textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Red Flag Warning issued for the mountains of northeast Georgia for Sunday. Increased Fire Danger statement issued for the rest of northeast Georgia for Sunday.

KEY MESSAGES

1. Gusty northwest winds develop behind a cold front late tonight into Sunday morning, with the strongest winds in the mountains. Much cooler air follows, ending the recent stretch of summer-like warmth. 2. Temperatures return to near or below normal Sunday night through Tuesday. Some frost is possible over the mountains and/or northwest North Carolina Piedmont Sunday night and especially Monday night, so precautions might be needed to protect sensitive vegetation. Warm weather returns Wednesday into next weekend. 3. Rain chances remain very low from Sunday afternoon through Friday, with relative humidity becoming very low each afternoon. These conditions will exacerbate Fire Weather concerns and drought conditions.

DISCUSSION

Key message 1: Gusty northwest winds develop behind a cold front late tonight into Sunday morning, with the strongest winds in the mountains. Much cooler air follows, ending the recent stretch of summer-like warmth.

The main feature of interest in the near term is a cold front currently positioned across the Ohio, Tennessee, and Lower Mississippi Valleys. The front will approach quickly tonight, likely reforming east of the mountains as it moves into the area. A weak surface low is expected to develop along the boundary near the northern Blue Ridge late this evening and track northeast toward the Mid-Atlantic coast by early Sunday morning.

Most guidance, including the 12Z HREF and REFS, keeps the bulk of shower activity west of the mountains through midnight. A low probability exists for an isolated shower or storm near sunset across the central and northern mountains along a pre-frontal trough, but forcing is weak and instability is limited. Any precipitation that does develop overnight will weaken as it moves east into the foothills and Piedmont early Sunday morning, resulting in minimal rainfall and little relief from ongoing dry conditions.

The primary impact with this system will be the surge of gusty northwest winds and the arrival of much cooler air behind the front. Strong pressure rises on the order of 5 to 9 mb in six hours are expected immediately behind the boundary between roughly 09Z and 15Z Sunday. This will support widespread gusty winds, especially across the mountains where gusts of 30 to 40 mph are expected, with 40 to 50 mph likely along the highest ridges. A Wind Advisory is in effect for elevations above 3500 ft in the western North Carolina mountains from 6 AM to 2 PM Sunday. Probabilistic guidance from the NBM and REFS supports the highest likelihood of advisory level gusts along the Blue Ridge Escarpment during the morning hours. Outside of the mountains, gusts of 20 to 35 mph are expected.

Key message 2: Temperatures return to near or below normal Sunday night through Tuesday. Some frost is possible over the mountains and/or northwest North Carolina Piedmont Sunday night and especially Monday night, so precautions might be needed to protect sensitive vegetation. Warm weather returns Wednesday into next weekend.

High pressure will continue to build in from the west Sunday night into Monday. CAA and weakening winds should allow temperatures to fall several degrees below climo, reaching the mid to lower 30s in portions of the mountains and northern foothills, so frost may be able to form in areas sheltered from the winds, though dry dewpoints could be a limiting factor. On Monday, CAA will be reinforced as a stronger area of continental high pressure pushes across the Southern Appalachians from the Ohio Valley, beneath the confluent area upstream of the departing upper trough. This should bring a dry backdoor cold front into the CWA. Exceptionally deep mixing is shown on prog soundings for Monday afternoon, suggesting the redevelopment of gusty winds. Gust potential is not quite as high as Sunday, though some ridgetops could approach Wind Advisory criteria. CAA will continue Monday night as the backdoor front pushes southward. Enough wind is expected to linger over the southern half of the area to offset cooling, such that those areas should see overnight mins similar to those the previous night. The mountains and northwest NC Piedmont (generally near/north of I-40) are more likely to decouple and thus trend colder, with more widespread temps in the mid to lower 30s. Dry air again will be a limiting factor for frost east of the mountains, so the mountain valleys would appear to be most at risk, based on the current forecast. Max temps Tuesday however will rebound several degrees in the mountain valleys as SW flow redevelops by afternoon; other areas will remain a few degrees below normal thru the afternoon.

With heights rising aloft and conditions supporting airmass modification, temps trend warmer Tue night onward. Maxes Wed-Fri end up 7-10 degrees above normal under continued mostly sunny skies. The next front approaches from the west Friday night. Global ensembles are in relatively good agreement on timing, though probabilities for enough rain to improve fire weather or drought are very small. Small PoPs do however return to the western zones.

Key message 3: Rain chances remain very low from Sunday afternoon through Friday, with relative humidity becoming very low each afternoon. These conditions will exacerbate Fire Weather concerns and drought conditions.

With high pressure being the dominant feature across the Southeast from Sunday through late next week, precip chances over our area will remain near zero. The predominantly northwest flow occurring Sunday and Monday will help advect dry continental air into the region, and also promote development of gusty winds during those afternoons, enhancing fire danger. Any improvement in fuel moisture associated with rainfall early Sunday morning is likely to be very short-lived.

Gusty northwest winds will arrive behind the front before daybreak Sunday morning and continue throughout the day. While the strongest winds are expected to occur in the morning, before the really dry air moves in during the afternoon, there will be enough of an overlap for winds and RH values to potentially meet red flag criteria, especially in NE GA on Sunday. A Red Flag Warning was issued for Rabun and Habersham Counties, where land management officials are especially concerned about the dry fuels in the higher elevations where spring green-up is not yet as prominent as in the the piedmont. An Increased Fire Danger statement was issued for the rest of northeast Georgia as well.

Though winds will be generally less of a factor on the subsequent afternoons, as temperatures trend back well above normal and RH continues to dip below 30% each afternoon, fuel moisture is likely to remain on a downward trend, even though spring green-up will continue. Hence fire danger is likely to remain near the top of our concerns through the rest of the forecast period (Friday). A frontal system may reach the area from the west next weekend, though confidence is low that any meaningful rainfall will result.

AVIATION /01Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Increasing VFR stratocu expected ahead of a cold front overnight. Scattered showers also move in ahead of the front, so retained the PROB30s for SHRA. KAVL has the highest probability of MVFR restrictions with the SHRA. Low end gusty SW wind continues ahead of the front, then shifts to the NW behind the front increasing in speed and becoming very gusty. N to NW gusty winds continue through the day Sunday but do diminish somewhat by late afternoon. Low clouds scatter out quickly behind the front as well.

Outlook: Winds diminish Sunday night with dry and VFR conditions continuing through most of next week.

GSP WATCHES/WARNINGS/ADVISORIES

GA...Red Flag Warning from noon to 8 PM EDT Sunday for GAZ010-017. INCREASED FIRE DANGER from Sunday afternoon through Sunday evening for GAZ018-026-028-029. NC...Wind Advisory from 6 AM to 2 PM EDT Sunday for NCZ033-049-050- 052-053-059-062>064-505. SC...None.