textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The Aviation portion of the discussion was updated.

KEY MESSAGES

1. Anomalously cold temperatures continue tonight. A Cold Weather Advisory remains in effect for Mitchell, Yancey, and Avery counties through 10 AM Thursday. Patchy black ice will develop in some spots again tonight through early Thursday morning. 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 will bring impacts to life and infrastructure. 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 tonight. A Cold Weather Advisory remains in effect for Mitchell, Yancey, and Avery counties through 10 AM Thursday. Patchy black ice will develop in some spots again tonight through early Thursday morning.

A reinforcing cold front located just east of the mtns will continue to move southeast across the Piedmont/Upstate this evening. Elevated winds from within the cold advection layer will continue well into tonight, so 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.

More patches of black ice can be expected again late tonight and Thursday morning as temperatures drop below freezing, though the extent will not be as great as the past two nights.

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.

The majority of guidance continues to depict a significant winter weather event across much of the forecast area Friday night through Saturday night. Forecast confidence in accumulating snow is high, but confidence in amounts (spatially) remains low to moderate. There are are still several scenarios on the table, but at a minimum a solid winter weather advisory event appears likely with increasing confidence for warning criteria snowfall.

By tomorrow night, a tall west coast ridge will be established from the Great Basin through the Cascades with a subtropical jet extending beneath the ridge across Baja California to the Gulf. An area of confluent flow will also exist across Texas. Farther north, the mainly player will be a potent Tropospheric Polar Vortex (TPV) that will be rotating around a larger gyre over southeast Canada. The TPV is progged to be located over Lake Superior as a positively tilted trough. This feature will dive across the Midwest and dig through the Mississippi Valley on Friday. At the same time, an upper low will close off as the trough takes on a neutral tilt in the vicinity of central Tennessee. By Saturday, guidance digs the potent upper low in the vicinity of the Upstate as it evolves into a negative tilt and instigates subsequent surface cyclogenesis along a coastal baroclinic zone. The upper low will be anomalously deep with H5 heights in the bottom 1st percentile to near all time record low for the southeast. A rapidly deepening surface low then shifts offshore and moves east/northeast of the coast.

This will be an unusually cold snow event for the western Carolinas and northeast Georgia with very cold temperatures in place as the snow occurs. Some members of guidance depict temperatures in the low 20s to teens during peak snowfall rates on Saturday and even the warmer solutions keep temperatures at 30 degrees or lower. Furthermore, forecast soundings indicate incredibly cold profiles with a deep dendritic growth zone and plenty of forcing in part from strong DPVA with the upper low. This will result in rare SLRs ranging from 15:1 to 20:1. Surprisingly, guidance is in very good agreement with SLRs with only small differences between the 10th and 90th percentiles. Thus, it won't take much liquid QPF to realize several inches of snow. The very cold temperatures will also increase impacts to travel with snow sticking very efficiently to roads.

So, the biggest question of the forecast is how does the system evolve and how much moisture do we have to work with. There are several scenarios on the table that will eventually be dictated by the timing and strength of the TPV and how it interacts with several other pieces of energy emanating out of the west coast.

-A stronger and slower TPV that is able to take on a neutral to negative tilt before crossing the Appalachians would favor surface cyclogenesis closer to the coast with an overall slower progression of the coastal low. This scenario would favor more QPF to work with across much of the area.

-An intermediate scenario would be for a wetter upper low that is able to work with more moisture from a lead wave along the Gulf coast. If the instigation of the coastal low and track aren't favorable for moisture transport farther west, the upper low and associated moisture would still be sufficient to squeeze out warning criteria snow.

-And finally, the third scenario would be for a faster and weaker TPV that takes longer to transition to neutral/negative tilt. This would be a more moisture starved scenario with much more meager QPF to work with.

The global guidance and their respective ensemble members, namely the GFS/GEFS/AIGFS, the ECMWF/EPS/AIECMWF, and to some extent the CMC support solutions with sufficient QPF for warning criteria snow across much of the CWA. The UKMET/ICON/NAM/RRFS, on the other hand, are notably drier and would mainly support an advisory level event. That being said, confidence in accumulating snow remains high - it's just a matter of how much and where. Confidence is highest east of I- 26 and especially along and east of I-77 at this time. Also to keep in mind, with the high SLRs it won't take much QPF (~0.15-0.2") to squeeze out warning criteria snow (3" or greater). Another unusual aspect to this event to keep an eye on is the potential for blowing snow from gusty winds. Low-level mass response to the deepening coastal low will support 25-30mph wind gusts on Saturday. High SLRs will favor dry powdery snow with the potential for rarely seen blowing snow. Consideration was given to hoisting a winter storm watch with this forecast package, but given some lingering uncertainty regarding QPF and placement have opted to hold off for now and reassess the overnight model guidance.

Key message 3: Dangerously cold temperatures and wind chills will impact the region Saturday night through Monday morning. This will bring impacts to life and infrastructure.

In addition to the snowfall, another shot of brutally cold arctic air is forecast to arrive through the day Saturday and persist through Monday morning. Strong cold advection will ramp up on the backside of the deepening coastal low and behind the departing upper wave with strong northwest flow. 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 -25 with -5 to 0 along and north of I-85 and low single digits south of I-85. These values would be near or below 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 temperatures may plummet, especially if there's sufficient snowpack still on the ground. The mountains will likely see widespread single digit lows again with some single digits possible east of the mountains where the deepest snow is. These lows will be dependent on snowpack, however, and should a drier scenario play out they will likely be several degrees warmer, but still rather cold. These expected temperatures and wind chill values, especially Saturday night will prove dangerous and will impact life and infrastructure.

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 /00Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR at all terminals through the period. A few patches of mid-level wave clouds were noted across the mountains and western Piedmont, but any mid-level ceiling should scatter out by mid-evening. Additional patches of mid and high-level clouds could move overhead again Thursday morning, then clouds will start to increase Thursday evening. Still seeing some gusty winds at issuance time in some places, but they should abate. Direction should mainly be W to NW. Weak high pressure will build in Thursday and then wind should go light/variable in most places for several hours.

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

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Cold Weather Advisory until 10 AM EST Thursday for NCZ033-049- 050. SC...None.