The Eyewall

In brief: In this update on Hurricane Isaias we provide the latest information on the track and intensity forecast, as well as look at impacts, which could range broadly across a few southeastern US states.

Isaias status

As of 11 am ET on Thursday, Hurricane Isaias has reached sustained winds of 85 mph, and its central pressure continues to fall, now dropping to 975 mb. The storm is starting to accelerate, and is now moving to the east-northeast at 10 mph. It also appears to be in the middle of turning toward the north. So what happens next?

Where Isaias will go

By now our confidence in Isaias’ track is fairly high over the next few days. Observations are starting to closely match predictions, and the major uncertainty is whether there are any wobbles in the center’s position. Isaias should approach the northern Gulf coast on Friday evening, a little more than a day from now. Its landfalling position will likely be somewhere near Mobile, Ala., or Pensacola, Fla. From there the storm should accelerate to the north, likely tracking near Tuscaloosa, Ala., during the mid-morning hours on Saturday. This forward motion will be helpful in mitigating some of the worst inland flooding, but could bring damaging wind gusts to a larger area.

How strong Isaias will get

This is a trickier proposition. The storm is, and will continue to move over, some exceptionally warm water during the next 24 hours. Given the hurricane’s falling central pressure and sea surface temperatures, one would anticipate rapid intensification. However wind shear is not zero, and should really start to increase 12 to 24 hours from now as the storm gets closer to the northern Gulf coast. The National Hurricane Center predicts Isaias to reach 110 mph in sustained winds by Friday morning, before weaking a bit prior to landfall on Friday night. This is a reasonable forecast, but subject to some uncertainty. Isaias could be a bit weaker than this at landfall as the wind shear looks to be pretty stout.

What about impacts?

We are going to see the usual hits from Isaias in terms of hurricane threats: storm surge, wind damage, and inland rainfall. None of these look to be pleasant, of course, but absent the aforementioned shear they could definitely be worse.

Storm surge is obviously dependent upon the landfalling location, and the area of maximum surge (5 to 7 feet) could shift a little to the west or east as a result.

Much of Alabama and eastern Mississippi could end up seeing wind gusts of 50 to 60 mph from Isaias as it moves in on Friday night, into Saturday morning. This is likely enough wind to cause some widespread power line issues.

Inland rains from Isaias will be mitigated to some extent by its forward movement. Most of these totals should be manageable.

Due to the shear environment surrounding Isaias some of its strongest thunderstorms could occur well east of the system over Alabama, Georgia and northern Florida on Friday night and Saturday.

In brief: Tropical Storm Isaias is slowly strengthening in the Gulf this evening, as it likely becomes a hurricane by tomorrow. It will turn north and race toward the Gulf Coast between Biloxi and the Florida Big Bend on Friday night, making landfall, likely as a hurricane with wind and storm surge, as heavy rain and severe weather spread across the Southeast this weekend.

Tropical Storm Isaias has spent the day getting organized, and just around sunset this evening, it may look actually the worst it has all day. It’s pretty evident that wind shear is impacting the western flank of the storm right now. Satellite estimates are that it’s battling about 20 kts of shear near the center and substantially more than that on its northwest flank, hence the rather ragged appearance of the storm this evening.

Isaias is moving east northeast around 8 mph, away from the shear axis over the western Gulf, which means it should gradually find a little less shear over the next 12 to 24 hours. That’s when we should see Isaias reach its prime and strengthen into a formidable hurricane. Shear will once again begin to hammer Isaias on Friday morning, and this should be when we see it begin to weaken a bit.

First key point: While Isaias is going to fluctuate in intensity over the next 48 to 72 hours, the Gulf Coast, especially east of Mississippi and west of the Big Bend in Florida should be preparing for a hurricane, possibly a borderline major hurricane landfall.

In terms of how strong it gets, Google’s 1,000 member ensemble is still showing about a 75 percent chance of Isaias strengthening into a major hurricane at peak. Google’s operational ensemble is around 50 to 60 percent odds of peaking as a major hurricane. The tropical models lag Google by a bit, though even most of those still peak Isaias as a category 1 or 2 storm. Either way, this is sort of like debating the difference between calling it aluminum or aluminium. There are scientific and classification considerations here, but for all intents and purposes for the eastern and central Gulf Coast it seems it’s going to peak as a pretty decent hurricane.

So what about landfall intensity? As noted, on Friday, we’ll see Isaias likely get hammered by wind shear that worsens throughout the day as it approaches landfall in the evening. If you notice the chart below, the same intensity chart as above, I have us flagging the intensity dispersion right at or immediately before landfall.

Indeed, at or around landfall, about 25 percent of members maintain Isaias as a major hurricane and roughly half have it as a Cat 1 or tropical storm. But here’s the thing about that. Weird things often happen in October in the Gulf. This is why I would encourage folks to prepare for the worst case scenario, not hope for the best case outcome. We’re good at hurricane forecasting today, but we’re far from perfect. And there’s about a 1 in 4 chance that Isaias will come in near its peak intensity, assuming it can get to Cat 2 or 3. It should also be noted that Isaias will be booking it north at 15 to 18 mph in all likelihood, which may limit how much time there is for Isaias to substantially weaken before hitting land.

Key point 2: In this instance, given the accelerated timeline and fast forward motion of Isaias at landfall, it’s wise to prepare for the higher end outcome risk on the Florida Panhandle and in Alabama.

Where will Isaias make landfall? The tropical models like Biloxi, MS through Destin, FL as the landfall window. Google’s model seems to like Mobile Bay through Panama City, FL. This matters to a point because the worst impacts of Isaias are going to be exaggeratedly on the eastern flank of the storm. But basically, anyone east of Biloxi is on notice, with a special notice for folks between Mobile and Destin, FL right now. This could further change between now and tomorrow. Average track errors at this point in the system’s life cycle are still fairly meaningful.

This also matters for storm surge, as the surge near and east of where the center makes landfall will see surge levels of 5 to 7 feet above ground level, formidable. Keep in mind, we have lost more people to surge and flooding over the years than any other element of a hurricane. Take it seriously.

The good news is that the rainfall threat seems to be a little lower overall with this storm. Some areas east of where the center comes in could still see 8 to 10 inches of rain, but it appears that the flash flooding threat and inland extent of flooding, while a noteworthy element of this storm is not likely to a catastrophic one.

One thing I am becoming a little more concerned about with respect to Isaias is severe weather. As Isaias moves northward into the Tennessee Valley, it will spread rain and thunderstorms over a wide area of the Southeast. There are multiple areas within Isaias’s overall circulation that may be primed for some severe weather. Shear is going to be outrageously high. While wind shear is detrimental to a tropical system’s organization, it is an element that can enhance isolated to scattered severe thunderstorms by creating more potential for rotating thunderstorms.

Instability is going to be a big limiting factor, but sometimes with a tropical cyclone and very high shear, that lack of instability may not matter as much.

Key point 3: Some form of a severe weather outbreak may occur east of where Isaias tracks on Saturday or Sunday after landfall.

So the bottom line in all this:

1. * Folks on the Gulf Coast should prepare for a landfalling hurricane, possibly a borderline major hurricane, somewhere between Biloxi, MS and the Florida Big Bend, with the highest odds of landfall right now between Mobile and Destin.

• * Storm surge of 5 to 7 feet above ground at peak will be possible near and east of where Isaias comes ashore.

• * Landfall is likely to be Friday night in the affected area.

• * Heavy rain is likely across the Southeast resulting in pockets of flash flooding.

• * Severe thunderstorms and tornadoes are likely to some extent on Saturday and Sunday across the Southeast and perhaps into the Carolinas, but details are very fuzzy right now.

We’ll have more for you in the morning.

In brief: A quick update late Wednesday morning on Tropical Storm Isaias, which has an improving satellite appearance and could undergo rapid intensification during the next two days before landfall along the northern Gulf coast.

Isaias status

As of 11 am ET on Wednesday, Tropical Storm Isaias has 45 mph sustained winds, and is moving relatively slowly to the east-northeast at 8 mph. It is clear from the storm’s appearance on satellite that Isaias is getting better organized. We should expect significant intensification.

Put another way, in the modern era (post-World War II) the Atlantic basin has never gotten this deep into an Atlantic hurricane season without an actual hurricane forming. But we are likely to break the streak in the next 12 to 24 hours.

Current thinking on track

There continues to be a divergence in the physics-based models (i.e. Euro, GFS) and the AI-based model guidance. In terms of a landfall along the northern Gulf coast, the former models are closer to southwestern Louisiana or Mississippi, and the latter are more focused on areas such as Mobile, Alabama or Destin, Florida.

Following a reformation of Isaias’ center, I expect the various suites of models to come into better alignment today, which should allow us to increase our confidence in the track, and where the final landfall location will occur on Friday afternoon, evening, or early Saturday morning. Matt will have a more comprehensive post later today that I suspect will better be able pinpoint Isaias’ track over the next 48 hours.

What about intensity?

We are definitely concerned about the potential for rapid intensification with Isaias. Although it is early October, seas over the central Gulf are a few degrees Fahrenheit above normal for this time of year, and plenty warm to support strengthening. Isaias will also not encounter much dry air along the forecast track, and should enjoy a relatively low shear environment until later on Friday when it nears the coast.

So what does that mean? The National Hurricane Center presently forecasts that Isaias will reach a maximum of 110 mph winds on Friday morning before weakening slightly ahead of landfall. This makes Isaias a strong Category 2 hurricane, but it could easily be a category stronger or weaker at landfall. This will bring the potential for significant wind damage, and to the right of Isaias’s center, storm surge along the northern Gulf coast.

Coastal residents and businesses from Gulfport, Miss., to Apalachicola, Florida, should be paying close attention to Isaias. And inland residents, particularly in Alabama, should keep taps on the storm for winds and heavy rains on Friday and Saturday.

Much more later today.

In brief: Invest 92L is now Tropical Depression 9 and likely to become our next named storm, and it has a decent chance to become the season’s first hurricane. The storm is likely to impact the central or eastern Gulf Coast by the weekend, with a focus between Biloxi and Destin, FL right now. The potential exists for a landfalling hurricane or borderline major hurricane by the weekend on the Gulf Coast.

The forecast has slowly come into better view today for Invest 92L, which is now classified as Tropical Depression 9, and it appears, as we have been kind of warning about as a possibility, this one may be a potent one for the Gulf Coast.

The area of thunderstorms associated with TD 9 looks frazzled but robust this evening. We’re coming out of the daytime minimum for tropical system thunderstorm development and should see the depression begin to perhaps more aggressively produce storms overnight.

The center of Tropical Depression 9 is expected to drift eastward over the next day and a half as it gradually organizes. From there, we hit the inflection point where this possibly turns into something bigger. And indeed, support for something bigger is increasing today. Let’s start with what we know. We know that some of the best hurricane modeling overall, Google’s AI forecast has bumped up the potential for a hurricane and a major hurricane to approximately 90 and 45 percent respectively.

We know that whatever this is, assuming it generally follows the Google track, will eventually run into a lot of wind shear near the Gulf Coast by Friday night or Saturday, when it presumably approaches landfall.

That wind shear should be detrimental to the storm’s intensity (allowing for weakening on approach to the coast), but as I’ve noted on a couple occasions this week, we have seen late season cases where southwest to northeast wind shear can actually help the outflow of a storm and keep it fairly strong. So I think you need to watch this storm and assume it may come in close to peak intensity and prepare around that. That said, while 55 percent of Google’s broader (not operational) 1,000 member ensemble has the storm peaking as a major hurricane, only 22 percent make landfall as such. That seems to suggest it will come in past peak intensity. Still, it is wise to be cautious when any ensemble is flagging 22% of its members making landfall as a major hurricane.

We also know that it’s going to be traversing some of the highest oceanic heat content in the Gulf. While the Gulf itself is below average in terms of heat content (but above average in terms of water temperature), where this is likely to track will hit some of the most heat-rich water of the Gulf.

It won’t linger long over that patch of water, so it’s unlikely to be super meaningful, but it could be enough to tip the scales one way or another from tropical storm to hurricane or hurricane to major hurricane depending on the storm’s evolution.

We know a boatload of moisture is headed for the Gulf.

Rain totals aren’t eye popping, but they are over a broad area. Flash flooding is likely to be an issue in spots along the Gulf Coast, as well as inland.

We know that dry air could have a big impact on the storm’s western flank.

As the storm comes north and pulls in much drier, autumn air southward across Texas and Louisiana and into the western Gulf, we could see it begin to eat away at the western semicircle of the storm, and it’s possible we end up with a sloppy hurricane that has much greater impacts on the east side than the west, even more than usual.

We also know that this is probably going to lead to a bit of a potent setup for strong to severe storms in the Southeast after coming ashore. While details are not going to be worked out for a few days yet, the SPC did mention the potential for expanding severe risks Sunday and Monday in their discussion today.

So what does this all equate to?

• Tropical development is likely in the Gulf, with advisories almost certain to begin tonight or tomorrow morning.
• The system we are watching is likely to become a tropical storm, a strong tropical storm, and probably a hurricane at some point.
• There is about a 50/50 shot that this becomes a major hurricane over the open Gulf before the storm likely weakens a bit at landfall. (However, the risk of the storm making landfall at peak intensity is not zero).
• Heavy rain is likely along the Gulf Coast and inland, along with the potential for strong to severe storms Sunday and Monday across the Eastern U.S.
• The main target area for landfall appears to be between New Orleans and Panama City, FL, with a slight edge on areas between Biloxi and Destin.
• The storm may have exaggerated impacts on the east side of the center versus the west, meaning landfall, while not the only thing that matters, will be an important delineation for this storm’s worst impacts.
• The timing appears to be between Friday night and Sunday morning for the worst impacts and landfall.

Folks on the Gulf Coast, particularly from Biloxi through Destin really should be preparing now for the potential of a hurricane by Friday night. While details and specifics will change in the coming days, there’s a lot we can see already that’s concerning enough to make it make sense to begin to make preparations. More to come.

Let’s get this out of the way now: The next storm’s name is Isaias, a name that is recycled from the 2020 list, which did a lot of damage on the East Coast, and in some circles was a surprise that it was not retired. It replaced Hurricane Ike on the lists in 2014. It is pronounced “ees-ah-EE-ahs.”