textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Latest trends have lowered on the threat of freezing rain for the northern mountains and adjacent foothills.

Rain chances have lowered for Tuesday.

KEY MESSAGES

1. Areas of fog, possibly dense, returns along and south of I-85 through sunrise this morning. Dry and warm today. 2. Cold air damming develops Monday and Tuesday, bringing much cooler and well below high temperatures as well as the potential for freezing rain across portions of the northern mountains and adjacent foothills Monday into Monday night. 3. Cold air damming replaced by much warmer conditions by Wednesday afternoon, allowing well above normal temperatures to return through the rest of the workweek. Rain chances should return by Thursday, lingering through the weekend.

DISCUSSION

Key message 1: Areas of fog, possibly dense, returns along and south of I-85 through sunrise this morning. Dry and warm today.

Good radiational cooling conditions will allow ground fog to form across mainly the lower Piedmont. Guidance continues to trend toward more coverage of fog, mainly along and south of I-85 where the ground remains moist. Areas of fog will be forecast in these areas, and some of this may become dense. A Dense Fog Advisory may be needed at some point. Lows will be 5 to 10 degrees above normal.

The warm air mass remains in place today, with an increasing downsloping westerly wind helping bump temps up to around 15 degrees above normal. Breezy conditions are expected in the mountains this aftn. A backdoor cold front will slip thru the area from the NE this evening, bringing winds around to northeasterly and producing increasing clouds. Temps will fall to near normal.

Key message 2: Cold air damming develops Monday and Tuesday, bringing much cooler and well below high temperatures as well as the potential for freezing rain across portions of the northern mountains and adjacent foothills Monday into Monday night.

Classical Cold Air Damming will be in place by Monday as a ~1040mb surface high sets up shop over the northeastern CONUS, leading to a drastic change in temperatures compared to Sunday as afternoon highs drop 20-25 degrees. The surface high quickly shifts offshore Monday night into Tuesday over the northwestern Atlantic as CAD takes on more of an in-situ wedge and residual cold pool as the synoptic support for wedge is cutoff and the flow begins to turn out of the south-southeast later Tuesday. Highs continue to trend warmer on Tuesday, but skeptical of how warm based on the presence of a residual cold pool by the afternoon. Highs on Tuesday remain slightly below normal as a result, leading to the consensus of a weaker damming scenario.

The southern jet stream continues to stretch from the Rockies through the Central Plains, and into the OH Valley and Mid-Atlantic on Monday as a series of shortwaves traverse within the mean flow. The initial shortwave arrives on Monday, bringing better chances for precipitation, especially across the North Carolina zones. With CAD in place, the typical coldest spots during CAD have the potential for freezing rain at the onset of precip thanks to wetbulb temperatures near freezing and another shot of light freezing rain Monday night once temperatures cool close to freezing. Latest guidance have backed off on freezing rain probabilities of >0.01" with LREF indicating only 15-25%, while HREF probabilities soar to 65-85% in the northern mountains and adjacent foothills in the northern Blue Ridge Escarpment. Needless to say, the threat for light accumulation is still present, but the current forecast isn't nearly as bull-ish compared to previous forecasts, which lowers confidence overall. The NAM continues to hold strong on freezing rain potential (which is likely influencing the HREF probabilities the most), while other high-res guidance are not as excited. Lingering PoPs are apparent for early Tuesday, but temperatures should be too warm to support any wintry p-types as temperatures will be too warm. Drier conditions will return by Tuesday afternoon.

Key message 3: Cold air damming replaced by much warmer conditions by Wednesday afternoon, allowing well above normal temperatures to return through the rest of the workweek. Rain chances should return by Thursday, lingering through the weekend.

Anomalously high thicknesses will be present as anything left of a residual cold pool will quickly be eroded by low-level WAA and daytime mixing, leading to much warmer temperatures by Wednesday. Upper ridging offshore the Southeast Coast will keep the area well above normal through the end of the workweek with continued southwesterly moisture advection from the Gulf of America. As a result, daily PoPs remain in the forecast starting Thursday, but likely in the form of WAA showers and isolated thunderstorms, especially with enhanced diurnal coverage. Wednesday remains mostly dry as that will be the transition day from the CAD setup into the WAA regime as it becomes fully established by Thursday into the upcoming weekend. Well above normal temperatures stick around through the end of the forecast period.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Fog has formed across the major valleys of the Piedmont, impacting KAND and KCLT as expected. This fog should burn off by late morning with full sun and a SW wind picking up across the area. At KAVL, winds will remain light and VRB to calm well into the morning and then pick up from the N/NW by late morning. They will strengthen during the afternoon with low- end gusts likely. A backdoor cold front will slip in from the NE this evening, bringing winds around from SW to N/NE across the Piedmont sites. MVFR to IFR cigs are expected to develop and NE winds increase with low-end gusts possible as cold air damming strengthens overnight tonight.

Outlook: A strong cold air damming setup is expected Monday into Tuesday, which may bring showers and associated restrictions. VFR conditions will likely return on Wednesday, while another cold front may bring showers and associated restrictions Thursday into Friday.

GSP WATCHES/WARNINGS/ADVISORIES

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