textproduct: Greenville-Spartanburg

SYNOPSIS

Drier conditions briefly return to the area today before widespread precipitation returns tonight into Tuesday. Behind the system, high pressure will result in chilly and dry conditions Wednesday and Thursday. Unseasonably cold and unsettled weather is expected Friday as another system brings more precipitation to the area.

NEAR TERM /THROUGH TUESDAY/

As of 117 AM EST Monday...

Key Messages:

1) Scattered dense fog may persist through much of the overnight hours, and possibly into dawn Monday.

2) Quiet but chilly conditions Monday as high pressure passes by.

3) Another round of rain picks up Monday night, with potential for more freezing rain across the northern NC mountains. Winter Weather Advisory has been issued for the northern Blue Ridge from 7 PM Monday through 10 AM Tuesday.

Radar imagery indicates it's now dry across the forecast area, although some very light drizzle is still being reported at a couple of ob sites in the Upstate. Still seeing widespread cloud cover on satellite, and widespread patchy fog and low cloud cover, which was...unexpected based on the bulk of guidance. It would seem that postfrontal CAA is simply too weak - as evidenced by light to calm winds across most of the area - to effectively scour out moisture. In light of this, we can expect at least intermittent patchy fog throughout much of the night unless winds pick up, and have issued an SPS for Dense Fog through daybreak. If visibility issues continue closer to rush hour, will likely need to upgrade to a Dense Fog Advisory in the next few hours. Temperatures, true to form, continue to run above guidance, but we can still expect to make it into the 30s across most/all of the area by sunrise.

Mostly quiet conditions are expected today, as surface high pressure currently analyzed over the lower Midwest migrates into the Ohio Valley and eventually off the Mid-Atlantic coast by tonight. Should see a definite period of dry, quiet weather, but hi-res guidance suggests we'll never quite get rid of upper-level cloud cover, so at least thick cirrus should stick around most of the day. This will once again limit solar insolation, and so highs today will only make it into the mid-40s once again.

Active weather will once again pick up tonight as the axis of a stronger z500 trough dips down out of the Great Lakes region, resulting in an uptick in synoptic-scale forcing across the area after sunset. As noted by the previous forecaster, the NBM has consistently depicted onset of rainfall earlier than nearly all the other guidance...especially the CAMs, which suggest only light rain before 03z or so, with onset of more consistent, heavier, and widespread rain holding off until closer to 06z for most of the area. The setup for Monday night/Tuesday looks eerily similar to that of last Saturday night/Sunday, with low-level WAA and moisture advection resulting in a rapid increase in moisture as well as the development of a deep warm nose across most of the area. One of the main differences looks to be surface temperatures, which are forecast to be a little warmer Tuesday morning than they were Sunday morning; as a result, accumulating wintry precipitation looks to remain more confined to the northern Blue Ridge...where a few hundredths of an inch of ice are expected. Once again, the NBM depicted disturbingly high ice accums across Avery, Mitchell, and Yancey County...so had to tamp these down at higher elevations which are likely going to be inside the warm nose. Rather, expect the bulk of accumulations to occur on mid-level slopes, where at least light freezing rain will continue through much of Monday night. As a result, a Winter Weather Advisory has been issued for the Appalachians north of the French Broad Valley, beginning 7 PM Monday and ending 10 AM Tuesday. It currently appears unlikely that upgrades/expansions will be unnecessary, but should they become necessary, they'll be handled with the next forecast package on Monday.

SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/

As of midnight Monday morning:

Key message 1: Precip will change to snow or rime ice along the TN border Tuesday afternoon as northwest flow develops behind the departing system.

As colder air reaches the high elevations along the TN border Tuesday afternoon, profiles cool and wintry precip again will become possible. NW flow implies mainly snow or rime ice, though prog soundings cool at differing rates with altitude, and brief freezing rain or sleet may occur in transition to snow/rime. Precip looks to taper to flurries by around sunset. Min temps Tue night will fall into the lower 20s and upper teens across much of the NC mountains, with mid to upper 20s otherwise.

Key message 2: Dry weather Wed and Thu. Temperatures trend slightly cooler Wednesday but warmer Thursday.

Weak sfc high pressure will migrate across the Southeast beneath a very low amplitude ridge between the departing low and the next trough, comprised of loosely phased northern and southern stream shortwaves. On Wednesday, despite sunny skies temps now look to end up a little cooler than Tuesday owing to the freshly arrived continental airmass, with highs mainly in the upper 40s to around 50.

The northern wave will be associated with a cold front, but dynamic forcing is weak enough that its arrival Thursday should be dry and airmass change does not actually occur. The incoming high remains centered in the Ohio Valley thru the end of the day Thursday. Slight downslope flow develops which should bring max temps a few degrees warmer--almost back to normal for the Piedmont--despite increasing high altitude cloud cover.

LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/

As of 100 AM Mon:

Key message 1: Wintry precip may return to some of the area late Thursday night into Friday as a result of another cold-air damming event.

Continental sfc high will migrate out of the Midwest and set up over the Eastern Seaboard Thursday night, as the weak front settles around the I-20 corridor. The southern shortwave mentioned in the short term discussion will dig across the SW CONUS, itself shearing into a transient wave inland and a deep almost cutoff component near Baja California. The net height falls are shown to reactivate the front Thursday night, with Miller-A cyclogenesis taking place again by late Friday near the Carolina coast. Though the sfc high has trended weaker in some solutions, in-situ CAD would appear to develop in our CWA by Friday morning; models overall have come more in line in depicting precip beginning around that time, with temperatures near freezing over much of western NC and the northern tier of Upstate zones.

Felt the NBM-derived temp and PoP trends Friday morning were acceptable as-is; 00z GFS and 30/18z ECMWF prog soundings depict moistening from the top down, and snow currently would appear to be the main p-type for the areas that are sufficiently cold, likely mixed with rain/sleet where temps are just above freezing. FZRA could also occur early on in areas below freezing; once again the northern mountains may see subfreezing temps lock in after the warm nose strengthens and predominantly FZRA later in the day, with most other areas all rain by early afternoon. New 01/00z ECMWF coming in as of this writing appears to have shifted back to a drier solution more like it had shown 24 hours ago; the GFS solution has been consistent in being wetter, fwiw. Friday is expected to be a very chilly day even if the mixed p-types don't materialize, with maxes in the lower 40s throughout the CWA, except for the colder mountain/foothill locations.

Key message 2: Confidence remains low for next weekend's forecast, both for the occurrence of precip and the potential for winter weather.

Major models continue to differ in their handling of the trough(s) in the western CONUS, which leads to differences in how and when they develop the next precip event. Assuming the Miller-A develops Friday, it should be pulling away that night, and PoPs would decline accordingly. As heights fall in the south central and/or western Gulf regions, models appear in agreement that the coastal front again will activate south of I-20, which could induce yet another upglide driven precip event for our area Saturday. Then there's the GFS like solution that phases a northern Pacific shortwave with the southern stream, induces a Midwest cyclone, and develops prefrontal precip for our area by Sunday night. The result of these differences remains PoPs declining very slowly Friday night and Saturday with more chances returning late Sunday. Some freezing rain falls out of the grids in the mountains as a result of PoPs lingering when temps cool nocturnally and those models producing PoPs featuring shallow upglide. This should be taken as a very low confidence forecast. Daytime temps probably will trend back to within a few degrees of normal for Sat-Sun, though CAD could continue if the wetter solutions verify, and temps could remain locked into values closer to those expected Friday.

AVIATION /06Z MONDAY THROUGH FRIDAY/

At KCLT and elsewhere: Widespread IFR to LIFR fog has developed, somewhat unexpectedly, across the Upstate and Piedmont zones, and sans any clear mechanism to erode it, seems likely to continue through at least the first part of the overnight. So, have introduced TEMPOs for (V)LIFR at most sites...and will need to monitor the situation closely as the night wears on. Otherwise, expect a return to VFR after daybreak, and for winds to pick up out of the NE at this time. Some potential for gusts overnight and into the first part of the morning, especially at KAVL, before winds become lighter during the day Monday. VFR should persist through Monday, before the next round of rainfall arrives Monday night, with -RA expanding from west to east across the terminal forecast area, with associated flight restrictions developing in short order. Guidance indicates MVFR is to develop at all terminals by the end of the 06z TAF period, with potential - as yet too low to depict in the TAFS - for IFR. Likelihood of IFR or worse restrictions, however, doesn't start increasing in earnest until after 06z Tuesday, so for now, have kept this relegated to PROB30s at the very end of the TAFs. Once again, light FZRA is expected to develop across the NC mountains before daybreak Tuesday, but likelihood of impacting KAVL remains low, so no mention at this time.

Outlook: Widespread MVFR to IFR conditions continue into at least the first part of Tuesday. VFR conditions return Wednesday. Another system may affect the region Friday into the weekend.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Winter Weather Advisory from 7 PM this evening to 10 AM EST Tuesday for NCZ033-049-050-501-503-505. SC...None.