textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

An SPS for Dense Fog was issued for the entire forecast area through daybreak Sunday.

The aviation forecast was updated to reflect the new 06Z TAFs.

Rain chances for the remainder of tonight and the first part of Sunday were adjusted to better match recent hi-res model guidance.

Low temperatures were raised somewhat on Sunday night. Highs on Sunday were lowered to account for persistent cold-air damming.

KEY MESSAGES

1. A relative lull in rainfall is expected the rest of tonight through most of Sunday, before the next round of rainfall arrives Sunday night. Dense fog will also be an issue through Sunday morning. An SPS for Dense Fog is in place for the entire forecast area through daybreak. 2. Unsettled weather continues for the foreseeable future, as the synoptic pattern remains stagnant and the Carolinas remain trapped within a moist air mass.

DISCUSSION

Key message 1: A relative lull in rainfall is expected the rest of tonight through most of Sunday, before the next round of rainfall arrives Sunday night. Dense fog will also be an issue through Sunday morning. An SPS for Dense Fog is in place for the entire forecast area through daybreak.

Conditions are mostly quiet across the forecast area, but low stratus and fog as thick as pea soup has enveloped much of the region as in situ cold air damming continues. Our resident wedge is now completetely divorced from its parent high, which has meandered well off the Atlantic coast...and given what appears to be a lack of any upper forcing as per recent water vapor imagery, there's little impetus for any widespread showers to develop the rest of tonight. Though a few popup showers and scattered light drizzle will likely persist through the next 8 hours, the bigger story will undoubtedly be thickening fog and low stratus in place across the region. Visibility below 2 miles is now reported at virtually all our ob sites, and increasingly, we're starting to see sub-1 mile visibility at many locations. Accordingly, an SPS for Dense Fog will shortly be issued and remain in place through daybreak...and it's definitely within reason that parts of the area could be upgraded to a Dense Fog Advisory later tonight.

Mostly quiet conditions will persist through the bulk of Sunday, with frequent drizzle and a few isolated showers expected the first part of the day. Can't rule out a few afternoon thunderstorms, though these would *most likely* be elevated in nature as the wedge will persist for much of the day. The big question for Sunday will indeed be: how much will the wedge erode, and how quickly? There's little remaining synoptic support, and guidance doesn't really depict any upper features that'd prompt any periods of better upglide throughout the day. Without much rainfall to support diabatic cooling, the only reason to think it'll remain in place is that there's no discernible mechanism to scour it out. Referring back to the "pea soup" analogy above, however, the most likely scenario seems to be that some shallow afternoon mixing will help to somewhat improve fog and cloud cover conditions again today, but not enough to break the wedge-top inversion at most locations, and therefore not enough to expose us to any good surface-based instability.

Sunday night, a weak shortwave embedded within SWly z500 flow will slide across the Appalachians, reinvigorating low-level upglide. Hi-res guidance variously depicts a loosely-organized band of convection making tracks across Georgia late Sunday and arriving in our southwestern zones after sunset, then pushing across the area through the first half of Sunday night. QPF response looks limited - generally less than inch for most zones, and perhaps up to 1.5 inches in parts of the upper Savannah River Valley, per the 00Z HREF - but even without heavier precipitation or widespread convective rain rates, it should help rejuvenate whatever is left of the wedge by this point, and another night of widespread low stratus and patchy dense fog is looking increasingly likely.

Key message 2: Unsettled weather continues for the foreseeable future, as the synoptic pattern remains stagnant and the Carolinas remain trapped within a moist air mass.

Long-range models are coming into increasing good agreement that a more potent shortwave will arrive on Monday night and spur another round of heavier rainfall, perhaps with some elevated convective rates developing within it. Can't begin to express more than moderate confidence until we start getting hi-res ensemble guidance that extends past 00Z Tuesday, but the guidance we do have pretty consistently depicts the ingredients needed for our next wave of rainfall. To wit, the operational models tend to depict anywhere from 0.5 to 1.5 inches of rain falling across the area on Monday night, with corridors of closer to 2.5 inches possible mainly in the SC Upstate. Long-range ensembles mostly agree here, and the most recent LREF cycle doesn't really depict ANY scenarios where we don't get at least a healthy half inch plus.

Otherwise...there's no end in sight, as the synoptic pattern shows no sign of quieting. By mid-week, a weak upper low initially centered near the Gulf Coast should lift into the central Mississippi Valley and begin to weaken. This signal suggests a possible downtick in these successive shortwaves that keep bringing us round after round of rainfall, but as stubborn subtropical ridging and a stagnant Bermuda High remain in place off the Southeast US coast...moisture will continue to advect into the Carolinas, and the potential for more rain will continue.

AVIATION /06Z SUNDAY THROUGH THURSDAY/

At KCLT and elsewhere: Conditions remain largely on track as we approach the 06Z TAF issuance, with widespread IFR visibility and IFR to LIFR ceilings reported across the terminal forecast area. Generally think that these murky conditions will be maintained through daybreak, as in situ cold-air damming lingers across the region. Have already seen vis outpace guidance in some places, falling as low as 1/2SM in some sheltered locations. Shower coverage, meanwhile, has continued to dwindle, and though patchy drizzle and an occasional full-fledged shower can't be ruled out through daybreak, we're not looking at the scattered SHRA that developed last night. Rather, impacts will mainly be confined to excessively low vis and cig restrictions. Light NE winds are expected to continue through the overnight.

After daybreak on Sunday, the thinking remains that the wedge will at least partially erode through the day, permitting slow but steady improvement to MVFR during the afternoon hours...perhaps even very brief, intermittent, low VFR. Isolated showers, maybe even a thunderstorm or two, will be possible during the latter half of the afternoon into the evening. This'll further assist in raising ceilings, though any areas of heavier rain would reduce visibility there. Wherever the wedge erodes and conditions improve, there's potential for winds to become more ESE instead of NE...but they'll remain light 3-6kts through the day.

Conditions should crash again Sunday evening. Most hi-res guidance depicts a band of heavier SHRA with embedded TSRA arriving in the Upstate by 03Z or, then pushing northeast across the area through 07Z. In its wake, widespread LIFR ceilings and reduced visibility will once again develop at all the TAF sites.

Outlook: The pattern will remain unsettled and murky for the foreseeable future, with widespread diurnal convection, often persisting into the overnight hours, and lowered visibility and ceilings each night.

GSP WATCHES/WARNINGS/ADVISORIES

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