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WHAT HAS CHANGED

Hurricane Isaias strengthens and turns northward toward the Gulf Coast. Wind Advisory out for the CSRA Saturday. Aviation discussion updated for 18z 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 is fairly similar and in line with forecast thinking. High cirrus and now some virga is pushing in and will continue to grow in thickness as the hurricane approaches the Florida/Alabama coastline this evening. 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. 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 HRRR, HREF, and REFS all paint a similar picture with this banding setup, leaving the 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. Trusting the entire hi-res ensemble a bit more, the concern is that any of these heavy bands push into our area, especially in the CSRA which would present a increased flood risk. So the I-20 corridor in western SC and eastern GA should remain on guard if this precip banding decides to shift east; LREF-HREF-REFS probs of over 4" are about 30% in these areas, decreasing quickly to the east. 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. Confidence is increasing that the northern Midlands 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 /18Z FRIDAY THROUGH WEDNESDAY/

VFR conditions expected into Friday evening followed by deteriorating conditions Friday night and Saturday.

Mid-level cloud cover will remain in place as moisture steadily pushes in the from the south this afternoon and evening. Rain chances will hold off until later this evening and overnight, with cigs steadily falling after 00z eventually down to IFR Saturday morning. Rain will overspread the area throughout much of Saturday but will be periodic in nature with waves of heavier rain bringing IFR-LIFR conditions throughout the day. Winds will be strengthening out of the east overnight and especially Saturday with gusts over 15 knots tonight and over 25 knots Saturday morning and afternoon. Conditions will continue to remain IFR-LIFR through the end of the TAF period with gusty winds, rain, and low cigs lasting into Sunday morning.

EXTENDED AVIATION OUTLOOK...Strong gusty northeasterly winds likely throughout Saturday evening and Sunday along with periods of heavy rain and IFR conditions.

CAE WATCHES/WARNINGS/ADVISORIES

SC...Wind Advisory from 5 AM to 11 PM EDT Saturday 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.