textproduct: Columbia

WHAT HAS CHANGED

Widespread showers and thunderstorms again expected as we get into the weekend, with a focus on the central/western Midlands and CSRA. Updated for the 18z tafs.

KEY MESSAGES

- 1. Increasing chances for showers and thunderstorms are again expected beginning Friday in the CSRA and then expanding over the area into Saturday. Flash flooding is a threat during this period.

DISCUSSION

Key Message 1: Increasing chances for showers and thunderstorms are again expected beginning Friday in the CSRA and then expanding over the area into Saturday. Flash flooding is a threat during this period.

A complex pattern over the CONUS is forecast to yield another round of heavy rainfall, particularly during the Friday night through Saturday night period across the area. This is forecast to generally be driven by multiple shortwaves passing through the area atop a northward lifting surface front amidst impressive PWs of 2.0"-2.2" across the area.

Before we get to that, we have the actual cold front (that is the impetus for many of the forecast difficulties over the next few days) pushing through today. This feature is currently in the Pee Dee region of SC per surface observations, slowly sinking southward at the moment. While CAMs are mixed on overall convective coverage with this, we have seen CAMs struggle with placement and coverage of showers and storms over the past several days. Thinking it is reasonable to expect a band of showers/storms to develop this afternoon ahead of this front, especially since we'll likely see plenty of instability on the order of 2000 j/kg of SBCAPE. Some strong storms are possible but will be reliant on robust, initial updrafts for some precip loading & subsequent downbursts with that. The overall heavy rain threat seems lower than it has the past few days. However, given how saturated the soils are, it will not take much to cause flooding issues.

This front is forecast by guidance to continue sliding south and southwestward as the day and night goes along, with a pretty large spread (especially for this timeframe) amongst guidance on where the front itself sets up. Some CAMs have the front pushing clear into central Georgia by midday Friday, which I suspect is unlikely. The surface high across the NE US is not particularly strong so it seems that it will be unable to push the front that far southwest of us. Most guidance is coming towards a consensus of the front hanging up somewhere in the Atlanta-Augusta- Orangeburg location & beginning to lift back northward on Friday and Friday night. Driving this northward movement is a series of shortwaves forecast to emanate from a longwave trough across the central US. So while there may be a reprieve in thunderstorm activity on Friday (especially across the northern Midlands where PWs look to fall to less than 1.5"), this will be short- lived.

The environment forecast to then overspread the area as front lifts northward on Friday night is quite primed for problems locally given how much rain we have received across the entire area. PWs of >2" & long, skinny CAPE profiles will favor efficient rainfall rates atop soils that are already primed for flash flooding. The presence of multiple, focused shortwaves pushing atop this surface boundary is quite concerning, specifically Friday night through Saturday night. While CAMs are somewhat of a mixed bag right now, the global models are picking up on an extended period of rainfall in this period. ECMWF EFI for QPF has shot up run to run in this period, adding confidence to this. If the trends continue, we may need a Flash Flood Watch for portions of the forecast area on Friday night and Saturday.

Thereafter, the forecast becomes a bit trickier. Guidance is generally showing a bit of a drying trend on Sunday, with another shot of rainfall on Monday ahead of a more robust cold front. That front is generally forecast to result in dry and cooler weather by the middle of next week. However, at this point, not completely confident that it actually comes to fruition. We shall see!

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Predominantly VFR conditions expected with brief restrictions possible each morning. Diurnal thunderstorms are expected to develop and may periods of bring heavy rain, reduced visibility and gusty winds.

Cold front will be pushing into the forecast area this evening before stalling out over the southern Midlands and CSRA early Friday.

From the afternoon into the overnight hours, mainly vfr ceilings and visibilities should persist outside of any thunderstorm activity that may develop. Thunderstorms could bring brief ifr visibilities in heavy rainfall to any taf location impacted. Confidence remains low at this time for widespread convection, so have mainly continued with PROB30 wording at each taf site later this afternoon and into the evening.

Late tonight into Friday morning at ags/dnl, guidance is indicating the possibility of some lower end mvfr or upper end ifr visibility restrictions developing. Have mainly gone with a tempo group at those two locations for the lower visibilities.

EXTENDED AVIATION OUTLOOK...Shower and thunderstorm chances increase over the weekend as a front meanders across the region. The greatest rainfall and early morning fog/stratus chances will be favored across the CSRA and southern Midlands.

CAE WATCHES/WARNINGS/ADVISORIES

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