textproduct: Columbia

WHAT HAS CHANGED

Lake Wind Advisory has been allowed to expire. Updates to Key Message 1. Aviation discussion updated for 00z TAF.

KEY MESSAGES

- 1) Rain and a few thunderstorms are possible this evening into tonight where periods of heavy rain are likely along and south of the I-20 corridor. Rain may persist into Friday, especially south and east of I-20.

DISCUSSION

KEY MESSAGE #1: Rain and a few thunderstorms are possible this evening into tonight where periods of heavy rain are likely along and south of the I-20 corridor. Rain may persist into Friday, especially south and east of I-20.

A batch of moderate rain is pushing southeast of the FA at this time, but the next round can be seen off to our west where meager instability has developed leading to some weak convection as well. To this point, 0.25-0.75" of rain has been seen generally north of I-20, while areas south have seen under 0.25". The strong 45-50 kt LLJ is pushing northeast of the FA, but strong moisture transport persists with PWAT's around 1.20-1.40" across the region. The sagging front is slowly approaching the mountains and is expected to move into the FA overnight, though there is uncertainty is how far south it will push. The slow moving front, forcing from the southwesterly LLJ, and flow aloft generally parallel to the front should yield moderate to heavy rain tonight as showers and weak thunderstorms train along this front. Where this training occurs, a swath of 1.5-3" of rainfall will be possible as shown in both the HREF and REFS LPMM for QPF through Friday morning. This solution is also shown in high-res guidance like recent HRRR solutions. The main uncertainty is how far south the front will make it and thus where this swath will line up. At this time, locations along the I-20 corridor (including both the Augusta and Columbia metros) and south toward the southeastern Midlands appear the most likely to see this heavy rain and locally higher amounts. Flash Flood Guidance (FFG) is fairly high across the whole FA due to persistent drought conditions and thus widespread flooding concerns are not anticipated, but localized nuisance flooding in low-laying area or urban spots could be possible.

At least light rain is expected to carry into Friday, especially in the southeastern Midlands, before gradually diminishing through the day as the front slowly move further south. Drier conditions then should move in by the evening hours Friday and into this weekend.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Restrictions are expected for much of this period, but the extent, timing, and depth of them remain uncertain. Both vis and cig restrictions are expected.

This TAF issuance proved to be a challenging one as there are multiple moving parts making for a difficult near term forecast. Lets get what we are confident in out of the way: forecast confidence is high that widespread rain and embedded thunderstorms will impact the TAF sites tonight, especially the Augusta and Columbia TAF sites. Numerous showers and thunderstorms have developed upstream of the forecast area at this time and are pushing east southeast within west- northwesterly flow. These are forecast to continue propagating over the area through the night, slowly sinking southward as the surface cold front (currently meandering across the upstate) sinks southward as well. The heaviest rainfall will likely reduce visibilities to 1sm or lower when they move through. The heaviest rain should be in CAE/CUB by 02z and exiting sometime in the 06z-08z timeframe. AGS/DNL will be about an hour later, with orangeburg about an hour later than them. Overall, this is the highest confidence portion of the TAF. Rain is forecast to continue into at least the first half of Friday before shifting eastward Friday afternoon.

Ceilings are another story. We have struggled to see consistent MVFR cigs this afternoon after some mixing cleared the cigs from this morning a bit. However, guidance is still fairly insistent on widespread low MVFR and IFR ceilings tonight and into Friday. This forecaster has a healthy amount of skepticism in this scenario, as the cold front is far slower than guidance had been forecasting it to be. This is the crux of the IFR forecasts, as we will need to be behind the front and see the lower levels of the atmosphere cool a bit to yield in-situ wedging that had been previously expected. Given the widespread rainfall that is expected overnight, it seems reasonable to expect MVFR ceilings for much of the period. IFR ceilings are just less certain at this point. Still forecasting them for a few hours or longer, especially at the Columbia sites which may end up north of the front by Friday morning. Overall, the weather is still expected to be impactful throughout the period.

EXTENDED AVIATION OUTLOOK...Widespread restrictions and rain expected Friday afternoon and into Friday night. Some improvement back to vfr possible on Saturday.

CAE WATCHES/WARNINGS/ADVISORIES

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