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WHAT HAS CHANGED

Hurricane Isaias strengthens and turns northward toward the Gulf Coast. Aviation discussion updated for 06z 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. The 00z suite of guidance is fairly similar to yesterday's 12z suite. Isaias is beginning to accelerate to the northeast as it interacts with the background synoptic environment surrounding it. The hurricane is compact and intense, but it is expanding in size. High cirrus is overhead and will continue to grow in thickness as the hurricane approaches the Florida/Alabama coastline this afternoon. These clouds will continue to keep temperatures cooler than they would be otherwise. 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 & should begin 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. The question remains just how much rain we get and how/when the rain ends & begins again Saturday evening. There is some guidance (especially the RRFS but some other models too) that maintain the idea that a more focused area of convection will develop along the coast of SC due to a coastal low that develops along the wedge boundary. This would then focus a branch of moisture transport more along that boundary and less so pushing into Isaias' overall circulation. While the presence of a strong coastal thermal gradient will likely result in enhanced precipitation totals in the coastal plain, it seems unlikely that it will limit our overall rain totals like RRFS is showing. The guidance has tended to shift the most robust moisture transport slightly east over the last 24h, making this forecaster think that 1-3" of rainfall is the most likely outcome across the area. There could be locally higher totals especially in the coastal plain or western Midlands/CSRA - both the area closer to that coastal thermal gradient and the better moisture transport. But for now, guidance right now is in good agreement on that 1-3" rainfall total.

There are other potential hazards at play with this event. Tornadoes are certainly possible in this event, especially on Saturday 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, 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 it won't really be much of a threat. It should just be breezy across the area, with gusts of 25-35 mph possible especially Saturday morning and early afternoon. A Lake Wind Advisory has been hoisted for area Lakes, beginning 5a Saturday and running through 2p Saturday. Isaias should quickly move northeastward and out of the area by Sunday afternoon. Warm and dry conditions are expected for the remainder of the period.

AVIATION /06Z FRIDAY THROUGH TUESDAY/

VFR conditions expected throughout the forecast period.

High level clouds continue over the area overnight and into the day Friday. These clouds are forecast to gradually lower through the day, particularly late in the TAF period. Some guidance is indicating the potential for MVFR ceilings over the Augusta terminals at the tail end of the period. Rain chances should hold off until after 06z tomorrow. However, can't rule out some shower activity reaching AGS or DNL late. Light and variable to calm winds overnight pick up out of the northeast after about 16z, with gusts to around 15 kts possible. Winds are then forecast to increase after about 00z, gusting to 20 kts, as pressure gradients begin to tighten.

EXTENDED AVIATION OUTLOOK...The threat of restrictions and rainfall increases early Saturday morning as tropical moisture surges into the Southeast with Isaias. Strong gusty northeasterly winds likely throughout Saturday and Sunday along with periods of heavy rain and LIFR conditions.

CAE WATCHES/WARNINGS/ADVISORIES

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