textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Fire weather concerns extend through Tuesday as near-critical RH is expected.

KEY MESSAGES

1. A brief return of winter weather with very gusty NW winds, accumulating snow for portions of the mountains, and below normal temperatures Sunday through Tuesday. 2. Near-critical fire weather conditions are expected each afternoon through Tuesday. 3. Temperatures warm throughout the week, with a possible cold front increasing rain chances across the area early Thursday into Friday.

DISCUSSION

Key message 1: A brief return of winter weather with very gusty NW winds, accumulating snow for portions of the mountains, and below normal temperatures Sunday through Tuesday.

NW flow significantly ramps up across the fcst area today, as a coastal low rapidly deepens over the Mid-Atlantic coastal waters. A tight pressure gradient and strong low-level CAA will support very gusty winds across our area by the early afternoon and con-tinuing into tonight. Peak gusts of 30 to 40 mph will be possible across most of the foothills and Piedmont this afternoon, with peak gusts into Advisory-criteria range of 45 to 60 mph possible above 3500 ft heading into tonight.

There could be a brief lull in snow shower activity over the mtns later this morning thru about midday, but moisture should increase again this afternoon with snow showers ramping up along the NC/TN border. Snow accums will be highly elevation-dependent for the first few hrs of the event due to warmer valley temps. There may also be a band of rain developing within a deformation zone associated with the coastal low that could set up just east of I-77. Any QPF from this would be minimal. Otherwise, skies clear out and temps remain below normal today.

The NW flow snow machine will be cranking up this evening, as a strong mid-level speed max digs from the Corn Belt to the SC Coast by Monday morning. In response, the depth of post-frontal moisture covering the TN Valley and much of the southern Appalachians will increase to near 700 mb, with temperatures at the top of the moist layer cooling to -16 to -20C. This will allow for a protracted period (at least 24 to 30 hrs) of optimized dendritic ice crystal growth and above-average snow to liquid ratios, all within persistent NW upslope flow. Solid Winter Wx Advisory criteria snowfall is likely from the higher elevations of the Smokies northeast across the mtns and valleys of the other NC counties bordering TN. Current fcst storm totals are generally in the 1 to 4 inch range in the valleys north of I-40, and about twice that range expected above 3500 feet. Especially favored upslope/high elevation areas along the state line will likely see a foot or more. Thus, a Winter Storm Warning is in effect for Avery County as well as the high elevations of Mitchell County. A Winter Weather Advisory is in effect for Yancey, Madison, Haywood, and the high elevations of Swain.

With very windy and cold advection continuing thru Sunday night, widespread wind chill values in the single digits are expected across the NC mtns, with sub-zero readings likely above roughly 4000 feet. At present, those areas don't quite reach Cold Weather Advisory criteria, but if they trend any colder an Advisory may be needed for the higher elevations and/or the northern NC mtns. Snow showers will finally taper off Monday night, with dry and below normal temperatures expected thru Tuesday. Mountain wind chill values will be low again Monday night, but with winds steadily weakening wind chills shouldn't be as cold as Sunday night/early Monday.

Key message 2: Near-critical fire weather conditions are expected each afternoon through Tuesday.

With minimal rain accumulations ahead of the cold front, fire weather concerns once again return Sunday and Monday for locations east of the mountains. Near critical RH of 25-30% is expected today and Monday afternoons as much drier air mixes in from the NW. Tuesday, even lower RH of upper teens to 25% becomes more widespread. Additionally, very strong gusty winds out of the NW of 25-35 mph on Sunday and 20-30 mph for Monday pose another concern for any burning. Though higher RH is expected for the mountains, sustained winds of 15-25 mph, with gusts of 35 to 50 mph are forecasted over the mountains. Outside of some NW flow snow at the TN/NC border, no precipitation is forecasted for locations east of the mountains during this time frame. Though the overall fire danger remains elevated, colder temperatures could be a limiting factor.

Key message 3: Temperatures warm throughout the week, with a possible cold front increasing rain chances across the area early Thursday into Friday.

High pressure settles in over the southeast by midweek, suppressing rain chances and increasing temperatures. Flow aloft becomes flatter before an amplifying trough develops again over the eastern portion of the CONUS. Meanwhile, guidance depicts a surface low forming and ejecting northeast, dragging a potential cold front across the area toward the end of the week. Ahead of the next potential shot for precipitation, surface winds turn S/SW and increase moisture advection into the area by midweek. Guidance at this time pushes a cold front through the area sometime Thursday into Friday, increasing precipitation chances. At this time, the probability of receiving at least 1" of rain is 40-50%, mainly for the mountains with probs decreasing elsewhere. Additionally, guidance has a 15-20% chance of at least 100 J/kg of sbCAPE east of the mountains, meaning there is a low chance for enough instability being present to produce convective concerns. However, this is very much subject to change as the system develops. Confidence is low this far out as how this system evolves is dependent on how far the high pressure retreats off shore, the amount of moisture returns ahead of the front, and where the low pressure system develops in reference to the CWA. Will continue to monitor.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: The immediate concern will be the patches of low clouds noted over northern SC and northeast GA and how they might affect KCLT and KAND in the next few hours. The initial development of this ceiling restriction to the south of KCLT has pushed off more to the east over the past hour or two, so a change to MVFR was not started until 08Z when some light precip should also begin. Note that confidence in the precip is not great, so the vis was kept no lower than 6 sm. KAND is a bit more of a problem because of the patches of IFR and LIFR about the region, but for now the TEMPO was kept MVFR. All other terminals also get a TEMPO for the light rain in the pre-dawn hours, but stay VFR. Once the precip moves out, the wind should shift to NW and VFR conditions are expected for the rest of the period at most terminals. The exception will be KAVL beginning late Sunday evening, with the possibility that snow showers will blow up the French Broad Valley and reach the terminal at some point. This will be handled with a PROB30.

Outlook: Gusty NW winds will persist across the area through Monday night with VFR conditions expected, except for possible low clouds and a few occasional snow showers at KAVL. Quiet weather returns to region by Tuesday.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Winter Storm Warning until midnight EST Monday night for NCZ033-050. Winter Weather Advisory until midnight EST Monday night for NCZ048>052. SC...None.