textproduct: Greenville-Spartanburg

SYNOPSIS

High pressure persists through late Thursday, resulting in dry and chilly conditions. Precipitation returns Thursday night as temperatures grow even colder, resulting in a wintry mix across the NC mountains and Foothills. Precipitation chances and below-normal temperatures linger through the first part of next week. Temperatures trend back toward normal by mid-week.

NEAR TERM /THROUGH 1 PM FRIDAY/

As of 119 AM EST Thursday...

Key Messages:

1) Quiet but chilly weather continues through Thursday evening.

2) Wintry precip expected across the NC mountains and even parts of the NC Foothills Thursday night into Friday morning, with a mix of snow and freezing rain expected in the mountains, and mainly snow expected in the Foothills.

3) Winter Weather Advisory, valid beginning 7PM Friday, has been issued for most of the NC mountains.

A quiet night but cold night is underway across the forecast area, as weakening high pressure remains in place across the Southeast. Patches of cirrus continue to stream across the region, but conditions are otherwise dry and clear. Models briefly trended toward dense fog developing across the Upstate overnight, but have trended back away from that. Dewpoint depressions of 1-3 degrees are being reported south of NC-SC state line, however, so at least patchy fog is still expected. Morning low temps should drop into the 30s across most NC zones, but may hover around 30 for much of the Upstate.

A quiet day is in store for Thursday, as high pressure presently centered over the northern Great Plains migrates eastward, centering itself over New England by late this evening. By this time, it'll be perfectly positioned to induce classical cold-air damming...providing much better support for a deep, synoptically-enhanced surface wedge than the previous few systems. Deep layer SW flow aloft, meanwhile, will intensify through the day, with a lobe of southern stream energy pushing out of the lower Mississippi Valley by tonight. Strong isentropic ascent looks to begin in earnest as early as 00z Friday, resulting a rapid influx of low-level moisture within a baroclinic zone draped across the Southeast. So, expect weather to become increasingly active after sunset.

As a result, a warm nose originating in Alabama and north Georgia will advect into the Carolinas, arriving first in the southwest mountains. Here, precipitation will begin as rainfall, and by the time the temperature drops below freezing, the warm nose will already be in place, and so precipitation will change over directly from rain to freezing rain. Farther north, across the northern Blue Ridge and NC Blue Ridge Escarpment, temps are likely to cool down enough to support some snow before the warm nose arrives, so precipitation will begin as rain or a rain/snow mix, then transition to freezing rain after midnight. Farther east, much of the NC Foothills and even some parts of the northern Piedmont will not see the arrival of the warm nose until near or after daybreak...resulting in nearly an all-snow forecast for these zones. Thus...have hoisted a high-elevation Winter Weather Advisory for most of the southwest mountains, valid from 7PM this evening until 10AM Friday; for the northern Blue Ridge, issued a zone-wide Winter Weather Advisory from 7PM tonight through 1PM Friday. For the Foothills, snow accumulations fall below an inch, too little to warrant an advisory; but, will still highlight these zones in the HWO, and may eventually issue an SPS as confidence grows closer to the event.

Expect precipitation to continue into the first part of Friday as CAD remains entrenched across the area, but it change over to rain everywhere by late morning. As the z500 forcing and low-level isentropic ascent weaken and pivot eastward, some measure of drying can be expected across the NC mountains by the end of the period at 18z Friday.

SHORT TERM /1 PM FRIDAY THROUGH SATURDAY NIGHT/

As of 1215 AM Thu:

Key message 1: Precip chances diminish Friday afternoon from northwest to southeast, and remaining chances Fri night are for only rain.

Dynamic lift gradually weaken through the latter half of the day Friday with core of 250mb jet streak pushing off the Atlantic coast, and the trailing, weaker streak being in an unfavorable position to support lift over our area. 850mb winds veer such that advection is more neutral or even cold-advective. Global deterministic models and CAMs support little to no PoP after 18z, but the NAM and some ensemble members keep precip going longer. Drying at 700mb implies decreasing ice nuclei and lesser rates after that time, regardless. CAD should be able to weaken slightly and temps warm enough to end any wintry p-type threat east of the mountains. Still looking at an exceptionally cold day though not as cold as previously forecast; afternoon maxes mainly in the lower 40s aside from high elevations and our northern couple tiers of NC Piedmont counties. Parts of the northern NC mountains and Escarpment probably will remain wedged in and below freezing much of the afternoon. Profiles aloft lack ice nuclei and suggest only freezing rain for those areas after 18z, amounts taper off and chances fall unmentionably low overnight.

Most models still largely depict the stalled front to our south being activated yet again by the secondary jet streak and/or the next shortwave. However, in most solutions that occurs too far south to bring the QPF response into our CWA. For those that do show precip coming back, however, we carry a 20-30% PoP mainly along/south of US 74. These models tend to be warmer so this PoP mention is carried as only rain, with temps mainly in the mid 30s, and closer to 30 in our north where PoPs are too low to mention. If precip were to spread farther north than temps would likely be above freezing there also.

Key message 2: Precip chances ramp up again Saturday night. Light freezing rain again may occur in the mountains, but otherwise only rain.

Small PoPs Saturday persist in about the same arrangement as Friday night, dropping slightly for a time in the US 74 corridor but remaining 20-30% south of I-85, again because of a few guidance members showing light precip via the frontal activation. The current slate of deterministic models is split as to whether warm upglide continues aloft, even if too shallow for precip. Cloud cover will keep temps limited to the upper 40s and very low 50s over most of the area, with wedge potentially hanging on.

Though the current slate of operational models is largely dry Saturday night, yet another speed max develops at 250mb and once again the stalled front could activate to our southwest, if not Saturday night, Sunday morning. There are enough ensemble members showing light precip creeping into the area to advertise a increase in PoP to 20-25% in our north, and 30-40% in our south, Saturday afternoon and night. On account of low confidence and the same idea as the previous night--the wetter models tend to be warmer--have confined FZRA mention to the northernmost mountains and Escarpment where subfreezing temps are most likely to persist.

LONG TERM /SUNDAY THROUGH WEDNESDAY/

As of 115 AM Wed:

Key message 1: Small chances for rain remain in the forecast for the Piedmont Sunday and Sunday night before the stalled front finally exits the region.

Broad but full-latitude trough will cross the Mississippi Valley Sunday and Sunday night, with embedded shortwave south of the Ohio River. Models largely feature one last coastal cyclogenesis event along the longstanding front which by then looks to be near the Gulf Coast. There remains spread as to when the low spins up, and some solutions exemplified by the 04/00z GDPS which show the shortwave digging farther south, and generating precip north of the front and thus bringing activity into the CWA from the west as well as on the periphery of the coastal low to our southeast. Chance (30-40%) PoPs result for all zones Sunday and Sunday night primarily for that reason. Previous model runs mostly were dry east of the mountains so confidence insufficient to warrant PoPs any higher. Temps Sunday similar to Saturday's.

Key message 2: Next cold front looks to arrive late Sunday, bringing a chance for northwest flow snow near the TN border, and colder temperatures for all of the forecast area Monday and Tuesday.

Those models which had been showing precip Sunday-Monday had been depicting the shortwave to cross the Appalachians of TN/NC in clipper-like fashion and produce northwest-flow snow, where temps are cold enough near the TN border. Some solutions still feature that without depicting chances from the coastal low, but the snow is still featured late Sunday given that has been the more consistent solution. Some FZRA mention on the eastern side of the mountains where profiles are less favorable for snow. Elsewhere, all rain. PoPs taper off Monday behind the front; max temps trend several degrees colder than Sunday, being about 10 below normal. Monday night temps reach the lower 20s in portions of western NC. Slightly warmer but still cold max temps Tue.

Small PoPs redevelop Wed-Thu with low possibly pushing across the Midwest and carrying another cold front into the area. Temps will have rebounded close to normal and no wintry precip currently expected east of the mtns.

AVIATION /06Z THURSDAY THROUGH MONDAY/

At KCLT and elsewhere: Next round of thick cirrus is just streaming into the upper Savannah River Valley and should arrive at KAVL and Upstate sites within the next 1-2 hours. Model guidance has backed off on the notion of dense fog in the Upstate tonight, likely in response to thicker forecast cloud cover; nonetheless, sites across the area are reporting 1-3 degree dewpoint depression...which means we're close enough that it's still a concern. So, went with MVFR to IFR fog/stratus mention in the Upstate and at KCLT, rather than LIFR. Can otherwise expect mostly light and variable winds overnight, with continual periods of high cirrus. Cloud cover will thicken on Thursday, becoming BKN by late morning. Mid-level cloud cover will develop as expected through most of the day. Conditions will begin to deteriorate from west to east after sunset, as rain steadily pushes in across the area and leads to a quick reduction in flight category from VFR to at least IFR everywhere by daybreak Friday. This was handled in the TAFS mainly with PROB30s for MVFR. KCLT, given its longer 30 hour TAF period, instead gets to prevailing MVFR with PROB30 IFR.

At KAVL: Expect more or less the same sequence of events as described above, but it's worth noting that toward the very end of the TAF period, there's a non-zero chance of FZRA and/or PL. Chances for actually recording this are higher than with the previous two winter precip events over the last week, so although no mention of anything other than RA is currently carried in the 06z TAF, it's not out of the question that it could be added at some point.

Outlook: A lull in rainfall and flight restrictions is expected late Friday into Saturday. Unsettled weather will remain, however, through the weekend and into the first part of next week.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Winter Weather Advisory from 7 PM this evening to 1 PM EST Friday for NCZ033-049-050-501-503-505. Winter Weather Advisory from 7 PM this evening to 10 AM EST Friday for NCZ048-052-053-059-063-064. SC...None.