textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated the aviation discussion to reflect the impending 12Z TAF issuance.

KEY MESSAGES

1. Patchy fog sticks around this morning. Another round of rain will develop late this afternoon into tonight. 2. Rainy weather with potential for afternoon storms continues through at least Wednesday. A cold front and some drier air may arrive by Thursday, but the pattern is highly uncertain past this point.

DISCUSSION

Key message 1: Patchy fog sticks around this morning. Another round of rain will develop late this afternoon into tonight.

As expected, rain has largely ended across the region, with only light patchy drizzle in its wake. Patchy fog and widespread low stratus have once again also developed, but fog has not been nearly as widespread nor as dense as the previous few nights. Consequently, it's looking like any dense fog products will not be necessary.

Quieter conditions are expected for most of today. Upper flow will remain largely unperturbed with deep layer S/SW winds persisting, and no discernible vort lobes in operational models. We can expect some decent instability to develop as the wedge mostly mixes out during peak heating...with hi-res guidance depicting on the order of 1500 J/kg sbCAPE across much of the area. Triggering will be an issue, but at least a handful of showers and storms may develop, particularly over the mountains. A potential wrinkle in the forecast comes from what look like a series of subtle speed maxxes embedded within the upper flow during the afternoon period, though none of the models can really pinpoint when they'll arrive, or exactly where they'll be maximized. As a result, while shear profiles generally look tame, there's a possibility that localized higher shear could develop beneath one of these speed maxima and result in an isolated stronger storm developing. And, as always in these situations, convection occurring near the wedge boundary could see a localized increase in SRH, increasing the severe risk. Nonetheless, with such low coverage of afternoon convection expected, the severe risk remains low, if perhaps nonzero.

Late in the day, a second, more focused shortwave will eject off the eastern periphery of an upper low currently centered over the Gulf Coast. It'll reach the Carolinas by late evening, in turn spurring another round of stronger isentropic ascent through the first half of the night. The 00z CAMs are really struggling to get a handle on the potential for this feature, variously depicting anywhere from minimal coverage, to widespread rain-producers generating as much as a half inch of rain in some locations between sunset and perhaps 3 AM. The 00z HREF and REFS guidance don't offer much clarity - so confidence remains limited in how rain will evolve for most of the night. This also makes the stratus/fog potential questionable, since the trend toward lower coverage in recent guidance would also mean less reinforcement of any lingering wedge.

Key message 2: Rainy weather with potential for afternoon storms continues through at least Wednesday. A cold front and some drier air may arrive by Thursday, but the pattern is highly uncertain past this point.

Diurnal convection is expected to continue through the first half of the week as a moist and moderately-unstable air mass remains in place across the area. The upper low previously centered over the Gulf coast will lift north and merge into the mean flow through Tuesday, and the subtropical ridge will actually expand westward, such that the source of these shortwaves that have been giving us the heavier rain periods will vanish, and moisture fetch will become subtly weaker. Still, moist profiles and diurnal instability will maintain daily rain chances through at least Wednesday.

An area of height falls is depicted across New England and the upper Ohio Valley on Thursday, and there's an increasingly strong signal in the synoptic guidance for a late week backdoor cold front arriving Thursday or Thursday night. This is really strongly supported in the 00z LREF...to the point that all four dominant clusters depict at least a weak FROPA during this period...suggesting the possibility of some drying late in the week.

Guidance really goes haywire by next weekend, however. Will we have sustained weak CAA and drying? Another shortwave and associated upglide over a stagnant frontal boundary, regenerating CAD and putting us right back where we are now? Will the boundary lift back north and put us in a more warm sector-like air mass? You can find a few ensemble members that support each of these scenarios, and probably others as well. So at this point, confidence is very low for the forecast past Thursday.

AVIATION /12Z MONDAY THROUGH FRIDAY/

At KCLT and elsewhere: Mostly IFR to LIFR ceilings and IFR visibility have developed across the terminal forecast area (with the notable exception of CLT itself, owing in part to repeated showers that have crossed the area overnight and disrupted lower ceilings observed in most other locations). Improvement to MVFR, and eventually VFR, is advertised this afternoon, during which time coverage of showers will initially be quite low. Scattered convection will develop a little later than the usual diurnal maximum, extending from mid/late afternoon into the late evening, and persisting well into the first part of tonight. Some deterioration in conditions is expected again on Monday night, but not nearly to the extent we've seen the last several nights...with most sites only dropping to IFR, followed by faster improvement again on Tuesday morning.

Outlook: The pattern will remain unsettled and murky for the foreseeable future, with at least scattered diurnal convection, possibly persisting into the overnight hours, and lowered visibility and ceilings forecast each night through at least the end of the work week.

GSP WATCHES/WARNINGS/ADVISORIES

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