textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

No changes to extent of Frost Advisory or Freeze Warning this morning.

Rainfall amounts for the middle of the week system continue to trend slightly higher.

KEY MESSAGES

1. Widespread frost across the NC mountains and foothills this morning, as well as freezing temperatures in Avery County and at very high elevations of other mountain counties. 2. Temperatures trend warmer through Tuesday as skies remain mostly clear. However, cannot rule out very patchy frost in mountain valleys tonight into Monday morning. 3. Another cold front will bring rain to the area during the middle part of the upcoming week. The threat for severe weather with this system, while appearing low at this time, has started to trend upward in the new guidance.

DISCUSSION

Key message 1: Widespread frost across the NC mountains and foothills this morning, as well as freezing temperatures in Avery County and at very high elevations of other mountain counties.

Colder air is filtering into the region early this morning on light northwest winds aloft, associated with continental high pressure centered in the mid-Mississippi Valley. Skies are clear across the CWA except for some upslope clouds along the NC/TN border, which are on a diminishing trend and should not be a major inhibitor of radiational cooling. The gradient has already relaxed enough that decoupling has occurred in many rural areas. Confidence is high for frost occurrence within the existing Frost Advisory. Based on updated hourly temps and seeing a few sites have cooled more quickly than forecast, can't rule out some very patchy frost in the NC I-40 corridor outside the Advisory, but think it is too isolated to warrant an expansion, particularly at this late hour. Dewpoints do not look likely to fall much more from current readings near freezing in those areas, so cooling probably will stop before a widespread freeze occurs in the mountains, though a freeze has already occurred on a few ridgetop obs and a few valley locations could experience a light freeze. Freeze Warning remains for Avery County where the zone average is likely to bottom out below 32.

Key message 2: Temperatures trend warmer through Tuesday as skies remain mostly clear. However, cannot rule out very patchy frost in mountain valleys tonight into Monday morning.

Surface high pressure will migrate across the area through tonight, then remaining offshore through Tuesday. Aloft, heights rise gradually over this period. Overall, both min and max temps trend warmer each day, with a particularly noticeable 7-10 degree difference Sunday versus Monday. That said, winds will be light tonight into Monday morning under still mostly clear skies, supporting good radiational cooling and decoupling. Some particularly sheltered mountain valley locations could dip into the mid 30s, and it is not out of the question a few areas may see patchy frost again.

Key message 3: Another cold front will bring rain to the area during the middle part of the upcoming week. The threat for severe weather with this system, while appearing low at this time, has started to trend upward in the new guidance.

Next chance for rain comes again later in the week as a developing trough ejects over the eastern CONUS Wednesday into Thursday and brings a cold front through the area. For the CWA, this enhanced area of forcing increases the chance for showers and thunderstorms with a non-zero severe risk. Taking a look at the parameters, moisture starts to ramp up when southerly winds return to start the week and remain steady through Thursday. Latest long-term guidance spins up an area of low pressure associated with the trough and swings it into the southeast and CWA Wednesday night into Thursday. Model soundings show an uptick in instability during the overnight, which could help to fuel the severe risk for locations east of the mountains. Strong shear of 50-60kts looks to drive the system as well. Additionally, QPF response is higher, especially with any strong convection. At this time, rain totals for the 24-hr period from Wednesday to Thursday have a 40-60% chance to exceed 1.00", which is a decent signal in guidance for much needed rainfall. The higher probs are located in the western portion of the CWA, but if this trend continues, could be seeing some beneficial rainfall. As with any storm system this far out, a lot can change, including how deep the trough digs, timing of the better forcing with instability, and how much moisture actually returns. Given all this, confidence is increasing that a system should cross the area and bring rain with the potential for thunderstorms. With these parameters, cannot rule out a few severe storms with the primary threat being heavy rainfall and damaging winds. This can and will likely change as the event gets closer.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

At KAND early this morning: The rapid drying which occurred at other Upstate SC sites Saturday evening has not yet occurred at KAND as of 06z Sun, and fog has formed mainly along the nearby rivers. Periods of MVFR fog, or a possible brief bout of LIFR cigs, could occur in the first hr or two of the period. Mentioning only SCT002 with the vsby being the more likely impact. Drying should occur and end the threat of fog before daybreak.

Otherwise: VFR. Aside from a few relatively high based stratocumulus over the Appalachians this morning, SKC much of today. Light N to NW winds early this morning, where not calm. Winds strengthen a bit from the NW by 13-14z but turn SW by early afternoon, probably remaining SW'ly tonight if not going calm again. Some frost is possible at KAVL this morning.

Outlook: Expect dry/VFR conditions to linger through the first half of the work week. Restrictions are possible with next frontal system Wednesday night into Thursday.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Frost Advisory until 9 AM EDT this morning for GAZ010. NC...Frost Advisory until 9 AM EDT this morning for NCZ048>053-058- 059-062>065-501>506. Freeze Warning until 9 AM EDT this morning for NCZ033. SC...None.