textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated precip onset timing based on radar and observations and adjusted temps based on current conditions.

Updated aviation discussion.

No change to snow amounts from the previous forecast.

The Winter Storm Warning remains in effect for the entire area thru 7 AM Sunday. An Extreme Cold Warning remains in effect for the entire area, starting at 1 PM Saturday over the NC mountains and running thru 1 PM Sunday, and starting at 7 PM Saturday elsewhere and running thru 10 AM Sunday.

KEY MESSAGES

1. A Winter Storm Warning remains in effect through 7 AM Sunday as a significant winter storm will bring heavy snow to the western Carolinas and northeast Georgia. Cold temperatures during the event will lead to significant travel impacts across most of the region. 2. Gusty winds combined with very cold temperatures will lead to dangerously cold wind chills today through Monday. The bitterly cold wind chill could result in hypothermia or frostbite if precautions are not taken. 3. Temperatures gradually warm through early next week with dry conditions returning across the region. Precipitation chances return by mid week.

DISCUSSION

Key message 1: A Winter Storm Warning remains in effect through 7 AM Sunday as a significant winter storm will bring heavy snow to the western Carolinas and northeast Georgia. Cold temperatures during the event will lead to significant travel impacts across most of the region.

Bottom line up-front...only minor changes to snow amounts in most places, but some parts of the NC Foothills were bumped up an inch or two. No changes to the Winter Storm Warning.

A significant winter storm is developing as snow develops across the area. Accumulations are taking place and will continue to spread across the area as well. The main driver of this event is a compact and deep upper low centered near the confluence of the MS and OH rivers as of Midnight, which the guidance continues to roll downhill on a track that would take it to the south of the fcst area late this afternoon and this evening. Note this 500 mb low is unusually deep for this time of year...on the order of five standard deviations below normal. The trend in the latest guidance was to orient the low on more of a neutral tilt, which may afford our fcst area a little more in the way of mid/upper forcing and a better chance to bring some Atlantic moisture westward into the fcst area compared to earlier runs, as the 850mb low moves over northeast GA and the Upstate this afternoon. In fact, the QPF seen on the HRRR and NAMNest has crept upward over the last few runs, while the ECMWF/GEFS/Canadian ensemble means have stayed relatively consistent. All remain well within warning criteria, even with modest QPF, because of the abnormally high snow-to-liquid ratio expected due to the deep cold air. No further Warning decisions need to be made, so now we just have to deal with some of the details and nuances of how the fcst will work out.

The CAMs continue to suggest banded structures in the simulated reflectivity though this might not be a strong case for mesoscale banding. If that were to develop, there could be sharp stripes and gradients of snowfall. Either way, there will probably be at least one axis of heavier precip/snow where the better frontogenesis sets up on the north side of the 850 mb low. What is interesting is how several guidance sources want to place the best snow potential somewhere out in that triangle area bounded by GSP/CLT/CAE later today. Guidance also has been consistent with a stripe of better snow potential somewhere in northeast GA, but confidence in placement will be elusive until/unless we see it start to materialize. Note that if we have banding, some places will get less than expected. However, the air mass should be cold enough...generally in the low/mid 20s outside the mtns...and the rates high enough...upwards of 1/2 to 1 inch per hour...that even lower accums will result in significant challenges to transportation as the snow easily sticks to roadways. There could also be some blowing snow concerns through the event that could result in brief near-blizzard conditions in some spots, particularly during the daylight hours.

Signals still point to the main precip shield moving out to the east late this afternoon and this evening as the upper low rolls east. It stands to reason that we should hold onto some upslope snow shower activity as the low level flow becomes more northwesterly with the departure of the 850 mb low. A chance of snow showers was added to the TN border NW Flow upslope zones through early Sunday.

Once the snow begins later this morning, consider staying put and only travel if you absolutely must. If you must travel, keep blankets, an extra flashlight, food, and water in your vehicle in case of an emergency and in case you get stranded.

Key message 2: Gusty winds combined with very cold temperatures will lead to dangerously cold wind chills today through Monday. The bitterly cold wind chill could result in hypothermia or frostbite if precautions are not taken.

There were some subtle changes in the new guidance with regard to the low wind chill values expected across the fcst area later today and especially tonight, but not enough to affect any course of action, since the entire forecast area is already under an Extreme Cold Warning beginning later today. There might be less confidence in reaching criterial in the lower Piedmont/Lakelands early Sunday because the wind guidance is not as strong, but it will still be really cold at any rate. Note that guidance continues to support strong wind gusts from the NW across the mountains beginning this afternoon as the low starts to pull away. Many locations over the mtns could get close to Wind Advisory criteria and the ridges and peaks above 4000 feet could approach Warning Criteria gusts (above 55 mph). The wind will only gradually diminish on Sunday, but that should allow for the Extreme Cold Warning to run out and be replaced by a Cold Weather Advisory for the rest of Sunday and Sunday night. Temps may struggle to climb above freezing on Sunday, but a warming trend will begin early in the week. For now, we will roll any Wind Advisory/High Wind Warning concerns into the Winter Storm Warning and mention the winds in the Extreme Cold Warnings as well.

These wind chills could result in hypothermia or frostbite if precautions are not taken. Dress in layers including a hat, face mask, and gloves if you must go outside. To prevent water pipes from freezing; wrap or drain or allow them to drip slowly. Make frequent checks on older family, friends, and neighbors. Ensure portable heaters are used correctly. Do not use generators or grills inside.

Key message 3: Temperatures gradually warm through early next week with dry conditions returning across the region. Precipitation chances return by mid week.

By Monday, low-level flow weakens and becomes more westerly as the broad/deep upper trof translates off the the Atlantic Coast and robust, 500mb NW flow lingers over our region. On Monday, temperatures should warm above freezing for all but the higher elevations with low 40s across most of our lower terrain. Over- night lows will continue to fall well below freezing, however, and will support refreezing of melting snow/ice each night.

By Tuesday, a gradual modification of the airmass should be evi- dent. An embedded upper shortwave from the Pacific NW will dive down the eastern flank of a stout west coast ridge through the Rockies and into the Great Plains by Wednesday morning. Guidance varies as to how this trof evolves with some members maintaining a more positive tilt while others transition it into a neutral to near negative tilt with an upper low trying to close off over the TN Valley. Eventually, this brings a slug of moisture back into our area early Wednesday thru at least early Thursday. The exact timing of this system, the track of the subsequent sfc low, and the availability of cold air will have major implications wrt precipitation type. The latest model guidance is trending towards more snow early Thursday over western NC, with mostly rain over our SC and GA zones. This type of outcome would result from the sfc low tracking farther south with a more favorable upper trof interaction and colder air in place over western NC. If this type of scenario does pan out, we can expect more winter weather for at least part of our fcst area. At present, there's still too much uncertainty to place much confidence on this scenario.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Snow with MVFR to IFR restrictions ongoing at the NC sites. Still more flurry like with brief restrictions for the SC sites. Snow remains steady with MVFR to IFR restrictions at the NC sites and develops this morning then continues through the day for the SC sites. Snow and restrictions begin to taper off by late afternoon and ends during the evening. Mainly VFR expected overnight but there could be vsby restrictions if blowing snow develops. Gusty N to NE wind through the day turning more NW to NNW by evening into the overnight.

Outlook: Dry and VFR conditions return Sunday but gusty NW winds will linger through late Sunday afternoon before gradually diminishing Sunday evening into Sunday night. Dry and VFR conditions will linger through at least Tuesday. A cold front may bring precip and associated restrictions Wednesday.

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 Warning until 7 AM EST Sunday for GAZ010-017-018- 026-028-029. Extreme Cold Warning from 7 PM this evening to 10 AM EST Sunday for GAZ010-017-018-026-028-029. NC...Winter Storm Warning until 7 AM EST Sunday for NCZ033-035>037- 048>053-056>059-062>065-068>072-082-501>510. Extreme Cold Warning from 7 PM this evening to 10 AM EST Sunday for NCZ035>037-056-057-068>072-082-502-504-506-508- 510. Extreme Cold Warning from 1 PM this afternoon to 1 PM EST Sunday for NCZ033-048>053-058-059-062>065-501-503-505-507- 509. SC...Winter Storm Warning until 7 AM EST Sunday for SCZ008>014-019- 101>109. Extreme Cold Warning from 7 PM this evening to 10 AM EST Sunday for SCZ008>014-019-101>109.