textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Slight increase in rain chances and precip amounts tonight through Sunday.

Confidence in accumulating northwest flow snow showers is improving, warranting increases in PoPs and accumulation forecasts over the NC mountains Sunday through Monday.

KEY MESSAGES

1. Gusty winds and low relative humidity this afternoon. Winds will become light this evening. 2. Two rounds of precipitation chances over the weekend with generally light amounts and minor to no impacts expected. 3. A brief return of winter with very gusty NW winds, accumulating snow for portions of the mountains and below normal temperatures expected Sunday through Tuesday. 4. Temperatures return to near-to-above normal levels during the latter half of next week, with a cold front bringing showers and perhaps some thunderstorms next Friday.

DISCUSSION

Key message 1: Gusty winds and low relative humidity this afternoon. Winds will become light this evening.

A cold front has pushed thru the area, ushering in much drier air to the forecast area this afternoon, but with limited CAA. Temps are spiking into the mid to upper 70s east of the mountains (which should break record highs for GSP and CLT). Gusty winds will linger thru the aftn, with gusts of 50-60+ mph on the highest peaks. But sub-advisory-level of mainly 20-30 mph across the lower valleys and the Piedmont. RH values are already falling below 25% in the immediate lee of some of the Blue Ridge, and winds will remain gusty. This would normally produce an increased fire danger. Fortunately, most areas received a wetting rain to limit wildfire potential. Winds will weaken this evening and RH will recover. Lows will be 15-20 deg above normal under increasing clouds.

Key message 2: Two rounds of precipitation chances over the weekend with generally light amounts and minor to no impacts expected.

Quasi-zonal flow aloft with a shortwave digging into the Midwest will cause a frontal boundary to stall out across the Southeast. A surface wave develops along the boundary over the central Gulf states tonight, which will back the low-level flow out of the SW and advect moisture atop the frontal zone. Guidance has trended wetter with this wave late tonight thru Saturday morning, solid chance PoPs across the area, highest in the south. The high-res models agree on fairly decent coverage of light to moderate showers breaking out across the region overnight. With temps so warm, no wintry precip types are expected. QPF should be light, due to weak forcing, dry sub-cloud layer to overcome, and limited instability. The wave is expected to quickly track east along the boundary and start deepening as a coastal low near the Outer Banks during the day SAturday. This should result in showers tapering off/exiting east by early aftn. There should also be some clearing skies, but clouds may linger across the Piedmont along and southeast of I-85. Highs should not be nearly as warm as the last couple days, but still 5-10 deg above normal.

A very strong speed max is expected to quickly dig from the northern Great Plains into the TN Valley Saturday through early Sunday, resulting in cyclogenesis across the Southeast coastal states. As a result, precip is expected to redevelop across at least a portion of the forecast area Saturday night into early Sunday, with PoPs returning to likely during this time. With the stronger forcing expected to develop south=>east of the area, rainfall amounts are again expected to be tame with this short-duration event... generally in the 0.10-0.33" range. Temps will remain above normal Sat night, but much cooler than in previous days as cold advection becomes established on the west side of a deepening coastal cyclone by Sunday morning.

Key message 3: A brief return of winter with very gusty NW winds, accumulating snow for portions of the mountains and below normal temperatures expected Sunday through Tuesday.

Low pressure will bomb off the NC and Mid-Atlantic Coast on Sunday, with strong NW low-level flow/cold advection overspreading our forecast area throughout the day, resulting in very gusty surface winds. At least Wind Advisory conditions appear likely across the eastern slope of the Blue Ridge throughout Sunday into Sunday night, as a mountain wave pattern matures coincident with deepening of the northwest flow. Even outside the mountains, gusts of 25-35 mph appear likely into the Piedmont.

Meanwhile, precip along the back edge of the synoptic forcing associated with cyclogenesis is expected to change to snow early Sunday as freezing levels plummet across the mountains. Synoptically-forced precip will quickly transition to northwest flow snow showers, as NW flow interacts with lingering low level moisture across the TN Valley and western slopes of the Appalachians. Ingredients for accumulating snow showers will be somewhat marginal Sunday morning, but will improve significantly during the afternoon and evening as the moist layer deepens in response to a strong mid-level speed max digging west of the Appalachians late Sunday into Sunday night. Temperatures at the top of the moist layer cool to -15C and below...suggesting efficient dendritic ice crystal growth and high snow:liquid ratios. This situation lingers into early Monday before ingredients gradually become less favorable for accumulating snow showers throughout the day. Both the setup and the duration of this event are indicative of at least a solid Winter Weather Advisory-level event, with widespread 1-4" snowfall amounts possible across the lower elevations...and 6+ inches in the higher elevations along the TN/NC border...with a chance of widespread Winter Storm Warning criteria above 3500'. We don't have the confidence for Watch consideration at this point...especially since we are likely more than 48 hours away from possibly reaching criteria, but this will certainly need to be entertained over the next 12-24 hours.

Otherwise, temps will return to below-normal levels Sunday through Tuesday...with the departure from climo being in the 10-15 degree range Mon and Mon night. Lingering very gusty winds will likely yield wind chills in the single digits across much of the mountains Sunday night, with some sub-zero readings likely above 4000 feet or so. Wind chills will be slightly higher Mon night, but still in the single digits across much of the high terrain.

Key message 4: Temperatures return to near-to-above normal levels during the latter half of next week, with a cold front bringing showers and perhaps some thunderstorms next Friday.

A deep upper trough will progress quickly away from the East Coast during the first half of next week. With rising heights, the late winter sun steadily modifying the air mass, and a return of SW low level flow, temperatures are forecast to warm slightly above normal by Wed and more or less linger within a couple of degrees of normal through the end of the period. Once the northwest flow snow showers taper off early Tuesday, conditions are expected to be dry until a short wave trough ejects into the East from the Central Plains Thu/Thu night. Associated front and precip band is likely to impact the forecast area late Wed night through Thursday. The pre-frontal warm sector is not expected to be especially warm or humid, but can't rule out some isolated thunderstorms within the frontal band.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: Winds are expected to weaken and lose their gusts this evening as the sfc pressure gradient relaxes. Increasing mid and high clouds overnight, with sct to numerous light showers expected across the fcst area Saturday morning. Guidance continues to keep conditions mostly VFR despite the precip, but there may be brief periods of MVFR visby and/or cigs thru the morning. Precip should move the east of our area by 17 or 18z Saturday, however another round will approach from the SW just after 00z Sunday. After the winds calm down this evening, they should remain light (ie, 5 kts or less) thru the rest of the 00z taf period. They will likely be VRB at times, but should favor a S to SE direction Saturday aftn/evening. At KAVL, winds will remain light and favor a NWLY direction overnight and thru the morning, and a more NLY direction tomorrow aftn/evening.

Outlook: Another round of rain chances and their associated restrictions returns Sat night thru Sunday morning. Gusty winds return Sunday thru Monday night, with any restrictions likely clearing out by Sunday afternoon for all but KAVL where low clouds may linger.

GSP WATCHES/WARNINGS/ADVISORIES

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