The Eyewall

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.”

In brief: Tropical development chances are trending up over the southwest Gulf this week. While we don’t have a fully coherent picture of what will occur, we can begin to speculate a bit on how dry air, wind shear, and the track will play out. Folks on the central and eastern Gulf Coast from just west of New Orleans through the Big Bend region of Florida will want to monitor the progress of Invest 92L.

Our disturbance to watch has been classified today as Invest 92L, and it’s an area of thunderstorms in the Bay of Campeche.

Nothing yet. Just some unsettled weather. Over the next couple days, this is expected to gradually become better organized and a bit stronger. As of this afternoon, the NHC has pushed up the potential for development to 80 percent.

So what’s going to go on here. Like I said, we expect this whole area to become a bit better organized through the week in the Bay of Campeche, and by Wednesday afternoon or so, we should have a fairly coherent area of low pressure there.

It’s at this point that the models begin to diverge wildly. The GFS goes ballistic. The Google AI model begins to gradually develop it, as does Google’s 1,000-member ensemble. The AIFS model seems to follow Google’s solution, albeit weaker (though the AIFS is known for poor intensity forecasting). The European operational model peaks it as a tropical storm and weakens it on approach to the Gulf Coast, and the Euro ensemble is a mixed bag of all of the above, though with a lean toward an AIFS type outcome. So the number of solutions on the proverbial board are plentiful.

I feel like the operational version of Google right now feels like a healthy compromise. Still a chance nothing comes of this, but also a 30 percent chance of a hurricane, or perhaps a 5 percent chance of a major hurricane. We’ve seen model guidance come into better agreement today, and we will want to study trends tonight for any sort of signals we can pull.

In the meantime, it seems like a significant rainstorm will precede a potential tropical storm or hurricane impact this weekend on the Gulf Coast, somewhere east of central Louisiana.

I don’t want to overlook the heavy rain impact of this system because that’s going to happen whether this is a tropical storm, hurricane, or nothing at all. So, from that point of view, we’re looking at a potential for some areas of flash flooding on parts of the Gulf Coast this week.

But whether or not this organizes still matters from a wind, surge, and honing of the heavy rain forecast. It appears the system will fester in the Bay of Campeche for a couple days before beginning to turn eastward or northward by later Wednesday or Thursday. There will be a lot of things influencing and dictating the outcome here. For one, wind shear, which is expected to be plentiful in the northwest Gulf but less so in the central Gulf.

Keep in mind that wind shear has not always acted as destructively on tropical system intensity as expected, especially along the Gulf Coast and in recent years. While shear is generally going to act against development and strengthening, there have been cases where northward moving storms toward the Gulf Coast have had their outflow aided by wind shear leading to maintained intensity or even strengthening. We can also get weird behaviors when storms are transitioning more toward extratropical cyclones than classic hurricanes. In other words, while wind shear is going to be a topic with this system, if it can organize in the next few days, I would not be fully convinced that is necessarily a negative. That said, this does look rather strong, and it may be a bit tough to fully overcome.

In addition to the wind shear as a result of the pattern over the Gulf and Southeast, there’s also the situation regarding the next Pacific storm, which will likely be named Simon. If that can outdevelop Invest 92L, it could impart more destructive shear on 92L, leading to weaker development. This seems be a low likelihood (but not an impossibility) right now.

Last, we always need to watch to see if dry air is going to be an issue. Much of Texas and the Gulf Coast has or will see rain, but some notably drier air pushing offshore of Texas is likely to get drawn into the storm in the Gulf. This will probably yield at least a somewhat lopsided system, with much greater impacts east of the center than west.

So, we have a long way to go here. At this point it looks like whatever it is will sweep north over the weekend. That leaves us a couple days to watch developments here before folks would need to prepare in earnest anywhere, if at all.

Bottom line:

• Gulf development risk is increasing.
• Potential ranges anywhere from a sloppy, disorganized system to a major hurricane.
• A “worst case” type outcome is unlikely here because of the hurdles related to dry air and wind shear.
• Likely track aims to between southeast Louisiana and the Florida Panhandle.
• Regardless of development, heavy rainfall is likely on the Gulf Coast this week and weekend.

Stay tuned here and to your other favorite sources of weather information for updates in the coming days.