textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The remainder of the fcst area was upgraded to an Ice Storm Warning. The onset time was moved slightly back. Some changes to precip types and amounts were made, generally more in favor of freezing rain/ice and less in favor of sleet, but well above warning criteria. Minor changes to temperatures.

KEY MESSAGES

1. Temperatures will remain below normal today and tonight with cold air damming developing later tonight and early Saturday. 2. The Winter Storm Watch has been upgraded to an Ice Storm Warning for the entire forecast area. We expect this to be a major winter storm across the entire forecast area with ice being the main precipitation type across most of the region, 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. 4. A reinforcing cold front is expected later next week with another chance for dangerously cold wind chills Wednesday night/Thursday morning, which could result in hypothermia or frostbite if precautions are not taken. There may also be some accumulating snow over the NC mountains near the TN border.

DISCUSSION

Key message 1: Temperatures will remain below normal today and tonight with cold air damming developing later tonight and early Saturday.

A weak band of low-level frontogenesis continues to interact with an increasingly moist airmass along and south of a weak frontal boundary just to our south. The resulting sct light showers have lingered along and just north of the I-85 Corridor thru the morning and into the aftn. This shower activity will continue to gradually move east of our fcst area this aftn, with little in the way of any accumulation expected.

Meanwhile, a low level ridge associated with sprawling/strong 1050+ mb arctic high pressure over the Great Plains/Canadian Prairie will continue spreading east of the central Appalachians today and tonight, allowing for the gradual establishment of cold air damming across our area. Otherwise, high temperatures today will be a few degrees below normal across most of our lower terrain. Our mtns and northern NC Piedmont will likely end up near climatology or just above, as dry air advection associated with deepening NELY flow will likely result in some aftn clearing. Low temperatures will be below normal early Saturday as low level cold advection steadily ramps up in association with the developing CAD.

Key message 2: The Winter Storm Watch has been upgraded to an Ice Storm Warning for the entire forecast area. We expect this to be a major winter storm across the entire forecast area with ice being the main precipitation type across most of the region, leading to hazardous travel and power outages that may last for days.

By now, it is a foregone conclusion that our Watch should be upgraded to a Warning for the rest of the forecast area, even with lingering sleet versus ice uncertainty.

The guidance continues to drift a bit on the onset timing and the precip types, but with a strong consensus on a major winter storm that will impact our area Saturday afternoon through Sunday night. The ingredients will be in place, with a strong, arctic high moving across the Midwest and Great Lakes, spilling east of the Appalachians and providing a cold and deep boundary layer as strong cold-air damming sets up by Saturday morning. Meanwhile, a southern stream trough will advect tropical Pacific moisture across Mexico into the Southeast US and atop the cold air mass. The guidance continues to drift a bit with the onset, to the point where precip may not reach our area from the west until after noon. Regardless, the precip will spread east quickly and decisively by late afternoon. The precip might begin as a very brief period of light snow, but more likely will start out as sleet and then ramp up from there as by Saturday afternoon most of the guidance shows the warm nose already having developed over top of the cold air damming wedge. It's at that point where the guidance differs on predominant precip type and thus a forecast challenge. Some of the model guidance has almost all freezing rain across the region, while others hold onto sleet well into Sunday morning. A compromise was reached that is more in favor of mixing and then changing to freezing rain from SW to NE overnight, with only the area along/N of I-40 east of the mtns holding onto the sleet into Sunday afternoon. The QPF still looks fairly impressive because of the mid-level moisture transport, so it's a matter of how that liquid falls out. The new fcst trended toward more ice and less sleet, to the point where much of the area outside the nrn foothills and nw Piedmont of NC will get maybe an inch of sleet at the most, with freezing rain being predominant.

The decision was made to upgrade to an Ice Storm Warning across the entire fcst area because the threat for a half-inch of ice is higher than the threat for significant sleet across the fcst area. Either way, it will be an icy mess, either from ice pellets or ice accretion. Note this may not be like other freezing rain events we have had in the recent past, because temps are expected to fall down into the 20s as this is happening instead of hovering around 32F as usual. As a result, there could be a quicker and greater impact to travel as ice starts to impact untreated roads almost immediately. Most of the region looks like it has good potential to accumulate one-half inch of ice through Sunday afternoon. The southern-facing parts of the Blue Ridge Escarpment from Rabun County GA and then along the NC/SC border have the best potential to see the most ice, perhaps up into the 0.75 to 1 inch range.

The exception will be the southwest mountains/Little TN valley, where wedges aren't as strong. Warm air advection will start to raise temps above freezing Saturday night, and by late Sunday morning most of that area will be above freezing. Some valley locations might never drop below 32F. However, there's enough potential for warning-criteria ice over the high elevations of Graham/Swain/Haywood late Saturday to support a positive warning decision. Expect that area to be cancelled early. Assuming we rise above 32F in these locations, there could be enough rain to cause some flooding threat.

The wedge begins to break down Sunday afternoon as strong warm advection continues from the southwest. In most cases, a strong wedge will hold on longer than most guidance indicates, but this storm system is not ordinary. We hold onto cooler temps over northeast GA and the western Upstate compared to the model blend. If the warm advection wins out in this case, the fcst ice amounts might be too high. If the wedge holds on longer than expected and keeps temps below 32F in northeast GA/Upstate SC and into metro CLT, our ice accum fcst might be as much as a quarter inch too low, especially if we get an elevated convective line coming over the top of a stubborn wedge. We will fine-tune this in the next few cycles as we have more output from the CAMs.

The system should finally get swept east early Monday morning. We might get temps to creep above 32F across northeast GA/Upstate SC to allow for a bit of melting by daybreak, but temps will fall back down to around 32F and keep the threats in place. Guidance has trended toward a period of upslope snow showers on the TN border through Monday morning, so a light snow accum can be expected there. The falling temps over the mtns combined with increasing NW winds could combine to produce wind chills below zero in elevations above 3500 ft Monday morning.

There are several possible failure modes in this forecast, related to QPF, sleet v freezing rain accum, and how quickly the wedge breaks down before possible convection moves through Sunday evening. Expect more changes as the guidance drifts.

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 or sleet, will 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 well. 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.

Not much has changed with our earlier thinking with regard to the cold temps early Tuesday morning. The axis of a deep mid/upper trof is expected to swing across the fcst area Monday evening and a continental polar sfc high pressure air mass quickly follows. The guidance has been consistent in dropping the low temps down into record territory, with all three climate sites reaching or tying record lows. We retain a strong enough NW wind overnight because of the lingering pressure gradient to drop wind chill values down into Cold Weather Advisory range across almost the entire fcst area, and much of the mountains above 3500 feet, Avery County especially, could drop down into Warning range. We have another few fcst cycles to see if the model trends any warmer before issuing products for this threat. There is some concern that model guidance might be counting on more snow/sleet cover to drop its temps down that much.

Temps are still expected to moderate through mid-week once the trof axis moves off to the east, the upper flow flattens, and the sfc high drifts eastward, but the temps will still be on the order of fifteen degrees below normal. High temps might be a couple of degrees higher than previously thought in some spots Tuesday and Wednesday afternoon, but still very cold at night, so melting of any lingering sleet and ice will be slow. We can expect a black ice problem each night/morning probably thru the end of the week.

The cold temps thru the week will exacerbate threats of hypothermia for anyone traveling or still without power. Make sure to check up on vulnerable loved ones and neighbors, pets, and have plenty of blankets and warm clothing. Remember to avoid burning fuels like propane or kerosene indoors as this increases fire risk and carbon monoxide poisoning.

Key message 4: A reinforcing cold front is expected later next week with another chance for dangerously cold wind chills Wednesday night/Thursday morning, which could result in hypothermia or frostbite if precautions are not taken. There may also be some accumulating snow over the NC mountains near the TN border.

Another strong short wave may drop down out of Canada on Wednesday to sharpen the upper trof and swing across the fcst area Wednesday night or Thursday morning. Some of the guidance brings a slug of deep moisture with the wave to fuel upslope snow showers on the TN border, and maybe even enough forcing for a few snow showers to break containment and move east of the mtns owing to the sustained cold air across the region. But, we will worry about this tomorrow or the next day. The passage of the wave will reinforce the cold air perhaps as early as Thursday morning, introducing another Cold Weather Advisory risk across the mountains.

AVIATION /19Z FRIDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: KAND is still observing MVFR cigs and these will likely persist for a few more hrs. Otherwise, pre- vailing VFR conditions are expected thru the 18z TAF period. Some amount of scattering has been seen over the NC sites this afternoon as drier air gradually filters into the area. None- theless, VFR cigs will fill back in overnight. Light NE winds will continue thru the day/evening before increasing to 8 to 12 kts with low-end gusts overnight/early Saturday.

Outlook: A major winter storm is expected to impact the entire area beginning Saturday afternoon and continuing thru Sunday. Significant accumulations of freezing rain/ice and sleet are likely at all TAF sites. Extended flight restrictions should be expected into Monday, with VFR returning by Tuesday.

CLIMATE

RECORDS FOR 01-27

MAX TEMPERATURE MIN TEMPERATURE STATION HIGH LOW HIGH LOW ------- ---------- ---------- ---------- ---------- KAVL 80 1999 17 1940 54 1916 2 1986 KCLT 75 1890 24 1940 58 1890 6 1940 KGSP 73 1954 29 1940 57 1949 8 1940

GSP WATCHES/WARNINGS/ADVISORIES

GA...Ice Storm Warning from 1 PM Saturday to 1 PM EST Monday for GAZ010-017-018-026-028-029. NC...Ice Storm Warning from 1 PM Saturday to 1 PM EST Monday for NCZ033-035>037-048>053-056>059-062>065-068>072-082- 501>510. SC...Ice Storm Warning from 1 PM Saturday to 1 PM EST Monday for SCZ008>014-019-101>109.