textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Minor increases in precip chances for patchy light rain across the southern half of the area today.

A Winter Storm Watch remains in effect from Saturday morning through early Monday afternoon for the entire forecast area.

KEY MESSAGES

1. Periods of scattered light precip through tonight, with small potential for very light/low impact wintry weather across the high elevations this morning. Otherwise, rain is expected. 2. Winter Storm Watch is in effect Saturday morning through early Monday afternoon as a storm system will bring heavy mixed precipitation to 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 through tonight, with small potential for very light/low impact wintry weather across the high elevations this morning. Otherwise, rain is expected.

The leading edge of a very broad baroclinic zone will settle across the forecast area later this morning. This baroclinic zone will steadily intensify through the period, 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 "false starts" in surface development near the Gulf Coast states are anticipated through tonight, with bouts of enhanced frontogenesis supporting precip potential across our forecast area...mainly in the form of patchy -RA across the southern half of the CWA, where mostly chance PoPs are advertised, except for a small area of likely across the SC Lakelands and upper Savannah River Valley this morning. In the very near term...there is small potential for a patchy wintry mix this morning across the higher elevations south of I-40, but the probability of noteworthy/impactful accums is very low. Max temps are forecast to range from around normal across the southern half of the CWA, to several degrees above climo along the I-40 corridor, where less cloud cover is anticipated. Min temps tonight will be around 5 degrees above normal.

Key message 2: Winter Storm Watch is in effect Saturday morning through early Monday afternoon as a storm system will bring heavy mixed precipitation to the entire forecast area, leading to hazardous travel and power outages that may last for days.

The big story is the continued evolution of this upcoming winter storm over the weekend. High confidence on widespread impacts prompted a Winter Storm Watch to be issued for the entire area. This is in effect for Saturday morning through Monday morning. As confidence in precip accumulations increases, warnings are likely to be issued. Regardless of precip type of amounts, the most important aspect of this system to understand are the widespread impacts, from hazardous travel conditions and power outages lingering into the beginning of next week. With that said, let's talk precipitation. Some of the latest model guidance has slowed the system down a bit, delaying the onset of precip to later on Saturday, rather than Friday night. While the system moves into the area, the colder air appears to get a slight delay in arrival, thanks in part to a developing shortwave moving over the southeast. This could create a brief period of isentropic ascent with moisture lagging, ahead of the colder air. What this does is help to create a warm nose sooner than originally expected. A warm nose means more ice/sleet than snow. The transition zone has shifted once again to the north, with model guidance honing in on a much higher chance for a significant ice/sleet event for locations south of I-40. So, as is more the norm for the south, the snow is heading north and the ice/sleet looks to be the dominant p-type as of now with the latest model guidance.

Now, let's look at the NBM 48-hour accumulation probabilities for this storm. For now, current data shows the sleet chances for Saturday into Sunday have increased to 40-50% with freezing rain chances 60-70% east of the mountains and south of I-40. Current projections of total ice accumulations for Saturday and Sunday being greater than 0.25" are 65-85%, so high confidence for warning criteria for ice being met east of the mountains. Locations of Upstate SC and into the NC Piedmont have a 40-50% chance of total ice accumulations greater than 0.5". As for snow, the likelihood of snow accums greater than 4" are 35-45% for locations north of I-40. At this point, ice/sleet are becoming the main p-types for the majority of the CWA as snow gets shunted further north.

Details will continue to be thoroughly looked over with each forecast cycle leading up to this weekend, including a better idea of total accumulations. But the main focus at this time should be preparations for a significant winter storm. Regardless of the individual p-types, the important thing to note is there will be plenty of QPF 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 well as an updated first aid/emergency kit. 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.

Behind this winter storm over the weekend, high pressure looks to remain the dominant feature with frigid temperatures lingering. Current guidance decreases temperatures even more and increases the chances for dangerously cold wind chills. At this time, current model guidance has wind chills below zero for the mountains and teens in most locations east of the mountains. A few locations north of I-40 could see wind chills in the single digits. If this trend continues, a widespread Cold Weather Advisory may be needed. Additionally, frigid temperatures could keep winter precipitation around longer, continuing impacts for travel. These extremely cold temperatures could also have severe impacts on those who experience an extended power outage. Again, ensure you have plenty of blankets and warm clothing to keep warm. Remember to avoid burning fuels like propane or gasoline indoors as this increases fire risk and carbon monoxide poisoning.

AVIATION /12Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: An area of showers will push across the southern part of the Terminal Forecast Area this morning, with tempos for -SHRA and MVFR cigs/visby warranted at the upstate SC terminals. Can't rule out a brief period of IFR visby...esp at KAND, as there is quite a bit of 1-2SM observed underneath the showers across central GA. Rainfall rates are expected to diminish as the showers move east into deeper dry air, so we opted to stick with MVFR for now. Showers could develop as far NE as KCLT later today, and a Prob30 is carried there from 17-20Z. Otherwise, VFR is expected to prevail through at least this evening.

While shower coverage is expected to diminish later this afternoon into the evening, another round is anticipated late tonight across the southern half of the area, and Prob30s for -RA and MVFR cigs return to the forecast at the Upstate TAF sites and KCLT...mainly between 06-12Z Friday. Light WSW winds this morning will become light/variable at most sites from late morning through the afternoon. Winds will become light NE across much of the area by late tonight.

Outlook: Some potential for cig/visby restrictions will persist through Friday morning, although shower chances will decrease. 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.

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.