, forcing vertical profiles to become supportive of a virtually all-snow event across the entire CWA. As time progresses and cold advection intensifies and deepens, profiles will become increasingly supportive of snow:liquid ratios of 15-20:1 during the day Saturday.

What remains the subject of much consternation is QPF. After multiple model cycles in which the overall trend in deterministic guidance was toward a snowier scenario for the CWA, 00Z guidance has generally come in with a bit lighter QPF. Ensemble guidance also provides plenty of ammo to give one pause as to the potential lack of QPF problem undercutting snowfall potential...especially across the western half of the area. The main issue as we see it is that much of the precip potential for this event would be dependent upon how quickly the mid-level deformation zone matures as the upper low passes near/over the CWA early Saturday. In general...deformation zone precip regions can be underwhelming (and under performing of guidance) in terms of precip production. We have therefore generally sided with the lower side of the guidance envelope for QPF, especially across our western half. Having said that, ratios are forecast to be so high within the def zone, that our deterministic forecast has fallen out with snow totals supportive of a Winter Storm Watch across the entire area, albeit only marginally so (2-4") across much of the western half of the area. Confidence is higher in a 4-7" range across the eastern half...as well as across the TN border counties, where strong N/NW low level upslope flow will enhance snowfall rates. 50% confidence that Warning criteria will be reached is the threshold for Winter Storm Watch issuance, and while I can't say I'm quite there across the western half, we have opted to go ahead and hoist one for the entire area after collaboration with neighboring NWS offices. It should also be stated that even if some areas don't technically meet objective criteria, roads are likely to become a mess due to the cold nature of the air mass...as increasingly powdery/dry snow will quickly stick to surfaces.

With the pressure gradient tightening on the west side of developing, strong coastal low, gusty winds will develop across the region on Saturday, with gusts of 25-30 mph likely...resulting in some areas of blowing snow, and possibly brief, localized blizzard-like conditions. The precip will taper off Saturday evening, leaving behind the coldest air mass of the year...nay one of the coldest air masses we've seen in a while.

Key message 3: Dangerously cold temperatures and wind chills will impact the region Saturday night through Monday morning. This air mass will pose potential threat to life and property if precautions are not taken.

Low temperatures Saturday night are expected to fall into the single digits over the mountains to low teens east. These cold temperatures combined with 20-40mph wind gusts (locally higher above 3500ft in the mountains) will yield incredibly cold wind chill values. Mountain wind chills are forecast to range from -10 to -20 in the valleys, with values of -15 to -25 expected in the high elevations...probably even lower than that on the highest peaks. Even outside the mountains, wind chill of 0 to 5 below is currently forecast along and north of I-85, with low single digits south of there. These values would be near or exceed criteria for extreme cold and an extreme cold watch will likely be needed with later forecast packages. High temperatures Sunday will struggle to warm even with full sunshine. A lot of incoming solar radiation will go into melting snow and temperatures may struggle to reach freezing over much of the area outside of the lower Upstate and Upper Savannah Valley. Some guidance is even colder with highs struggling to get out of the 20s. Winds will relax by Sunday night, but air temperatures may be as cold, if not colder than Saturday night. Much of the area may end up being continuously below freezing from Friday evening until Monday afternoon...possibly longer than that across some high elevations areas. The extreme cold and very wind conditions Sat night, combined with the prolonged nature of the cold weather will pose a threat to life and property if necessary precautions aren't taken.

Key message 4: Temperatures gradually warm through early next week with a return to dry conditions across the region.

Finally, by Monday low-level flow shifts to out of the west and cuts off cold advection. This will allow for the airmass to gradually modify with afternoon highs forecast to rise into the mid 30s to low 40s, but still well below average. This trend continues through the middle of next week with highs rebounding into the low to upper 40s by Wednesday. Overnight lows will still be cold in the teens to low 20s with refreezing of melting snow each night.

AVIATION /06Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR at all terminals through the period. A mid-level wave clouds continue across the mountains and western Piedmont, holding on longer than expected. Periods of mid and high clouds will persist thru the day. Gusty NNW wind lingers at KAVL into the overnight, but should abate by daybreak. Otherwise, light wind thru the period, mainly favoring a W to NW direction, toggling to SW across the Upstate this aftn.

Outlook: VFR conditions are expected to continue thru at least Friday under a cold and dry air mass. Confidence continues to increase regarding the potential for light snow across the area beginning Friday night and continuing thru Saturday, especially at KCLT. Low pressure 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

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 Watch from Friday afternoon through Sunday morning for GAZ010-017-018-026-028-029. NC...Winter Storm Watch from Friday afternoon through Sunday morning for NCZ033-035>037-048>053-056>059-062>065- 068>072-082-501>510. Cold Weather Advisory until 10 AM EST this morning for NCZ033- 049-050. SC...Winter Storm Watch from Friday afternoon through Sunday morning for SCZ008>014-019-101>109.