textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

The trend toward less snow accumulation over the NC mountains from Wednesday afternoon through Thursday morning continues with this forecast package.

The aviation discussion was updated this morning to concur with the latest 12z TAF issuance.

KEY MESSAGES

1. Low relative humidity and comparatively warm temperatures develop this afternoon, but weak winds during the afternoon limit fire danger. 2. Accumulating snowfall is expected Wednesday night through late morning Thursday over the mountains of NC, mainly along the TN border and at elevations above 3500 ft, which may result in some travel impacts. However, the expected amounts continue to trend downward. 3. Significantly colder temperatures will persist from Thursday through the weekend. Dangerously cold wind chills will be possible above 3500 ft Thursday night through Friday morning and again Sunday night and Monday morning.

DISCUSSION

Key message 1: Low relative humidity and comparatively warm temperatures develop this afternoon, but weak winds during the afternoon limit fire danger.

Elongated surface high pressure is presently analyzed over the Gulf coast, with a broad sfc ridge axis extending all the way into the Carolinas. The 00z observed soundings depicted a robust dry slot at around 850mb. Hi-res guidance generally suggests that southwesterly flow in the low-levels will advect a small amount of moisture out of the Gulf through Tuesday night, but that enough dry air should still remain in place this afternoon to support another round of low relative humidty. Guidance depicts a weak capping inversion atop this afternoon's boundary layer, but this shouldn't stop at least some dry air from mixing down to the surface...resulting in borderline or critical RH across much of the area. Temperatures this afternoon will be two categories warmer than the previous few days - with highs in the upper 50s, approaching 60 for much of the low terrain - which is another point toward better mixing and lower RHs, especially across the SC Upstate. Winds will be weak for most of the day, however, limiting fire danger, and as of the midnight forecast issuance, land managers have not coordinated any need for a Fire Danger Statement in any of our NC, SC, or GA zones on Tuesday.

Key message 2: Accumulating snowfall is expected Wednesday night through late morning Thursday over the mountains of NC, mainly along the TN border and at elevations above 3500 ft, which may result in some travel impacts. However, the expected amounts continue to trend downward.

These wintertime situations when you have to rely on the phasing of nrn and srn stream short waves to get the big snow are always tricky and fickle, and this one looks no different, as the latest model runs continue the trend away from anything significant Wednesday into Thursday. The problem appears to be the lead srn stream short wave moving across the Gulf Coast on Wednesday. This feature doesn't allow much in the way of moisture return ahead of the main deep mid/upper trof digging down over the Great Lakes/Midwest/MS Valley regions on Wednesday. The best forcing with this initial wave now looks as if it will skirt past the fcst area to the south and east early in the day on Wednesday before the better moisture can move up from the southwest. This is academic anyway, because the cold air will still be on the west side of the mtns during this time. The forcing associated with the digging upper trof axis should still reach the mtns in the afternoon on Wednesday and then move across the fcst area Wednesday night and early Thursday, but the forcing looks to be weaker than previous runs and mainly frontogenetic. Precip should move in from the west in the afternoon as an elevation-dependent rain-to-snow. Confidence is still relatively good that the cold air will arrive by Wednesday evening to change over the precip to snow down to the valleys. There's still enough going for it to expect a dusting to a half inch of snow over many of the mtn valleys mainly Wednesday evening. However, this continues to look like a more and more minor event as the forcing looks weaker and short-lived with the trof axis moving through quickly, and with the deeper moisture moving east of the mtns by 06Z Thursday. Although the remaining moisture will be shallow, the top of the moist layer will remain in the dendritic snow growth zone (below -12 C). Expect a quick transition to NW flow snow production by midnight, then continuing through daybreak along the TN border, and tapering off to flurries by midday. Note that the model blend doesn't handle NW Flow Snow situations very well with a bias toward ending too early, so precip chances were extended into Thursday morning which raises the QPF a bit. Even with taking that into account, the fcst amounts look unimpressive, with maybe some high elevation points near the TN border reaching Advisory criteria. Of note, however, will be the plummeting temperatures overnight, well down into the teens and 20s, which could result in some travel issues on slippery roads. Bottom line is that a Watch is not necessary, but we will monitor for a possible Advisory in some locations.

Key message 3: Significantly colder temperatures will persist from Thursday through the weekend. Dangerously cold wind chills will be possible above 3500 ft Thursday night through Friday morning and again Sunday night and Monday morning.

What should effectively be an arctic front should be east of the fcst area by daybreak Thursday with strong cold advection across the region. Temps will be held down 10-15 degrees below normal Thursday and Thursday night, meaning temps will not get out of the teens and 20s across the mountains for highs Thursday, and will bottom out in the single digits to lower teens over the mtns and teens east of the mtns early Friday morning. Altho the trend on the wind chill is up slightly because of less wind, it wouldn't take much of a fcst error in wind speed to push some of the high elevations down into the realm of the Cold Weather Advisory.

Some moderation is likely Friday and Saturday as the upper trof moves away and a progressive ridge moves through, but temps still stay down 5-10 degrees under average. There could be another brief shot of NW flow snow shower activity Friday night and Saturday with no significant accum indicated.

The guidance digs another upper trof down from the nrn Plains Saturday into Sunday which would drive a reinforcing cold front across the region at some point, dipping the temps back down to well below normal. Another possible dangerous wind chill situation could develop over the high elevations Sunday night and Monday morning as a result.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: VFR conditions expected to continue through the new 12z TAF period. Mostly calm winds across the area should gradually pick up after daybreak, with a steady 5-10kt winds developing by late morning. Sporadic gusts to ~15kts this afternoon. Generally expect SKC or FEW cirrus during the day, with cloud cover beginning to increase overnight from SW to NE. VFR ceilings above 10kft should gradually increase into the first part of Wednesday.

Outlook: Flight restrictions are likely to return Wednesday night into Thursday as a cold front and associated precipitation impact the region. VFR conditions should return over the weekend.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.