textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The aviation forecast was updated to reflect the 06z TAF issuance.

Remaining snowfall totals were updated for the ongoing northwest flow snow event in the Appalachians.

A Fire Danger Statement was issued for Thursday in the South Carolina Upstate and northeast Georgia.

Snowfall totals have increased slightly for the high elevations of southwest NC and the southern mountains Friday night and early Saturday.

Confidence remains low for snow east of the mountains Saturday night/Sunday morning, though precip/snow chances have crept upward over the southeastern edge of the forecast area and the Charlotte metro area.

KEY MESSAGES

1. Frigid Arctic air pushes into the Carolinas on Thursday, driving light NW flow snow along the NC-TN border, gusty winds, and very cold wind chills. 2. Low-end fire weather threat expected on Thursday. Low relative humidity combines with gusty winds to produce favorable conditions for fire behavior, but cold temperatures limit overall threat. 3. Light snow accumulations over the North Carolina mountains and extreme northeast Georgia Friday night through early Saturday could result in minor travel issues, mainly at high elevations. 4. We continue to monitor the evolution of model guidance for Saturday night and Sunday with some interest, given the consistent GFS depiction of snow potential, but a lack of consistency between models keeps confidence low. 5. Another cold air mass will settle over the region for early next week. A Cold Weather Advisory might be needed for parts of the mountains early Tuesday.

DISCUSSION

Key message 1: Frigid Arctic air pushes into the Carolinas on Thursday, driving light NW flow snow along the NC-TN border, gusty winds, and very cold wind chills.

The latest surface observations indicate that the advancing cold front is now located over the extreme eastern tier of the CWA, with extensive clearing underway across the rest of the forecast area. Lingering mountain stratocu is in place across the Appalachians, where light radar returns indicate that some snowfall is still underway. It appears that even the valleys are now cold enough to support snow; however, the strongest dynamic lift associated with an advancing shortwave axis has waned...with the shortwave now located over the NC Piedmont as per water vapor imagery. RAP soundings depict a rapidly-shrinking saturated layer near the surface between now and daybreak...certainly cold enough for ice nucleation but increasingly anemic overall. So, at this point, think the bulk of snowfall has fallen, and the heaviest rates are over. An additional 1-2" can be expected on some mountain slopes within the Winter Weather Advisory, with up to 4-5" of additional snowfall possible at extremely high peaks above 4500ft. Increasingly light snowfall can be expected to continue on favored west-facing slopes along the Tennessee border through mid-afternoon today, before tapering off entirely by sunset.

Currently also seeing wind gusts as high as 45-50kts at some higher elevations, within a region of robust postfrontal CAA. We're likely in the core of strongest CAA right now, so gusts shouldn't get much stronger than they already are, but gusts are expected to persist at their current intensity through daybreak, before diminishing somewhat during the day. Gusts as high as 25kts are possible through most of today, and should finally settle down to a steady 10-15kt breeze overnight. As a result, there could be isolated visibility issues in the mountains, where blowing snow kicked up by wind gusts creates problems. This will mainly be an issue over the next 12 hours, but isolated blowing snow pockets could continue into the day Thursday. Additionally, very cold wind chills - as low as 10 degrees below zero across many higher elevations, and as low as 20 degrees below zero at the highest peaks - are expected to continue tonight, and to return to some extent Thursday night, though they won't be quite as cold Thursday night as they are now. Consequently, a Wind Advisory will remain in place through 7 PM this evening, and a Cold Weather Advisory will remain in place through 7 AM on Friday morning.

Key message 2: Low relative humidity combines with gusty winds to produce favorable conditions for fire behavior, but cold temperatures limit overall threat.

Gusty winds as high as 25kts should continue through most of Thursday within the tail end of strong low-level CAA, as a strong Arctic air mass settles into the Carolinas. Hi-res guidance depicts the usual dry slot around 850-800mb, with a fairly weak postfrontal inversion to inhibit mixing into the dry layer. As such, think there's potential for dewpoints to mix out; forecast RHs crater to as low as 22% this afternoon, and even this could prove to be a conservative prediction if efficient mixing truly taps into the dry slot. Consequently, fine fuel moisture should have no problem dropping to critical levels this afternoon. The biggest limiting factor will be temperatures - with highs only making it into the mid- to upper-30s this afternoon (save for the Savannah River Valley, where they may hit 40), it'll be tough for fires to catch and spread quite as effectively, even despite dry and windy conditions. That's no magic bullet, however - these dry and windy conditions will contribute to increased fire danger, so if you burn, be extremely cautious!

After collaboration with neighboring WFOs and state land managers, the decision was made to issue a Fire Danger Statement for all of our Upstate South Carolina and northeast Georgia zones beginning at 10 AM.

Key message 3: Light snow accumulations over the North Carolina mountains and extreme northeast Georgia Friday night through early Saturday could result in minor travel issues, mainly at high elevations.

After a relatively quiet Friday courtesy of sfc high pressure passing to our south, which allows temps to rebound nicely back toward normal under sunny sky, a fast-moving short wave will approach from the west late in the day. This wave will be rotating around the bottom of a digging mid/upper trof over the Plains/Midwest associated with the new upper low dropping down over the upper Great Lakes on Friday. The model guidance has come into good agreement with shearing out this wave on the west side of the mtns Friday night on the leading edge of the trof. It doesn't look like much, but there should be just enough mid-level DPVA, just enough weak upper divergence from a jet streak developing over the mtns, just enough isentropic lift on the east side of the jet axis, just enough WSW upslope flow at low levels, and just enough moisture return from the western Gulf ahead of the wave to support precip developing late Friday over the TN Valley region and then moving east over the mtns overnight. Precip probs were raised a bit more and will be into the likely range over the Smokies and southwest NC into extreme northeast GA. Thicknesses/temps will be borderline for wintry precip, but fcst soundings from the NAM and GFS suggest an elevation-dependent rain/snow scenario, which seems reasonable. The forcing lifts past to the northeast fairly quickly while not translating much east of the mtns such that precip should be lifting out and drying up by mid-morning Saturday. The relatively short duration puts a limit on the QPF and there is uncertainty about how much Gulf moisture will be available, so the situation favors a high elevation Advisory at worst. Mountain valleys are expected to remain just warm enough for the precip to be mainly rain. Suffice to say as it stands right now, Warning criteria looks out of reach, but we will wait to get past the current winter wx event before entertaining a new Advisory for Friday night/early Saturday over parts of southwest NC including the Smokies and Nantahala mountains, but that's the direction the forecast is heading.

Key message 4: We continue to monitor the evolution of model guidance for Saturday night and Sunday with some interest, given the consistent GFS depiction of snow potential, but a lack of consistency between models keeps confidence low.

In the wake of the first wave, the upper trof axis will deepen Saturday to our west as a much stronger wave dives down to amplify the trof, which is a development that guidance can agree with. The resulting SW flow aloft Saturday should allow temps to get back to normal, but this will be short-lived as the approaching short wave swings the upper trof axis eastward Saturday night. That's when things get interesting if you prefer the GFS, which has shown some decent consistency over the past 24 hours with developing a stripe of precip across the Carolinas in response to greatly improving deep layer forcing and frontogenesis ahead of a dynamic-looking short wave sweeping across the Southeast. The GFS would have the cold air coming over the mtns Saturday night coincident with the development of precip spreading northeast across the region. What is interesting is that the operational run has considerable support from the GFS ensemble with having snow across at least the area east of the mtns to the point where the ensemble mean would be pushing warning criteria, but this could be another example of the ensemble not being dispersive enough. Meanwhile, the operational Canadian has most of its precip just to our east with much less support from its ensemble, with only two or three members giving snow over our east. We await the new ECMWF ensemble, but that model has been much less enthusiastic for our winter weather prospects and has consistently had the precip along the coast or offshore. So, as enticing as the GFS/GEFS looks, caution is advised to manage expectations because confidence will remain low until more support is garnered from the other model ensembles. Officially, the model blend has slowly moved in the direction of including some precip chances over our southeastern fringe for Sunday morning, falling as some combo of rain and snow. The precip should move out during the middle of the day. Either way, temps will cool off back to ten degrees below normal with clouds and a northeast wind.

Key message 5: Another cold air mass will settle over the region for early next week. A Cold Weather Advisory might be needed for parts of the mountains early Tuesday.

In the wake of the strong wave moving past Sunday, we should remain at the bottom of a deep long wave upper trof through the first half of next week, supporting a cold Canadian sfc high pressure air mass that settles over the Southeast Monday into Tuesday. This will keep temps down in the 10-15 degree range below normal both days. Altho winds will be light Monday night, low temps will be cold enough that wind chill temps could be down close to Cold Weather Advisory criteria for the nrn mountains and high elevations.

AVIATION /06Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: VFR everywhere with gusty NW winds tonight as a strong Arctic air mass pushes into the area. Intermittent -SN and associated MVFR ceilings have made it to KAVL over the last hour or so, and will be included in a TEMPO in the new 06z TAF. Precipitation is not expected to impact any of the other TAF sites; rather, sustained gusty winds will be the big issue with the Thursday forecast. Gusts of up to 25kts are expected at most terminals (as high as 35kts at KAVL), persisting well into Thursday evening and only tapering off after sunset. A round of FEW/SCT mid-level cloud cover may develop around and after daybreak Thursday, translating east by afternoon...after which generally SKC conditions should round out the end of the period.

Outlook: Dry and VFR conditions linger through Friday. Precipitation and associated restrictions may return this weekend, but confidence remains low on timing.

GSP WATCHES/WARNINGS/ADVISORIES

GA...INCREASED FIRE DANGER from 10 AM EST this morning through this evening for GAZ010-017-018-026-028-029. NC...Wind Advisory until 7 PM EST this evening for NCZ033-049-050- 053. Cold Weather Advisory until 7 AM EST Friday for NCZ033-048>052- 058-059-062-063. Winter Weather Advisory until noon EST today for NCZ048>052- 058. SC...INCREASED FIRE DANGER from 10 AM EST this morning through this evening for SCZ008>014-019-101>109.