textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No major changes to the forecast as confidence continues to increase for a storm system to affect the area this weekend but confidence on details and impacts are still low.

KEY MESSAGES

1. Dangerous wind chills below zero possible across the Northern NC Mountains through this morning. A Cold Weather Advisory will remain in effect through Noon today for Avery County and elevations above 3500' for Yancey and Mitchell Counties. 2. Very low relative humidity (12 to 25%) expected this afternoon across the mountain valleys and all of the foothills and Piedmont, possibly resulting in increased fire danger. 3. A weak cold front will cross the area Thursday with a slight chance of precipitation. 4. A potential winter storm system could affect the area this weekend but details remain highly uncertain.

DISCUSSION

Key message 1: Dangerous wind chills below zero possible across the Northern NC Mountains through this morning. A Cold Weather Advisory will remain in effect through Noon today for Avery County and elevations above 3500' for Yancey and Mitchell Counties.

Deep cyclonic flow aloft and a ~1035mb Canadian surface high spewing into the region from the west has allowed for a reinforcing shot of cold and very dry air to settle across the CWFA. The surface high will slide from the the Mid/Lower MS Valley into the TN Valley by daybreak. With lingering gusts tonight from continuous CAA, expect overnight lows to be 10-15+ degrees below normal across the mountains with sub-zero wind chills at the highest peaks and single digits across the rest of the mountains. As a result, a Cold Wind Advisory remains in effect for Avery County and above 3500' in Mitchell and Yancey Counties for through Noon today. Overnight lows will run 5-10 degrees below normal for locations outside of the mountains. Surface high sets up shop across the CWFA through today which will lead to mostly sunny skies with temperatures remaining 5- 10 degrees below normal in the mountains and up to 5 degrees below normal outside of the mountains.

Key message 2: Very low relative humidity (12 to 25%) expected this afternoon across the mountain valleys and all of the foothills and Piedmont, possibly resulting in increased fire danger.

A very cold and dry airmass will remain in place this afternoon, allowing dewpoints to tank into the single digits and even below zero during peak heating. Despite below normal temperatures, RH values will be below 25% across a good portion of the CWFA. A limiting factor to any fire weather concerns will be the lighter winds as the surface high sets up shop across the southern/central Appalachians. After a full day of drying yesterday, fuels have dried out enough to support a Fire Danger Statement for northeast Georgia, and will collaborate with North and South Carolina for a potential Statement today. Surface high remains in control tonight into Wednesday as it gradually shifts offshore in response to the next system. Expect temperatures to remain 4-8 degrees below normal for tonight and for afternoon highs on Wednesday.

Key message 3: A weak cold front will cross the area Thursday with a slight chance of precipitation.

By Wednesday night, a surface low develops over the Midwest and brings a weak cold front toward the CWA. At this point, guidance keeps the frontal boundary hung up across the mountains with a brief period of chance PoPs (50-60%), mainly concentrated in the far western NC mountains through Thursday morning. Temperatures are forecasted to be at or below freezing so snow is the primary precipitation type. However, model soundings near the TN/SC border show a large amount of subsidence aloft with shallow moisture in the boundary layer. Limited moisture in the snow growth zone, increases confidence that any precipitation that is able to develop would be snow, but at a minimum. The probability for snow amounts over 0.25 inch is less than 25%. So, overall, this first event looks uneventful and brief. The rest of Thursday and into Friday has a slight chance (15-30%) for a bit of precipitation outside the mountains. Warmer temps should keep it as rain, but again, amounts are minimal.

Key message 4: A potential winter storm system could affect the area this weekend but details remain highly uncertain.

The bigger story for the forecast continues to be the potential for a winter storm to sweep across a wide swath of the south, from the central CONUS to the Carolinas. What we know at this point is a signal in model guidance has pointed to a strong continental polar (cP) air mass spilling into the central portion of the U.S. and spreading eastward. Meanwhile, moisture appears to stretch over the south and southeast ahead of the frontal boundary. Somewhere along this frontal boundary, wintry precipitation will likely form. Now, this is where the uncertainty comes into play as there are many factors that will change and directly influence this system for our area. The primary factor being the location of the transition zone from snow to a wintry mix, involving sleet and ice. Current model guidance from the GFS has shifted the transition zone further north, bringing the potential for more of an ice event than snow. The EURO keeps the snow as the primary p-type. Many of the GEFS members show a mix of ice and snow. The main point is that yes, the model guidance is signaling for a potential winter storm stretching over the weekend. There is also the possibility for the area of high pressure to extend further south, which also could cutoff precipitation chances entirely. It's a low chance, but still in the realm if possibilities. But, despite these forecast tools, there is still much uncertainty to where the transition zone sets up, which directly impacts how much snow vs ice vs nothing there could be.

Another factor increasing confidence in this potential winter storm is the cold air that is expected to be in place. Unlike systems recently where there was a chance for snow, the environment lacked the cold air required for any wintry precip to develop. This is looking to not be the case for this potential event. Currently, the NBM has increased probabilities of accumulations that could bring about warnings for snow (40%) and ice (35%). So, overall, there is a strong signal for a possible winter storm that could impact the area over the weekend, but details on snow/ice/sleet remain unclear this far out.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: VFR conditions will prevail through the forecast period. A few high clouds will pass overhead this morning, but skies should remain mostly clear today and tonight. Winds will remain light with a west to northwesterly component outside of the mountains, even becoming light and variable at times, especially overnight tonight. KAVL will maintain a north-northwesterly wind through tonight, with some low-end gusts possible during the afternoon.

Outlook: Dry high pressure will settle over the region through midweek, maintaining VFR conditions across the area. A cold front may bring some light precipitation and associated restrictions, mainly to the mtns, Wednesday night into Thursday.

GSP WATCHES/WARNINGS/ADVISORIES

GA...INCREASED FIRE DANGER from 11 AM EST this morning through this evening for GAZ010-017-018-026-028-029. NC...Cold Weather Advisory until noon EST today for NCZ033-049-050. SC...None.