textproduct: Greenville-Spartanburg

SYNOPSIS

Low pressure lifts across the forecast area today bringing widespread precipitation. Behind the system, high pressure will result in chilly and dry conditions Wednesday into Thursday. Unseasonably cold and unsettled weather is expected Friday as another system brings more precipitation to the area.

NEAR TERM /THROUGH TONIGHT/

As of 1237 AM EST Tuesday...

Key Messages:

1) Winter Weather Advisory in effect for the northern and central Appalachians, owing to ongoing freezing rain and light ice accumulations.

2) Rainfall will continue through the first part of Tuesday before we dry out during the afternoon and evening.

3) Dry but very cold conditions should return Tuesday night.

Virtually the entire forecast area is now seeing at least light drizzle as profiles moisten beneath a baroclinic zone draped across the Southeast. Some locations along I-77 may still be dry enough that precip may not be reaching the surface, but generally think that rain is now making to the ground everywhere. ACARS data depicts a moist, deepening warm nose above a steep low-level inversion, indicative of strengthening in-situ cold air damming driven by intensifying rainfall. The result is another round of patchy fog and low stratus, though so far it's far less murky out there than it was this time last night. Meosanalysis would suggest that isentropic ascent over the developing wedge has still not peaked, with the strongest 850mb winds and moisture advection still analyzed over north Georgia and progged to continue into our southern and western zones over the next 2-3 hours.

This setup still looks like it'll contribute to freezing rain across the central and northern Blue Ridge between now and daybreak. Already, some reports of ice have begun coming in from across our northernmost mountain zones, and would expect this to continue more or less through daybreak. Higher peaks are likely to be inside the warm nose, so freezing rain accumulations are actually likely to be higher/most widespread across mid-level slopes below 4000ft of elevation...and above the larger mountain valleys, which by virtue of a borderline temperature forecast may not reliably fall below freezing at all tonight. Expectations for overall coverage of ice remain more or less the same, which is to say little to no accumulating ice is expected west of I-26, with much better coverage east of I-26 and especially north of the I-40 corridor. Temperatures will follow a very non-diurnal curve, which is to say we're likely already at our coldest in most places, with continuing strong WAA likely to lead to slow but steady warming through daybreak and beyond. A Winter Weather Advisory remains in place for the southern French Broad Valley, northern Blue Ridge, and NC Blue Ridge Escarpment zones through 10 AM today, by which point temperatures at even the coldest spots should be reliably warm enough to preclude further ice accumulation and convert remaining precipitation to an entirely-rain forecast.

Rainfall will move steadily eastward through late morning...with essentially the entire forecast area forecast to be dry by early afternoon. Cloud cover could linger through much of the day, but the influence of WAA across the area will help offset any loss of daytime heating; high temperatures are therefore expected to reach the low 50s across the non-mountain zones, only remaining in the 40s across the NC mountains. By tomorrow evening, we should see consistent clearing that'll continue into Tuesday night. Between a somewhat cooler postfrontal air mass in place tomorrow night, and a few hours of good radiative cooling after midnight once CAA abates and winds die down, lows Wednesday morning are likely to drop into the mid-20s across most of the low terrain, except for the Savannah River Valley, which may hover around 30.

SHORT TERM /WEDNESDAY THROUGH THURSDAY/

As of midnight Tuesday morning:

Key message 1: Dry weather Wednesday with temperatures trending slightly cooler in most Piedmont locations.

Weak sfc high pressure will migrate across the Southeast Wednesday, though 500mb flow already will be slightly cyclonic as the next trough digs into the northern Great Lakes region. Despite sunny skies weak CAA continues and max temps will be held 2-3 degrees cooler than Tuesday in most locations east of the mountains, the greatest change being in areas that experience strong downsloping/warming Tuesday afternoon. Temps will be about the same within the mountains or perhaps slightly warmer.

Key message 2: A weak front settling through the area Thursday could spawn a few showers in our far south, and should bring cooler temps to the mountains and warmer temps east.

Dynamic forcing with the aforementioned trough will remain far off to our north through Thursday, and the associated front looks to reach the CWA with minimal oomph. Midlevel cover will reach the area early in the day which will limit warming in the mountains, with CAA developing there during the day also. With the previous airmass modifying, and with downsloping beginning aloft with the arriving front, temps should trend slightly warmer east of the mountains.

The sfc high behind the front expands east out of the mid-MS Valley by the end of the day Thursday. The front in turn should stall along its southern periphery somewhere across GA/SC. Meanwhile a separate shortwave will exit the southern Rockies and begin to activate the previous stalled/coastal front in the far Deep South. It is hard to discern which front is responsible for the small PoPs depicted by some models over our Lakelands zones, but the idea is plausible enough for a slight chance in the official forecast, as rain Thursday afternoon.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

As of 1230 AM Tue:

Key message 1: Confidence remains very low in the forecast for Thursday night through Saturday night, but period(s) of light wintry precip cannot be ruled out in that timeframe, mainly in the northern CWA.

General expectation is the same as previous cycles, that the surface high that reaches the area Thursday will center over the Mid-Atlantic coast by Friday morning, and that amplifying jet streak and/or the weak trough crossing the southern Rockies will lead to frontogenesis along the baroclinic zone across the SE Coastal Plain to our south. The timing of that process, causing eventual development of a Miller-A cyclone, however remains difficult to pin down.

Cold-air damming had seemed like a good bet given the strength and position of the high, but recent GFS runs show the high being weaker and more transient, with less diabatic cooling and later onset of PoP which would suggest precip is likely to begin as rain for all but the coldest Escarpment areas. The ECMWF, which had once been consistent in Friday being dry, is now faster and colder than the GFS, clearly depicting CAD and freezing rain and/or snow over our northern mountains and I-40 corridor. With the wedge locked in, it continues to produce some wintry precip through Friday. The GFS eventually does produce QPF in a similar pattern, just starting later and almost all liquid. The Canadian GDPS is drier, showing a weaker high like the GFS but also keeping the precip associated with the frontal wave mostly to our south. The best consensus for precip occurrence is Friday afternoon through very early Saturday morning, and at least weak CAD appears likely, even if temps remain above freezing. Temps Friday look to be quite chilly. with most areas in the lower 40s, possibly not making it out of the 30s if the CAD is as strong as the ECMWF depicts. Regardless of which solution verifies, precip chances should be on the decline east of the mountains, at least temporarily, late Friday night as the Miller-A forms and pulls away from the coast. Northwest flow snow or rime ice could develop near the TN border at that time.

It remains unclear whether the aforementioned southern shortwave will phase with another shortwave in the northern Plains, so there could be another opportunity for coastal front to reactivate Saturday night, exemplified by the 02/00z ECMWF. The GFS had once depicted this sort of thing. PoPs do ramp back up as a nod to this possibility. P-type in this scenario probably would be predominantly rain, but more FZRA can't be ruled out in the mountain/foothill locations where colder temps persist. Temps should rebound back into the upper 40s or lower 50s Saturday--still several below normal.

Key message 2: Light northwest flow snow is possible near the TN border late Sunday thru early Monday.

Most models depict a clipper-like shortwave blowing across the Appalachians between late Sunday and Monday morning, and small PoPs redevelop near the TN border. Temps appear supportive of snow at that time. The GFS depicts some light snow showers developing in the NC Piedmont Monday morning but those often don't pan out with a clipper, or end up just being flurries and a novelty. Temps trend slightly cooler again on Monday.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

At KCLT and elsewhere: Widespread MVFR ceilings currently between FL020 and FL030 are expected to slowly deteriorate over the course of the tonight, with a pretty strong signal for IFR ceilings by daybreak at most terminals in both the statistical guidance and model sources. The possibility remains, but with lower confidence, on LIFR ceilings developing around daybreak. Currently seeing the heaviest RA south of I-85, but with much better returns west of the Savannah River Valley and inbound...think most sites will see increasing coverage/intensity of rain beginning within the next 2-3 hours. In conjunction with this, expect visibility to fall to MVFR/IFR levels by around 09z. Vis should improve fairly quickly once the sun comes up, but ceilings will be much more stubborn, with the GLAMP and short-range blended guidance both suggested that KCLT and the Upstate sites could remain socked into IFR conditions well past noon Tuesday, despite rainfall ending everywhere by late morning or early afternoon. Could be well into the late afternoon/evening hours before enough low-level moisture is scoured out enough to get us fully back to VFR. Light and variable winds overnight, at least somewhat favoring an ENE component, will gradually become more consistently NW by mid-day Tuesday. Expect VFR conditions and a steady NW wind of 5-7 kts the first part of Tuesday night, and for winds to then become light and <3kts across the terminal forecast area during the predawn hours Wednesday.

Outlook: Dry, VFR conditions expected to persist through Wednesday and the first part of Thursday. Rain could return as early as Thursday afternoon, but more likely Thursday night and Friday as the next system arrives. Rain and associated flight restrictions could continue into the weekend.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Winter Weather Advisory until 10 AM EST this morning for NCZ033-049-050-053-065-501-503-505-507-509. SC...None.