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WHAT HAS CHANGED

Hurricane Isaias continues to strengthen in the Gulf. Aviation discussion updated for 12z TAFs.

KEY MESSAGES

- 1. An active weekend ahead as Hurricane Isaias moves inland to our west. Confidence in the extent of impacts is increasing but remains lower than normal.

DISCUSSION

Key Message 1: An active weekend ahead as Hurricane Isaias moves inland to our west. Confidence in the extent of impacts is increasing but remains lower than normal.

Overview: Moderate, to at times, heavy rainfall appears likely as Hurricane Isaias passes to our west on Saturday and into Saturday night. Breezy, coolish, and cloudy conditions are expected otherwise.

Discussion: Hurricane Isaias has developed in the south-central Gulf as of the 11p 10/7 advisory. Guidance continues to be in fairly good agreement on it intensifying quickly over the next 24-36 hours as it treks northeastward and begins interacting with an upper level trough pushing eastward out of Texas. There is still some spread between the AI & Physics-Based models on how far east Isaias eventually makes landfall. The impacts to the Midlands and CSRA are expected to be relatively unchanged based on which path it eventually takes.

Isaias will overspread the area with widespread high cloud cover over the next couple of days as the system interacts with a remnant trough over Texas. This trough will continue to shift the Isaias northeastward and eventually northward as the northern/southern stream of the jet streak merge over the Mid-Atlantic and everything gradually amplifies. The right entrance region of this upper level jet streak is forecast to overspread much of the southeastern CONUS by Friday afternoon, helping moisture advection to begin going to our south. As Isaias move northward, it will begin interacting with the overall synoptic pattern & is expected to expand in size, pushing up against a low-level high pressure system & develop a strong easterly branch of the low-level jet. This feature, in some form or fashion, is expected to overspread the area during the day on Saturday. As it does so, significant PWs on the order of 2-2.5 standard deviations above normal (2.2"+) will overspread the region.

The synoptic pattern associated with this, featuring high PWs, a broad entrance region of an upper level jet, onshore 850 hPa flow atop a strong surface high pressure, should favor widespread and heavy rainfall on Saturday. However, guidance is fairly bearish on overall rainfall totals. Part of this derives from some guidance wanting to develop a sort of coastal low or more focused trough along the larger scale in-situ wedge boundary that will be in place along the coast. It is not entirely clear why this develops; the extra-tropical transition of Isaias will be well to the west of this boundary. This setup should not yield a coastal low, but in some model scenarios, a weak one develops and focuses moisture transport vectors much more in the coastal plain and nearer to the boundary along the coast as opposed to being more in-line & attached to Isaias' movement. This plays a role in how much rain we may receive, as we will need isentropic lift and warm advection to be the primary forcing mechanisms for rain development across our area.

Regardless of the magnitude, a rainy day does look to be on tap on Saturday. We should see rain overspread the area in the morning and last in waves through at least Saturday evening/overnight as the best warm advection and onshore low- level flow shifts northward. Overall totals, as mentioned before, are uncertain. Guidance is not tightly clustered around a solution, with LREF members presenting a fairly large spread amongst guidance. Of note is the long right-shift in the distribution of ensemble solutions, indicating still the potential for heavy rainfall (3"+) on Saturday. However, the majority of members suggest that 1"-3" of rainfall is most likely across the area, with the heaviest totals expected in the eastern Midlands. As with any tropical cyclone, a tornado threat appears possible, especially if the aforementioned front can shift to our north. We'll likely need 70F+ surface dewpoints to help any circulation make it to the surface. There will be plenty of shear in place, so if we can get convection south of this boundary on Saturday night a tornado threat could materialize. Emphasis on could there, as confidence isn't too high in getting convection south of the strong warm advection regime earlier in Saturday.

Isaias looks to remain well west of the region, and its greatest impacts are forecast to remain there as well. However, that doesn't mean that we won't see impacts here locally. Keep an eye on the forecast over the next couple of days!

AVIATION /12Z THURSDAY THROUGH MONDAY/

Expect predominantly VFR conditions with some patchy MIFG possible early this morning.

Mid-upper level cloud cover across the TAF sites has generally mitigated any issues with fog this morning. Still dew point depressions are not high across the area, so some morning MIFG around sunrise is possible, mainly at AGS-CAE-OGB with little to no impacts in terms of restrictions. The rest of Thursday will be dry with light and somewhat variable winds across the area today, varying from east to northeast. Mid-level cloud cover will likely hang around but remain above 10k feet.

EXTENDED AVIATION OUTLOOK...The threat of restrictions and rainfall increases late week and into the weekend as tropical moisture surges into the Southeast with Isaias.

CAE WATCHES/WARNINGS/ADVISORIES

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