Hurricane Helene strengthening as it enters the Gulf of Mexico

Helene squeezed through the Yucatan Channel on Wednesday morning, meaning that the inner core remained over very warm water.  As soon as it entered the Gulf of Mexico, it was upgraded to the season's fifth hurricane and is expected to intensify through landfall.  It is forecast to reach Category 3 status by Thursday morning, and could be a Category 4 hurricane by the time the center reaches the coast on Thursday evening.

Two important things stand out in that map above: 1) the wind field is expected to become very large which is why hurricane and tropical storm warnings extend so far away from the center and 2) it will be moving quickly on Thursday as it makes landfall, allowing extremely destructive winds to punch far inland before the storm's winds decay.  This is shaping up to be a catastrophic event for Tallahassee, and even up into Georgia as far as Atlanta and then northward into the Asheville NC region.

The suite of Hurricane Threats and Impacts graphics from early Wednesday morning are shown below, and illustrate the far-reaching impacts of the storm.  The cone is overlaid only for reference, but remember the cone is never used for showing where impacts will be experienced.  You can find the latest version of this on interactive maps at https://www.weather.gov/srh/tropical#hti

As far as preparation time goes, the map below shows the approximate time that tropical storm force winds could arrive (with probabilities in the colored contours), which is when outdoor work should be completed.

As I mentioned yesterday, I have some updating radar resources available at https://bmcnoldy.earth.miami.edu/tropics/radar/ to help monitor Helene.

The easterly wave out by Cabo Verde that I mentioned previously (Invest 98L) is still organizing and could become the next named storm later this week or weekend: Isaac.  Models continue to indicate a northward turn before it reaches any land.

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Helene forms in western Caribbean and rapid intensification is likely

On Tuesday morning, Invest 97L (later designated as Potential Tropical Cyclone Nine for the sake of issuing watches & warnings before it formed) was upgraded to Tropical Storm Helene.  This is the eighth named storm of the season, and will very likely become the fifth hurricane of the season by Wednesday afternoon.  Amazingly, this will be the fourth hurricane landfall along the US Gulf coast already this season, and it's not over yet!

The appearance on Tuesday morning is still lopsided and sheared, but as it moves toward the Gulf the wind shear is expected to relax and it should look much more symmetric.

The organizing storm is forecast to pass through the Yucatan Channel during the day on Wednesday, then encounter very favorable environmental conditions for rapid intensification.  It's quite rare for NHC to explicitly forecast RI (conventionally defined to be an increase in intensity of at least 35mph in a 24 hour period) because by definition, it's an outlier.  But they are, and not even as aggressively as some of the model guidance is.  Helene could easily go from a minimal tropical storm this morning to a Category 3 or 4 hurricane on Thursday before making landfall.

A nice alteration to the 22-year-old "cone of uncertainty" graphic is the inclusion of inland wind-related watches and warnings.  This experimental feature has been put to the test already this season, but not like it will be with Helene.  Helene is going to pick up speed in the Gulf and be cruising northward quite fast when it makes landfall.  This means that places hundreds of miles inland will still experience tropical storm and even hurricane-force winds before the storm's winds have a chance to decay. As the storm gets closer, these will extend further inland.

Helene's track, size, and intensity are going to help maximize the storm surge generated along the Florida coasts.  The Big Bend region of Florida is shaped to funnel water onto land when a hurricane approaches, and the current forecast shows the potential for 15 feet of storm surge in that area.

This set of four maps is the suite of Hurricane Threats and Impacts graphics produced on Tuesday morning.  Operational for every storm near land since 2015, these show threat levels for the four hurricane hazards: wind, storm surge, flooding rain, and tornado.  These excellent impact-based maps are interactive and zoomable, and you can find the latest version at https://www.weather.gov/srh/tropical#hti.  I've been a huge proponent of these since they were released experimentally in 2013 and I don't think they get enough visibility.

I have (and will add more) long radar loops to help track the structure and position of the storm at https://bmcnoldy.earth.miami.edu/tropics/radar/

By the way, Helene is still an original name in the set of rotating lists used for the Atlantic.  It first appeared on a list in 1982, so this is its 8th time being on the list of names, but it's only the 6th time being used (the 1982 and 1994 seasons never got to the H name).

Elsewhere in the basin, there is an African easterly wave out near Cabo Verde that NHC is giving an 80% probability of becoming the next tropical cyclone in the coming week.  Models generally indicate that this will turn to the north long before reaching the Lesser Antilles or any other land.  But there's plenty of time to keep an eye on it.  After Helene, the next name on this list is Isaac.

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Disturbance in western Caribbean taking shape and threatens Gulf coast

The disturbance that I mentioned last Wednesday is indeed organizing.  It's still not a tropical cyclone, but it has been designated as Invest 97L as of Sunday evening.  That designation triggers regional hurricane models to run on it, and facilitates aircraft reconnaissance data to be assimilated into the models.  Models are in excellent agreement on it forming soon (perhaps even later today) and intensifying into the season's fifth hurricane by Tuesday night or Wednesday morning as it enters the Gulf of Mexico.  The next name on the list is Helene.

Remarkably, this is likely going to be the fourth hurricane landfall along the US northern Gulf coast this season... Beryl, Debby, and Francine were the first three.  The timing of landfall is pretty tight among models: Thursday afternoon-evening.  But of course storm surge, heavy rain, and damaging wind will arrive before that and will be a concern from Jamaica to Cuba and the Yucatan, then the west coast of the Florida peninsula, then the Florida panhandle.

These model forecast tracks all agree on the worst case: the center remaining over the ocean the entire time and passing over the very warm Loop Current in the eastern Gulf of Mexico prior to landfall in the Florida panhandle.

A significant concern is the fairly high probability of it rapidly intensification in the Gulf of Mexico.  Not only is the sea surface temperature along its path a toasty 30-31C, the warm water runs deep, producing extremely high ocean heat content... an endless source of fuel.  A warm ocean isn't everything when it comes to hurricanes, but it's a lot.

https://bmcnoldy.earth.miami.edu/tropics/sectors/

After landfall, the storm surge threat diminishes quickly, but the threat of flash flooding from heavy rain will continue inland for a few days.  This map shows the latest rainfall forecast over the next week: an enormous area covered by 3"+ totals.  This will evolve as the storm's forecast evolves, but you can find the most recent version at https://www.wpc.ncep.noaa.gov/#page=qpf.

This could become the season's 8th named storm on Tuesday (September 24), the 5th hurricane on Wednesday (September 25), and if the models are right, the season's 2nd major hurricane on Wednesday or Thursday.  The average dates for those milestones are September 9, September 28, and September 19.

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