textproduct: Columbia

WHAT HAS CHANGED

Hurricane Isaias about to make landfall along the Florida panhandle this evening. Flood Watch issued for same counties as Wind Advisory for Saturday and Saturday night. Aviation discussion updated for 00z TAFs.

KEY MESSAGES

- 1. An active weekend ahead as Hurricane Isaias moves inland to our west. Confidence in the extent of impacts increasing with periods of heavy rain bringing an isolated flooding threat. Isolated tornadoes are possible on Saturday night as well.

DISCUSSION

Key Message 1: An active weekend ahead as Hurricane Isaias moves inland to our west. Confidence in the extent of impacts increasing with periods of heavy rain bringing an isolated flooding threat. Isolated tornadoes are possible on Saturday night as well.

Overview: Moderate to heavy rainfall is likely as Hurricane Isaias passes to our west on Saturday and into Saturday night.

Overall, the forecast has not changed significantly since yesterday afternoon with the 00z and 12z suite of guidance being fairly similar and in line with forecast thinking. Widespread clouds will continue to thicken and lower as Isaias landfalls and moves inland overnight. Moisture advection will be relatively quick this evening with PWs rising from where they are currently which is around 0.8" to well over 2" by Saturday morning.

Rainfall is expected to accompany this system & should become more widespread sometime after midnight tonight. Forcing this rainfall will be a combination of strong moisture transport & isentropic ascent atop an in-situ wedge across the area. It doesn't hurt that synoptic scale lift should be pretty good as well as the right-entrance region of an upper level jet streak pushes northward into the Mid-Atlantic. Guidance is effectively in two camps: narrow bands of heavy convective precip vs widespread isentropically forced precip. As expected, and what is likely more physically likely, the hi-res guidance focuses two bands of training convection. One across central GA and another along a secondary trough and heavy rainfall axis along the coast with rainfall totals 3-5", but outside of our forecast area for now. The 18z guidance is trending a bit more eastward with the band of training convection into the CSRA and western Midlands with potential for 3-6 inches of rain and possible locally higher amounts. The HRRR, HREF, and REFS all paint a similar picture with this banding setup, leaving the eastern Midlands as a relative minimum in the region. The global and AI guidance do not capture this banding, with a broad brush 2-4" across the region but is likely not a physically realistic solution. The hi-res LPMM generally does a nice job portraying the footprint well for heavy rain events as this time range, but usually struggles spatially. So shifts of 25-50 miles with this banding is highly plausible, bringing a boom or bust potential to much of the area. Trusting the entire hi-res ensemble a bit more, and with LREF-HREF-REFS probs of over 4" are about 30% in these areas, decided to go ahead and issue a Flood Watch for potential flash flooding in the CSRA including Edgefield county where confidence is highest. Confidence is increasing that the Pee Dee will likely be the driest spot, sandwiched between areas of strong convection and lifting dynamics by the time the deepest moisture arrives.

There are other potential hazards at play with this event. Tornadoes are certainly possible in this event, especially Saturday afternoon and night. As Isaias' circulation lifts northward, the aformentioned thermal gradient is expected to lift northward as well. The timing of this should be such that it occurs as the best moisture transport approaches behind the initial warm front, which could create a scenario on Saturday night that we see low- topped supercells develop in a highly sheared environment. LCLs will be quite low, and we typically don't need much more than a surface dewpoint of 70F to have some tropical tors develop. The RRFS and HRRR have both shown signs of developing the necessary convection as well as pushing sufficient low-level CAPE into the area on Saturday night. SPC has placed the area in a Slight Risk, driven by a tornado threat. Have multiple ways to receive warnings on Saturday night!

Winds are the other threat but likely the least impactful. Some gusts over 40 mph are expected in the western Midlands and as a result, a wind advisory has been issued to cover this potential throughout Saturday. Otherwise, it should just be breezy across much of the area, with gusts of 25-35 mph possible especially Saturday morning and early afternoon.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

Deteriorating conditions tonight into Saturday.

Ceilings are expected to lower overnight, eventually reaching IFR conditions by about 12z and remain there through the rest of the TAF period. There is potential for ceilings to reach LIFR at times during the day Saturday as well. Rainfall spreads across the area overnight from south to north. Visibilities with the rain are generally forecast to be in the 3-5 SM range with some lower vis at times. Northeast winds are expected to increase overnight into Saturday morning, with gusts nearing 30 kts at AGS, DNL, and AIK beginning around 12z and lasting for a few hours before gradually decreasing. The strongest winds at CAE, CUB, and OGB are anticipated to be slightly lower.

EXTENDED AVIATION OUTLOOK...Periods of rain continues overnight Saturday into Sunday, leading to ongoing restrictions. Conditions begin improving later on Sunday and into early next week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Wind Advisory from 5 AM to 11 PM EDT Saturday for SCZ018-025. Flood Watch from Saturday morning through Saturday evening for SCZ018-025. Lake Wind Advisory from 5 AM to 11 PM EDT Saturday for SCZ016- 020>022-026>031-035-037-038-041-115-116-135>137. GA...Wind Advisory from 5 AM to 11 PM EDT Saturday for GAZ040- 063>065-077. Flood Watch from Saturday morning through Saturday evening for GAZ040-063>065-077.