textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The winter storm watch has been upgraded to a winter storm warning across the North Carolina mountains. The remainder of the watch will likely be upgraded to a warning tonight.

Snowfall totals have increased with the highest totals still expected along and east of I-77, but confidence in heavy snow is increasing for portions of the Upstate and mountains.

An extreme cold watch has been issued for the entire area for very cold air arriving during and after the winter storm. For the mountains, the extreme cold watch runs from Saturday afternoon to Sunday afternoon. For locations east of the mountains the watch is from Saturday evening to Sunday morning.

KEY MESSAGES

1. Mostly cloudy, dry, and below normal through tonight. 2. A significant winter weather event will 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 will be significant. 3. Dangerously cold temperatures and wind chills will impact the region Saturday 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. Precipitation chances may return by mid week.

DISCUSSION

Key message 1: Mostly cloudy, dry, and below normal through tonight.

Dry high pressure lingers over the area through tonight keeping quiet weather around. Mostly cloudy skies can be expected through tonight outside of some brief clearing during the evening hours. Highs this afternoon will rise well above freezing across the lower elevations but will remain near to below freezing above 3,500 ft, ending up ~10-15 degrees below normal. Lows tonight will fall below freezing across the lower elevations and remain below freezing across the higher elevations, ending up ~5-10 degrees below normal. Lighter winds will keep wind chill concerns at bay.

Key message 2: A significant winter weather event will 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 will be significant.

While some variations in model guidance persist, the overall forecast trend is coming into better agreement with high confidence in a significant winter storm for the western Carolinas and northeast Georgia. A potent Tropospheric Polar Vortex (TPV) is currently analyzed just north of the Great Lakes within a broader gyre over southeast Canada. Farther south, a subtropical jet extends from the eastern Pacific to the Gulf Coast with a lead shortwave trough diving through the Southern Plains. And to the west, a tall western ridge resides from the Great Basin through the Cascades and into the Canadian Rockies. Heading through tonight and into tomorrow, the TPV will drop across the Midwest and dig into the Mississippi Valley. The trough will initially have a very positive tilt to it, but will quickly take on a neutral tilt over middle Tennessee before becoming negatively tilted as it swings across the Southern Appalachians through the day on Saturday. The latest trend in guidance has been for a slower evolution of the TPV, which would allow for the lead southern stream wave to get out ahead of the bowling ball upper low and initiate surface cyclogenesis along a coastal baroclinic zone along the Carolina coast.

Furthermore, guidance has latched onto the development of a mesolow in the lee of the Southern Appalachians with the mesolow moving southeast through the Savannah River Valley and into the South Carolina coastal plain. This would place much of the forecast area in a very favorable location north of the mesolow and north of the bowling ball closed upper low. Here, strong forcing for ascent will be present in conjunction with convergent low-level flow, some component of which will have a moisture connection to the Atlantic. This will also foster the development of a notable band of 850mb frontogenesis across the Upstate and into the North Carolina Piedmont and portions of the foothills. The result of which has been a notable southwest trend in QPF with an uptick along the I-85 corridor. Snow will start as early as tomorrow afternoon across the mountains as upper divergence increases as the TPV encroaches on the Tennessee Valley. A strong upslope component of the flow will also help force precipitation across the mountains. Snow will then overspread areas east of the mountains late Friday night into early Saturday morning. The event will peak through Saturday morning into the early afternoon before gradually tapering off from west to east late afternoon through the overnight hours with all snow coming to an end by daybreak Sunday morning.

This will be a very unusual and highly anomalous event in many ways with near record low 500mb heights, very cold temperatures and rare Snow-to-Liquid Ratios (SLRs) of 15:1 - 20:1. The dendritic growth zone will be very deep with plentiful forcing for efficient snow growth. Thus, it won't take much QPF to reach warning criteria snow across practically the entire forecast area. The winter storm watch across the mountains has been upgraded to a winter storm warning owing to earlier onset time tomorrow afternoon and evening. Snow will take longer to break out east of the mountains and the winter storm watch here will likely be upgraded to a winter storm warning with the overnight forecast package. Forecast confidence in where the highest snowfall totals occur is moderate and some variability still exists. Right now, the highest totals are still generally along and east of I-77, but this is subject to change pending the evolution of the mesolow. A band of heavy snow could setup along a corridor extending from Greenville/Spartanburg to Charlotte. Snowfall totals along and north of I-40 are a little more uncertain now with the heaviest snow potentially shifting south towards I-85. Nevertheless, warning criteria snow of at least 3" is still highly likely. There has also been a signal for a "dry slot" between snow bands in the general vicinity of northeast Georgia, although where this occurs is still uncertain - but even there warning criteria snow should still be realized. The ceiling is high with this event and a swath of 8-12"+ totals cannot be ruled out where the greatest banding is able to set up shop.

Temperatures will be very cold during the snow with significant impacts to travel and roads expected. Thankfully, power outages should remain isolated as the dry powdery snow from the high SLRs will have a hard time accumulating on power lines. Blowing snow has also been added to the forecast as gusty winds combined with the drier snow will support reduced visibilities during stronger gusts/heavier rates. It's not out of the question that isolated blizzard conditions may occur, but will likely remain below the criteria for at least three consecutive hours of 35mph or greater gusts and visiblity of 1/4 mile or less.

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

Intense cold advection will ramp up Friday night through Saturday as the winter storm is underway. Highs on Saturday will remain below freezing across the area with many locations in the teens to mid 20s, if not colder. Winds will also ramp up through the day Saturday in response to a deepening coastal low. Wind chills will crash by Saturday afternoon across the mountains with locations east of the mountains seeing dangerous wind chills by Saturday night. Low temperatures Saturday night will be very cold with widespread single digits across the mountains along with a number of single digits expected for portions of the North Carolina foothills and Piedmont. Very low teens will be common elsewhere. These temperatures combined with frequent wind gusts will result in dangerous wind chill values from -10 to -25 over the mountains and 0 to -10 east of the mountains. High temperatures Sunday will struggle to reach freezing outside of the Savannah River Valley with another night of brutal lows going into Monday morning as much of the area drops back into the single digits to low teens. With that being said, an extreme cold watch has been hoisted for the mountains from Saturday afternoon to Sunday afternoon and for the rest of the area from Saturday evening through Sunday morning. At least another round of cold weather advisories will likely be needed potentially through the day Sunday and then again Sunday night. Winds will relax by Sunday afternoon into the overnight, but temperatures will remain cold enough to support at least an advisory. These temperatures will be dangerous to life and property, especially if precautions are not taken.

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

By Monday, low-level flow weakens and shifts to out of the west which cuts off cold advection and finally allows for at least some degree of a warming trend. While still well below seasonal averages, temperatures are forecast to warm above freezing into the upper 30s to low 40s on Monday and eventually into the low to mid 40s by Wednesday. Overnight lows will still remain cold and well below freezing, however, with persistent melt/freeze cycles each day. By mid week, guidance indicates another positively tilted trough dropping across the Great Plains and into the Mississippi Valley. There's considerable uncertainty as to how this feature evolves and what degree of cold air remains nearby. Some ensemble members paint another wintry scenario, while other camps remain warm enough for a rain event. Will get through this weekend's winter storm first and then iron out details for later next week.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

At KCLT and elsewhere: Dry and VFR through the 18Z TAF period thanks to high pressure lingering over the area. BKN to OVC cirrus are expected to stream across the forecast area through tonight, with cirrus becoming more SCT briefly this evening. Cigs will gradually lower late Friday morning into Friday afternoon, but will remain VFR. Winds will be light through the period, gradually turning more S/SE this afternoon and evening east of the mountains. Winds at KAVL will remain NW through early this evening before going calm overnight. Winds at KAVL will pick up out of the SE Friday. Winds east of the mountains will toggle more E/ENE Friday.

Outlook: Confidence continues increasing regarding the potential for light to moderate snow across the area Friday evening into late Saturday. Low pressure pulls away from the area Sunday allowing dry and VFR conditions to return, but gusty N/NW winds will linger through late Sunday before diminishing.

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