Matt Lanza

A Monday evening update on PTC 9/Helene and a word about Tampa

Headlines

  • No major forecast changes with PTC 9/future Helene this evening.
  • A major hurricane is now explicitly forecast by the National Hurricane Center.
  • Watches now extend up to the Florida Keys.
  • Impacts in Tampa are expected to include tropical storm force winds and some degree of storm surge. This post examines how bad that could theoretically get and how that might happen.

Helene/PTC 9 forecast update

The overall forecast is generally steady this evening versus this morning, so not a whole lot has changed. The NHC is now explicitly forecasting Helene to become a major (category 3+) hurricane on approach to the Florida Panhandle.

PTC 9’s forecast is mostly unchanged this evening, except now showing a cat 3 major hurricane on approach to Florida. (Tomer Burg)

The current forecast track takes it to the west side of Apalachee Bay, which would likely produce a substantial, very bad surge in Apalachee Bay and points east around the Big Bend and Nature Coast. With some models indicating this could get even stronger, that surge risk is going to be extremely bad. I need to underscore that for folks in Florida’s Big Bend, we know you have been through several storms in recent years. This one has a higher ceiling than those it would appear, meaning the surge impacts could be as bad or worse than anything you’ve experienced in the last several years. Just keep that in mind.

The timing is mostly unchanged today, and we’re looking for a Thursday evening landfall in Florida right now. The cone extends from about Destin through Tampa. It’s important to recognize that 72 hour forecast errors are significant enough usually that storms can deviate considerably. Just ask Houstonians how Beryl’s expected track into Corpus Christi and South Texas changed this year. So if I lived in Tampa right now, I would be taking this as seriously as anyone on the Panhandle or Big Bend.

Let’s talk about Tampa Bay risks

Assuming a track as forecast by the NHC, tropical storm force winds are likely in Tampa, and there is some potential for hurricane-force winds on the immediate coast in Pinellas or Pasco Counties.

Current wind threat map from NWS for the Tampa-St. Pete and Sarasota areas. (NOAA NWS)

The issue is that if this shifts any farther east, which is something the ICON model hints at with a landfall more in the Big Bend than in Apalachee Bay, then the risks increase drastically in Tampa both from a wind and surge standpoint.

Speaking of surge, the NHC publishes storm surge inundation maps that show reasonable worst-case scenario outcomes. These maps are not the *likely* case, but they are the outcome that could occur in a worst-case, something that has about a 1-in-10 chance of occurring. For Tampa, this would sort of like if the ICON model came to pass, and the storm made landfall just up the coast rather than in the Panhandle.

Reasonable worst-case scenario flood inundation map for Tampa Bay if the storm track presumably comes much closer/stronger than is currently forecast. (NOAA NHC)

The map suggests some rather significant inundation potential in and around Tampa Bay in this worst case outcome. This is one reason why I want to just make sure that folks in Tampa take this very, very seriously. As noted, this probably won’t happen, especially if the current forecast remains unchanged, however there is still enough uncertainty out there that it’s not a scenario I would be willing to discount at this point.

Anyway, this is one of many different storylines we need to follow with this storm, including the coastal impacts in Florida at landfall, the inland impacts in Georgia and South Carolina and elsewhere from wind and rain, the east coast impacts, and so on.

Currently, Hurricane Watches are posted between Cabo Catoche and Tulum on the Yucatan and Pinar del Rio in Cuba. Tropical Storm Warnings are in effect for Grand Cayman, much of the eastern Yucatan, and Artemsia, Pinar del Rio, and the Isle of Youth in Cuba. Tropical Storm Watches are in effect for the Keys. Those will be expanded northward later.

We’ll have more for you in the morning.

Potential Tropical Cyclone 9 likely to become significant Hurricane Helene with widespread Florida & Southeast impacts

Headlines

  • Potential Tropical Cyclone 9 is likely to become a depression and/or tropical storm by tonight or tomorrow morning.
  • It will accelerate north toward the coast of Florida, with a landfall likely on Thursday somewhere between Pensacola and Tampa. Hurricane impacts will spread across Florida’s west coast and Panhandle.
  • PTC 9 or Helene is expected to be a significant storm and potentially a major hurricane (cat 3+).
  • Because of fast forward motion, PTC 9 or Helene will spread damaging wind far inland from the coast, across South Georgia, possibly into the Atlanta area (depending on track) and into the Carolinas.
  • PTC 9/Helene will be a significant, disruptive storm for the Southeast Thursday, Friday, and possibly beyond.

Give our site a bookmark and check back in later today and/or tonight for some additional updates to this post.

Potential Tropical Cyclone 9

The forecast track for PTC 9 suggests a storm tonight or tomorrow morning, and a hurricane by Wednesday, with impacts beginning in Florida by Thursday. (NOAA NHC)

We have our potential tropical cyclone now, and it will be known as PTC 9. Expect this to formally become a depression later today and a tropical storm tonight or tomorrow morning. This should take on the name Helene.

The first forecast track from the NHC is very much in line with strong model consensus right now, which takes Helene somewhere between about Pensacola and Tampa, with the majority of guidance favoring the Panama City to Cedar Key corridor, close to the Big Bend of Florida. One thing to be very, very clear about: There is still a fair bit of spread among individual ensemble and tropical model members. You can use the slider here to compare the 6z runs of the European and GFS ensemble member forecasts for Thursday around midnight.

The 6z European ensemble (white background) and GFS ensemble show clustering of tracks in the Panama City-Cedar Key corridor with some tracks as far southeast as Sarasota or Fort Myers and some back closer to Pensacola. (Weathernerds.org)

The latest tropical models are very clustered on the west side of Apalachee Bay, with a track very near Tallahassee once inland. So you can see that the model guidance is in great agreement right now on the broad picture of things. I would still watch this very closely in Tampa and Sarasota, as well as Pensacola, as those locations are in the “margin of error” as you might say.

There is also a fair bit of cross-track spread in timing, with some models showing landfall as early as Thursday afternoon and others more into Thursday night. Assume you have until Thursday morning to finalize your preparations on the coast. Anything beyond that is just proverbial icing on the cake.

PTC 9’s intensity outlook

One thing we are unfortunately continuing to be confident in is that this storm has a high ceiling in terms of intensity. There remains a significant spread within tropical model guidance with intensity, as some models suggest tropical storm intensity, while others are at the top end of major hurricane intensity, perhaps a higher end cat 3 or cat 4.

Model guidance shows a substantial spread in intensity forecasts with a handful just at tropical storm intensity, while some more hurricane-focused models show high end Cat 3 or Cat 4 potential. (Tomer Burg)

The NHC projecting a peak intensity of around 110 mph, just on the cusp of major hurricane strength is a good place to be right now, but do realize that the risk for a stronger storm is very much on the table. As we’ve been noting, this track is about the worst possible one a storm could take from a “fuel” standpoint in the Gulf of Mexico. PTC 9 is likely to travel over significant ocean heat content the majority of its life cycle, and water temperatures never really cool off much on approach to landfall. Given the anticipated forward speed of PTC 9, we are likely to see this storm come ashore at or near peak intensity.

The forecast track from the NHC underlaid by ocean heat content and water temperatures shows a storm that is likely to spend most of its time over some of the warmest water in the Gulf or Atlantic. (Tomer Burg)

Folks in Florida should be preparing for a major hurricane impact in the Big Bend or Apalachee Bay with all the threats you’d expect: Surge, wind, waves, flooding, etc. This has the potential to be as bad or worse than Idalia and much worse than Debby in the Big Bend.

Inland risks

The threats from PTC 9 are going to extend much farther inland than usual. The storm will be hauling as it moves inland. Even if it comes ashore weaker than forecast right now, it is likely to carry tropical storm and hurricane force winds well inland from the coast. Those impacts should begin Thursday night and continue much of Friday. This includes much of South Georgia, as well as possibly South Carolina, north Georgia, and portions of North Carolina or Tennessee. The map below shows wind gusts as forecast by the GFS model earlier today. Keep in mind that the final track will determine exactly where those winds occur. A track closer to Tallahassee will shift this farther west. A track closer to Cedar Key will shift this a little to the east.

Tropical storm force winds are likely to extend far inland from PTC 9, with widespread wind damage possible across Georgia and into South Carolina. (Weather Bell)

This is important because this type of scenario could mean widespread, significant power outages and wind damage across Georgia and South Carolina. Metro areas, including Tallahassee, Atlantic, Macon, Savannah, Columbia, Charleston, Greenville-Spartanburg, and Charlotte should be prepared for this scenario and prepare with at least a few days of food and water should this come to fruition.

In addition, heavy rainfall will occur with PTC 9, and flash flooding will be possible, especially in mountainous areas well inland from landfall.

Significant rainfall is likely well inland from landfall with PTC 9, and flash flooding is likely. Thankfully, the forward speed may help mitigate total rainfall a bit. (Pivotal Weather)

This is going to be a significant, disruptive storm for portions of Florida and the Southeast U.S., in addition to perhaps western Cuba and the Cayman Islands where 10 to 12 inches of rain may fall. Impacts will be widespread and damage could be significant. Folks should begin preparing immediately and follow forecast changes closely in the days ahead.

Increasing risk of a central or eastern U.S. Gulf Coast impact from a tropical system late this coming week

Headlines

  • Odds of tropical development next week are increasing, likely beginning Tuesday or Wednesday.
  • There is a growing consensus among modeling that this system has the potential to strengthen a good bit as it comes northward and is likely to accelerate toward the central or eastern Gulf Coast late in the week.
  • The odds of a system getting buried in the Bay of Campeche are lessening.
  • The odds of an impact to Texas are lessening.
  • People on the Gulf Coast between Louisiana and Florida should be following forecasts closely and beginning to think about their plans for the upcoming week should they need to evacuate or hunker down.

Odds of development increasing

The National Hurricane Center continues to raise the odds of tropical development over the next seven days. We’re now up to a 60 percent risk of development.

Tropical development odds continue to inch up, with development possibly initiating around Tuesday of this upcoming week. (NOAA NHC)

Again, this is a rather conservative view of things, and I anticipate we’ll see this continue to inch up today and tomorrow and push into the “2 day” development map tomorrow or Monday. Modeling is beginning to coalesce around the idea of development north of Honduras or just east of Belize, or on the Caribbean side of the Yucatan. The general theme of the models since yesterday has been to align more with the faster, stronger solutions which would be apt to bring a storm north or north-northeast across the Gulf of Mexico and toward the coast between Louisiana and Florida. There is still a good deal of uncertainty on specifics here, including whether or not it develops. There is still a minority of ensemble members in modeling that struggle to get this going. Not likely to happen, but it’s a non-zero possibility. I just want to highlight that there are reasons this isn’t a 100 percent chance of development right now.

A potential threat to the U.S. Gulf Coast

All that said, I just want to make clear right now that this has the potential to be a strengthening storm in the Gulf, moving north, fairly fast later this coming week. When Francine moved into Louisiana it passed over warm but not super warm waters in the central and western Gulf of Mexico. If this upcoming system tracks farther east of Francine as seems likely, it will be passing over much warmer Gulf waters.

The Gulf of Mexico as a whole is just shy of record warm levels, and most of that is being driven by the waters in the eastern Gulf. (University of Arizona)

The near-record warmth of the Gulf is being driven almost entirely by the eastern half of the Gulf of Mexico, close to where this system will likely pass.

I don’t want to overstate the risk at this point, but given the scenario (Quick moving, very warm water, generally favorable upper level conditions, and less dry air than Francine contended with), there is some legitimate risk of a significant storm on the central or eastern Gulf Coast late next week. You should be following this forecast closely if you live between Florida and Louisiana, and you should be ready to put your hurricane plans into motion quickly this week once we see further agreement on possibilities.

So what do the models say?

We have seen the operational models reduce the spread between each other considerably in the last 24 hours. We still have some variability on track and timing, but there has been a tendency to push toward a track northward between Louisiana and Florida sometime between Thursday and Sunday next weekend. The odds of the system getting buried off the Yucatan in the Bay of Campeche has dropped off some since yesterday.

Today’s European ensemble at 12z, hot off the presses still shows a very wide spread of possibilities, from a system getting buried in the Bay of Campeche to one tracking southeast into South Florida. But what I pay attention to with these runs is what has changed or where the “clustering” is occurring. If we compare the same image at the same time 24 hours ago, we can see a shift in both the ensemble mean (solid black lines) to the northeast and a decline in the number of Euro ensemble members that bury this system in the Bay of Campeche.

A comparison of yesterday’s Euro ensemble mean and individual member forecasts for next Thursday evening versus the same view today shows a marked shift to the northeast with the mean and fewer members in the Bay of Campeche, an indication that trends are shifting north and east in the Euro, closer to what the GFS suite has been showing. (Tropical Tidbits) (Editor’s note: This image was adjusted at 4:45 PM CT Saturday to align the forecast times which were previously askew by 6 hours)

This further indicates that there is a definite trend toward a faster and more progressive storm to the north or northeast within the modeling. We can see further evidence of this in a consistent GFS, the ICON, and the Euro AI model (AIFS) showing similar type of outcomes now.

Why is this? Since yesterday, we have seen a bit of a signal in modeling for a cutoff low or secondary trough to develop over Arkansas or Oklahoma. This is something of a new wrinkle.

The last 6 runs of the Euro ensemble mean showing a developing and farther east trough or cutoff low over Arkansas that is likely helping to “pull” the tropical system straight north. (Tropical Tidbits)

Whatever the case, this slightly stronger and farther east trough is helping to “pull” at the tropical system in the Gulf and force it to the north. This is what is likely shifting the Euro ensemble to the side of the GFS and other modeling now and why we’re seeing some additional agreement develop on a threat to the central or eastern Gulf Coast.

The bottom line: There is building agreement in the modeling that a tropical system is going to come north late this coming week toward the central or eastern U.S. Gulf Coast. While details remain elusive there is enough evidence of a threat to that region that folks should be paying close attention to forecasts and thinking about their plan for later this week should they need to evacuate or hunker down. We’ll have more on this tomorrow.

Trying to narrow down the development setup next week near the Yucatan

Headlines

  • We are likely to see development in the Caribbean or near the Yucatan next week.
  • Model guidance suggests that there are a few possible outcomes, including a quicker, stronger storm that tracks north or northeast or a slower, weaker outcome closer to the Yucatan.
  • Folks on the Gulf Coast, particularly between New Orleans and Florida should be flexible with their preparedness plans and be ready to implement them if necessary early next week.

We are still a few days from Caribbean development

I feel like we’ve been talking about this potential development in the Caribbean for days now. That’s one element of hurricane season that has gotten worse in recent years: The availability and democratization of weather data has kept any and every potential disturbance in the conversation for a week or two before they even form, then for a week or two as they do their thing. In some cases, you can be talking about one system from pre-inception to finish for a month! But the availability of all this information has made our job more important to help y’all make sense of what it’s saying.

The probability of Caribbean development remained at 40 percent this morning. We’ll see if the afternoon update in a bit increases those odds. (NOAA NHC)

Yesterday’s post has aged pretty well, thankfully. Frankly, not a whole lot has changed. But I think there’s a clear dichotomy taking shape now between the European modeling and the GFS modeling Yesterday, I noted how the Euro was apt to consolidate the disturbance next week near or over the Yucatan, whereas the GFS did so mostly in the Caribbean. Those models have not changed their view on this today. Trends over the last couple days have led to the European AI modeling, the AIFS to trend a little closer to the GFS solution of stronger, faster, more north and east. The ICON model is also taking that stand, so there is some slowly building support for the GFS here, I think.

The last 5 AIFS runs showing an erratic but general trend toward the more eastern outcomes for late next week. (Tropical Tidbits)

That said, if we look at ensemble guidance, there continues to be a gigantic split among the 30 to 50 ensemble members in both the GFS and European suite. The Euro seems to be split about 80/20 favoring the western/weaker outcome, whereas the GFS is more of a 40/60 blend favoring stronger and more east/north leaning outcomes.

Latest 12z GFS Ensemble outlook shows about a 60/40 or 70/30 type split in outcomes favoring a stronger, more east and north type track over a system that stays stuck near the Yucatan. (StormVista)

It may have even gotten more confident in that eastward lean in today’s 12z model guidance as seen above. The one big, big takeaway from both of these examples showing the AIFS and GFS Ensemble members is that there is still *very* little agreement on track, just many generalizations that can be inferred from this output.

I continue to think this sort of scenario favors either a Mexico or Yucatan impact or a Florida through New Orleans type impact on the northern Gulf Coast. I remain convinced that Texas is *not* the most likely outcome here, but I would continue advising Texans to monitor things heading into next week.

There are also gigantic timing differences here. The storms that come north and northeast tend to do so quickly, with impacts possible as early as next Thursday. So for folks in Louisiana through Florida, you would need to be ready to act as soon as Monday if this looks like a building threat. For Texas and Mexico, if the western outcome happens, this would likely remain a middling system for several days before organizing later next week and coming north next weekend. This would deliver impacts to the central Gulf Coast by next Sunday or so. That’s a large spread of timing options, so folks on the Gulf Coast should be flexible with their preparedness and be ready to act early next week once things become clearer.

We will have daily posts this weekend to keep you all informed.

Elsewhere in your tropical Atlantic!

Invest 96L has developed in the open Atlantic, and between it and the remnants of Gordon, we could maybe see something try to form over the next few days. Neither system is likely to impact land.

More interestingly, we have potential for a Cabo Verde system to develop later next week. The European ensemble has been quite bullish on this. There is plentiful support for this from other modeling as well. Expect this potential to show up around midweek next week.

The Euro ensemble, as well as other models show a growing likelihood of development later next week off Africa. (Weathernerds.org)

Now, is this system likely to be a land threat? It’s too early to say definitively, but the current thinking is that this would probably end up moving out to sea ultimately. Plenty of time to watch. No other threats are seen closer to home right now after next week’s Caribbean/Gulf story.