textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated discussion for 12z TAF issuance. No other significant changes at this time.

KEY MESSAGES

1. Likely remaining dry today, but with rain chances returning tonight and at least the first half of the day Saturday. Below normal high temperatures can be expected each afternoon, especially on Saturday. 2. Cooler temperatures for the weekend with frost possible for portions of the mountains and freezing temps at the higher elevations Saturday night. Dry weather and warmer temperatures expected through Tuesday. 3. Another cold front may bring rain chances to the area by midweek, but forecast confidence remains low.

DISCUSSION

Key message 1: Likely remaining dry today, but with rain chances returning tonight and at least the first half of the day Saturday. Below normal high temperatures can be expected each afternoon, especially on Saturday.

A reinforcing cold front has brought a weak but dry area of high pressure over the region this morning, characterized by relatively low dewpoints in the 30s and lower 40s. A more well defined, quasistationary front will remain near the Gulf Coast. The high will move offshore over the course of the day, and low level flow veers to southerly in advance of a digging trough to our west. A shallow layer of moist upglide will develop over the dry sfc airmass, maintaining some stratus over at least the southern portion of the CWA; patchy cirrus also will be seen. Some of the CAMs develop very light showers near/south of I-85 and this is reflected in a slight-chance PoP in those areas. Temperatures will be held mainly into the upper 60s and lower 70s by the cool airmass and to some degree by cloud cover

A weak southern-stream shortwave is shown to ride along the Gulf Coast boundary by this evening, activating it as a warm front and bringing moist upglide northward toward our area Friday night. This appears likely to result in a deep saturated layer, and dynamic lift from the shortwave and right-rear quad of jet streak does look to arrive early Saturday. For those reasons it is reasonable to reflect a 60-90% PoP with peak chances during Saturday morning, which then taper off from midday onward. The highest values should be along our southeast border nearest the front. However, the dry surface layer may still be present that precip would have to overcome. Furthermore, guidance members continue to disagree as to the precise location of the boundary, which lowers confidence in amounts and may be responsible for washing out what could be a fairly sharp gradient. It appears likely the best frontogenetic QPF response will be to our southeast, and still not much chance of seeing more than 1" total even south of I-85. NBM is running a little on the high side of the ECMWF and GFS ensembles, which together suggest little chance of exceeding even 0.5" in all but our immediate southeastern border. As noted previously, vertical profiles are generally unfavorable for convection and stratiform mode will prevail, so severe threat looks minimal assuming the boundary is to our southeast.

Though Saturday's pattern does not feature a sfc high indicative of any flavor of cold-air damming, the dry sfc layer at the onset of precip results in numerous models showing diabatic cooling that impacts temp trends Friday night and Saturday. There thus is potential for a rapid drop on the order of 5-10 degrees early Saturday morning (resulting in lows slightly below normal), which may not budge much while precip is ongoing. The lack of true CAD is reflected in even the cooler models depicting warming Saturday afternoon as precip ends; maxes nonetheless now are forecast to be around 15 below normal. Some bust potential exists for min/max temps Saturday, mainly in that if precip is insufficient to induce the diabatic effect, temps may prove appreciably warmer.

Key message 2: Cooler temperatures for the weekend with frost possible for portions of the mountains and freezing temps at the higher elevations Saturday night. Dry weather and warmer temperatures expected through Tuesday.

As the coastal low departs, high pressure from the north filters into the region, bringing much cooler and drier air. With this, a chance for a brief frost in the mountain valleys and foothills as temperatures dip to near freezing Saturday night. Current guidance shows the highest probabilities (50-70%) for temperatures dipping below freezing are at the highest elevations. How these temps trend will depend on how quickly the low departs, rainfall diminishes and skies clear out. Either way, not expecting much in the way of impacts except for any plants that are outside and sensitive to the cold. Another chilly night for Sunday night, however temps are trending warmer and while frost could occur in isolated locations in the mountains, confidence is much lower. A warming trend occurs to start the week as the air mass modifies and winds turn southerly. Dry weather through midweek.

Key message 3: Another cold front may bring rain chances to the area by midweek, but forecast confidence remains low.

High pressure keeps the weather quiet through most of the week. Model guidance hints at another possibility for a frontal boundary to cross the area towards midweek, but confidence is very low at this point. The general synoptic pattern looks to evolve a trough that swings southward and supports the potential for rain chances. Depending on how the ridge out west progresses throughout the week will either enhance or weaken the chances for a FROPA. Meanwhile at the surface, the high pressure slides offshore and depending on where it positions, could increase moisture advection into the region ahead of the front. Still too early to pinpoint details, but it's something to continue to watch.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

At KCLT and elsewhere: VFR through this evening with cirrus at FL200-250, but also a SCT-BKN layer of stratus at FL050-080 this morning. Winds start the morning mainly NE, except for KAVL which will be initially calm ut should come up from the SE. Winds veer to light southerly over the first few hours of the day elsewhere, going VRB or NE again after sunset. Developing frontal zone is likely to bring -RA up from the south tonight, possibly as soon as 03z over KGSP/KGMU/KAVL but more likely not until 07z or later. Dry surface layer at onset, and initially light precip rates, makes restrictions unlikely overall, but with heavier RA near the end of the period, MVFR cig/vsby is possible and reflected with PROB30s where the chance is sufficiently high.

Outlook: -RA tapering off Saturday afternoon. Dry and VFR conditions return by Sunday and linger through at least Tuesday.

GSP WATCHES/WARNINGS/ADVISORIES

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