textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The aviation forecast was updated this evening to reflect the 00Z TAF issuance.

Periods of light rain still expected tonight and Friday morning.

A Winter Storm Watch remains in effect from Saturday morning through early Monday afternoon for the entire forecast area. Upgrades to Warnings will be considered with the next forecast package. Snow/sleet accumulations continue to slip downward and ice accumulation amounts continue to go upward.

KEY MESSAGES

1. Periods of scattered light precip tonight and Friday morning. A small potential for very light/low impact wintry weather across the high elevations. Otherwise, rain is expected. 2. No upgrades to the Winter Storm Watch yet, but the precipitation types continue to trend more toward ice and away from snow/sleet. We still expect this to be a major winter storm across the entire forecast area, leading to hazardous travel and power outages that may last for days. 3. Dangerously cold wind chills may develop Monday night into Tuesday morning which could result in hypothermia or frostbite if precautions are not taken.

DISCUSSION

Key message 1: Periods of scattered light precip tonight and Friday morning. A small potential for very light/low impact wintry weather across the high elevations. Otherwise, rain is expected. A very broad baroclinic zone will remain across the forecast area through Friday morning. This baroclinic zone will steadily intensify through the night as the height gradient within a broad upper trough centered over the East strengthens in response to upper low passing just north of the Great Lakes. A couple of waves move east with weak isentropic lift and frontogenesis producing periods of showers across the area tonight and early Friday. Best chance will be along and south of the I-85 corridor but QPF will be light. Precip that moves across the NC mountains and I-40 corridor could include brief periods of a wintry mix; however, no significant accumulations are expected given the short duration and very light QPF there. Lows tonight will range from near normal to as much as 10 degrees above normal. Highs will range from near normal to as much as 5 degrees below normal.

Dry conditions expected Friday night before the winter storm begins affecting the area. Lows will be near 5 degrees below normal.

Key message 2: No upgrades to the Winter Storm Watch yet, but the precipitation types continue to trend more toward ice and away from snow/sleet. We still expect this to be a major winter storm across the entire forecast area, leading to hazardous travel and power outages that may last for days.

All signs continue to point toward a major winter storm across the region moving in from the west during the day on Saturday, taking the form of a Miller type 'B' configuration. The latest model guidance continues to drift later on the onset timing by a bit and the model blend reflects that, so it might not be until midday before precip begins over the mtns and perhaps mid/late afternoon over the wrn Piedmont of NC.

The model guidance continues to show two remarkable things. First, a very cold and very deep boundary layer will take hold across the region as a large Arctic high over the Great Lakes and upper Midwest spills down in a cold air damming wedge. The low level air mass is so cold that it should result in precip to begin as frozen types across the entire forecast area. Second, the system will be fueled by a band of Pacific moisture seen on Integrated Vapor Transport guidance that spreads quickly in from the southwest as the precip arrives. A brief period of light snow at onset can be expected, but with a quick transition to sleet as the warm nose comes in. Once that is established, the sleet may hold on longer than expected with such a strong warm nose because of the abnormally cold and deep boundary layer. Most places could get an inch or two of sleet during this period, before a transition to freezing rain begins.

That being said, the guidance continues to show a more stronger warm nose developing Saturday night, resulting in a mix then switch to freezing rain across much of northeast GA/Upstate SC into NC, eventually changing mostly to ZR in western NC as well. The stronger warm advection does two things to the fcst, first we end up with more ice accumulation and less sleet in most places, and second we are able to warm up above freezing by daybreak Sunday in the southwest upslope areas of southwest NC where cold air damming typically is not as strong, so ice amounts trend down there. The trend in the model blend is to bring the warm air up from the south Sunday afternoon, but this is treated with much skepticism. Strong CAD wedges typically do not erode that quickly, but the warm advection aloft also looks formidable. The freezing rain may persist longer into Sunday afternoon/eve over northeast GA and the western Upstate than indicated. In the end, our ice accumulation looks more like the neighborhood of one half inch in most places, and 0.75 inch along the Escarpment.

The bottom line continues to look like a mess either way with high confidence that all locations will get sleet and/or ice accumulations above our warning criteria. However, we will refrain from issuing warnings until our offices to the west are ready, which will allow us to have a better handle on expected types/amounts.

In the mean time, continue preparing for a major winter storm. Regardless of the individual p-types, the important thing to note is there will be plenty of wintry precip with this storm. Wintry precip, whether it's ice, sleet, or snow are going to have widespread impacts making for hazardous travel and power outages that could last for days. Prepare an emergency kit for your car/home and replenish fuel for your car and other heating sources such as generators. Remember to keep generator outdoors and at least 20 ft away from entry points to avoid carbon monoxide poisoning. Don't forget about your pets. Keep enough non-perishable food, water, and medications for at least 3 days. Ensure you have warm clothing and blankets as wel. Charge your phone and devices in advance so that you are able to receive alerts.

Key message 3: Dangerously cold wind chills may develop Monday night into Tuesday morning which could result in hypothermia or frostbite if precautions are not taken.

In the wake of the winter storm, a large continental sfc high that builds down over the Deep South will bring very cold temperatures down across the fcst area Monday night with clear sky and a light NW wind. Low temps are a little bit tricky in that some of the guidance might not be taking into account the snow cover (or lack thereof), possibly making incorrect assumptions about having a fresh snow cover all the way down across the fcst area that would depress the sfc temps in the model solutions. As the fcst is constructed right now, we have low temps in record low territory for Tuesday 27 Jan. I would not be surprised if this drifted slightly upward. However, there's enough of a steady light NW wind that our wind chills easily get down into Cold Weather Advisory criteria early Tuesday across the mtns/foothills/areas n/w of I-85. Even the lower Piedmont would stand an excellent chance. We might even have to consider an Extreme Cold Warning for Avery County and high elevations of the nrn mountains. We shall see how the fcst temps drift before we have to consider any watches for cold.

The air mass modifies relatively quickly early next week, but will remain well below normal on Tuesday, and still on the order of ten degrees below normal into the middle of the week. This won't allow for any accumulations of sleet and ice to melt very quickly. Travel impacts are anticipated through Monday over the mountains where temps don't rise out of the teens and 20s. East of the mtns, the high temps have trended a bit warmer, but not above 32F for any great lengths of time. It might take several days to alleviate all the wintry precip on untreated roads. The extremely cold temperatures could also have severe impacts on those who lose power. Make sure you have plenty of blankets and warm clothing to keep warm. Remember to avoid burning fuels like propane or kerosene indoors as this increases fire risk and carbon monoxide poisoning.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: Still seeing widespread light showers south of I-85, though not impacting any of the TAF sites currently. Expect generally VFR conditions to persist overnight, with no more than VCSH warranted at any of the Upstate sites through the period. Toward dawn, intermittent MVFR conditions could develop at KCLT alongside a surge in -RA. Periods of MVFR will be possible across the Upstate through much of Friday, with brief IFR even possible at KAND. Conditions should improve by evening. Light and variable winds overnight will turn decidedly NE Friday morning, continuing at 4-8kts through Friday evening.

Outlook: A major winter storm is forecast to impact the entire area beginning Saturday morning and continuing through Sunday. Significant accumulations of freezing rain/ice and sleet are likely at all TAF sites. Long term prevailing flight restrictions should be expected into Monday when the system finally moves east. VFR expected Tuesday.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Winter Storm Watch from Saturday morning through Monday afternoon for GAZ010-017-018-026-028-029. NC...Winter Storm Watch from Saturday morning through Monday afternoon for NCZ033-035>037-048>053-056>059-062>065- 068>072-082-501>510. SC...Winter Storm Watch from Saturday morning through Monday afternoon for SCZ008>014-019-101>109.