textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated for the 12Z aviation discussion.

KEY MESSAGES

1. The threat of patchy frost remains across the mountains tonight, and possibly east across the I-40 corridor Wednesday night and Thursday night. Protection of sensitive vegetation may be required in some locations. 2. Fire weather will gradually become more of a concern through mid-week as a dry air mass spreads across the region. Outdoor burning could become dangerous in places this afternoon and across the whole region by Wednesday afternoon due to low relative humidity values, gusty winds, and dry fuels.

DISCUSSION

Key message 1: The threat of patchy frost remains across the mountains tonight, and possibly east across the I-40 corridor Wednesday night and Thursday night. Protection of sensitive vegetation may be required in some locations.

A reinforcing dry cold front is still expected to drop southward across the forecast area early in the day today as a large continental sfc high in the nrn stream moves across the Great Lakes. Temps are expected to rebound back to normal this afternoon, but then fall off tonight to values about five degrees below normal. We've been watching the frost/freeze potential for tonight for several days, and although the fcst min temps tonight have been nudged downward a degree or two, the end result hasn't changed substantially. It would appear that we should retain just enough of a sfc wind and the air mass dries out so much overnight that frost will be patchy at best in sheltered valleys. As it stands now, the potential is too limited to warrant an Advisory. If there's a place that might stand the best chance of frost, it would be the Little TN River Valley where moisture remains the highest for the longest overnight. Note that temps above 5000 feet are likely to get below freezing, and parts of the northern mountains as well, particularly Avery County. However, we don't normally issue freeze warnings for elevations above 5000 feet, and the northern mountain zones are not yet active for frost/freeze this season. So, for the northern mountains, we will let the forecast speak for itself.

An inverted sfc ridge axis will nose southward along the east slopes of the Appalachians Wednesday, bringing cooler and drier air to the region. The air will be so dry that despite light wind and temps falling into the lower to mid 30s, the mountain valleys will likely have limited frost. A little better potential for areas of frost will be along the I-40 corridor of the northern NC Foothills and NW Piedmont. It's still not an ideal setup, as winds may struggle to decouple due to lingering pressure gradient between the high to our north and a developing low pressure system offshore. The air mass modifies somewhat Thursday, with temps a few deg warmer than Wednesday. That includes Thursday night, where temps will fall into upper 30s to lower 40s with good radiational cooling. Could see some patchy frost in the mountain valleys. No frost/freeze concerns after that, as an upper ridge builds over the Southeast and brings temps well above normal by the weekend.

Key message 2: Fire weather will gradually become more of a concern through mid-week as a dry air mass spreads across the region. Outdoor burning could become dangerous in places this afternoon and across the whole region by Wednesday afternoon due to low relative humidity values, gusty winds, and dry fuels.

The reinforcing high that builds in from the north later today and Wednesday will keep a dry air mass across the region through mid-week. In these situations, the model blend tends to not handle the amount of vertical mixing of dry air that ultimately occurs, and thus it does not reflect how low the RH often falls. Will undercut the model dewpoint/RH this afternoon and also Wednesday afternoon to account for this tendency. That downward adjustment brings the forecast RH down into the mid/upper 20s this afternoon, suggesting that some coordination with land management agencies will be in order because of uncertainty regarding fuel moisture across the region. Until we know more about how much the fuels dried out yesterday afternoon, we will hold off with any Fire Danger Statements until the day shift can get a better handle on that situation. Fire weather concerns look the worst on Wednesday, as drier air works in from the northeast and wind gusts of 15 to 20 mph will be possible. The winds should weaken in the afternoon by the time the RH dips below 25% in most of NC and the eastern SC Piedmont. But the Upstate and NE GA could see some overlap of those gusts with the low RH. Winds should be lighter Thursday and beyond, as high pressure settles over the area. However, temperatures warming to well above normal will result in RH values dipping below 25% in spots each aftn thru the weekend. Return flow will finally start to bring in some increasing dewpts and higher min RH values early next week.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: VFR at all terminals through the period, with only high cloudiness that moves off to the east during the middle part of the day. That leaves us with a wind forecast, which should be light N to NE once a reinforcing cold front drops south across the region by midday. The wind will diminish with sunset and veer slightly more to the NE. The threat of valley fog or frost at KAVL is too low to mention. Wednesday will have some low-end gust potential once the morning inversion is mixed out.

Outlook: VFR thru the late part of the week courtesy of high pressure. A developing easterly low level flow could bring a return of low clouds Thursday night, but more likely late in the weekend.

GSP WATCHES/WARNINGS/ADVISORIES

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