Kirk forms, watching western Caribbean (again)

Since my previous post on Friday morning, Hurricane Isaac has transitioned to an extratropical cyclone north of the Azores, Invest 98L did become Tropical Storm Joyce but that is already on its way out, the easterly wave near Cabo Verde became Tropical Storm Kirk today, and there's another strong easterly wave trailing behind Kirk that has a high probability of formation (Invest 91L). We are still cautiously watching a broad disturbance in the southwestern Caribbean for signs of development.

Overview image showing the five areas of interest, overlaid on the sea surface temperature.  Isaac and Joyce are essentially done, Kirk just formed, and there are two other areas of potential development in the coming week.  Plot from Tomer Burg.

Let's start with Kirk.  This was Invest 98L on Friday, then was upgraded to Tropical Depression 13 on Sunday.  It was just upgraded again to Tropical Storm Kirk on Monday morning, making it the 11th named storm of the season.

Kirk is going to be moving into a very favorable environment for intensification, and is forecast to become the season's 3rd major hurricane in just a couple of days.  Thankfully, models are in excellent agreement on it turning north and staying way out over the open ocean.

The wave behind Kirk, tagged as Invest 91L, is currently centered just south of Cabo Verde and NHC is giving that a 90% probability of formation within a week -- models show this also becoming a strong storm in a few days and also likely to turn to the north, though perhaps waiting a bit longer to make that turn.  There is enough spread in the model ensembles that the Windward Islands may need to keep a close eye on this in 10-11 days, for what that's worth.  It's way too far out to worry about any details.  The next name on the list is Leslie.

ECMWF model ensemble spanning 2-10 days into the future.  Small red numbers mark the central pressures of trackable low pressure systems in the ensemble.

Last, let's shift west and look at the disturbance brewing in the western Caribbean.  NHC is giving this a 40% chance of development within a week, and the models have backed off a bit on intensification in the Gulf over the past 1-2 days.  These maps below show the tracks from the GFS (left) and ECMWF (right) ensembles through next Monday.  While there are plenty of weak and dissipating members, there are a few that merit continued attention.  *IF* it develops, a track toward Florida next Sunday-Monday is not out of the question among the stronger members.  Assuming Invest 91L takes the name Leslie in the coming days, this (if ever named) would be Milton.

In terms of overall Accumulated Cyclone Energy, the season is at about 83% of average for the date, but that will increase notably once Kirk is a major hurricane later this week.  The last time we had such an inactive June-September period was 2016.  That's especially remarkable considering how explosively this season began.

 

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A hurricane, a tropical storm, and three areas of interest pepper the Atlantic

Today's post features Tropical Storm Helene, Hurricane Isaac, and what could be the next three named storms scattered all over the Atlantic.

First, Helene rapidly intensified on Thursday at an impressive rate.  Peak winds increased from 85 mph on Wednesday night to 140 mph on Thursday night, and amazingly, from 105 mph on Thursday morning at 11am to that 140 mph just twelve hours later.  To qualify for the conventional definition of "rapid intensification", the intensity needs to increase by at least 35 mph in a 24 hour period... Helene did that in just 12 hours.  The infrared satellite animation below spans 3am through midnight EDT on Thursday.

This is not only the fourth hurricane landfall on the US Gulf coast already this year, it was the most intense hurricane to ever make landfall in the Big Bend area of Florida.  The storm's huge expanse of strong winds helped it to generate record-breaking storm surges along the west coast of the Florida peninsula, and almost certainly the Big Bend area too (official numbers take time to get because ground crews need to carefully survey high water marks over a large area, sometimes in remote or hard-to-reach locations).

As expected, strong winds punched far inland, and as I write this the damage is still unfolding and being discovered.  Now that it has made landfall, it will rapidly lose its tropical characteristics, but it doesn't go away... there are still a couple days of trouble ahead in the form of flooding rains and multiple tornadoes.

Early Friday morning, Isaac was upgraded to the season's 6th hurricane and is still located in the far north-central Atlantic.  The NHC forecast is shown below... it now indicates that Isaac will stay rather far to the northwest of the Azores late this weekend.

Next, the African easterly wave I mentioned in the deep tropics east of the Lesser Antilles (Invest 98L) is extremely close to becoming the next tropical cyclone this season.  It could be upgraded to Tropical Depression 11 or even Tropical Storm Joyce any time today.  It is still very likely going to turn north into the central Atlantic well before reaching the Lesser Antilles, so no threat to land.

While on the topic of easterly waves, the yellow area we see in the overview at the top of the post is the next wave that has a low chance of developing in the coming week, so for now, I won't go into detail on it.  The wave just left the African coast yesterday.

Then, the area highlighted over the western Caribbean has a 30% chance of development in the coming week, according to NHC.  Model ensemble forecasts are very hazy on the long-term future of it, but for now, it's something the Yucatan peninsula and the Gulf coasts of Mexico and the US will want to pay attention to in the coming week.

Finally, here's an update on the overall activity so far this season.  We have now had 9 named storms, 6 hurricanes, and 2 major hurricanes.  The average by this date is 10, 5, and 2.

In terms of Accumulated Cyclone Energy, after an explosive start, the season at just 78% of average for the date.  

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Helene headed for Florida tonight, Isaac forms, and Joyce could form by the weekend

The Atlantic has sprung to life, and there's a lot to cover today.  The primary topic and only threat to land is Hurricane Helene in the eastern Gulf of Mexico.  Then there's newly-formed Tropical Storm Isaac in the north-central Atlantic, and possibly future-Joyce east of the Lesser Antilles.

The map below shows the Thursday 11am EDT advisory for Hurricane Helene from the National Hurricane Center, with the amazing spread of hurricane and tropical storm warnings covering a wide swath and penetrating far inland.

Helene has not rapidly intensified yet as was feared, but is a powerful Category 2 hurricane that should continue to intensify in the remaining hours before landfall on Thursday evening.  It is abnormally large, and tropical storm winds extend as far as 345 miles to the east of the center as of Thursday morning.  The Thursday morning set of Hurricane Threats and Impacts (HTI) graphics are included here for reference, but you can always find the most current version with interactive maps at https://www.weather.gov/srh/tropical#hti.  I overlaid the "cone of uncertainty" to illustrate that the cone is not (and never was) meant for showing where impacts will be experienced.  In fact, the vast majority of impacts shown are outside of the cone!

The storm will be traveling rather quickly tonight as it makes landfall, and that will allow hurricane-force winds to reach far inland, much more so than usual -- notice how far inland those hurricane warnings extend on the NHC forecast map.  On top of being a strong hurricane, the large size will help it generate extremely high storm surges in the Big Bend area of Florida.

Remember you can find a long, updating radar loop of Helene as it approaches the coast and moves inland at https://bmcnoldy.earth.miami.edu/tropics/radar/

Next, Isaac is a small storm that had a somewhat complicated evolution... it's been "on our radar" for several days, but clearly lacked tropical characteristics, until Wednesday night when it was named.  It's way up at 37N and headed east, so not a threat to land yet, but could reach the Azores by the end of the weekend.  The NHC forecast brings this up to hurricane intensity this weekend, which would make it the 6th hurricane of the season so far.

This the 9th named storm of the season, and Isaac is still a name from the original set of six lists, first appearing in 1982 and every six years since then.  However, the 1982 and 1994 seasons never reached the "I" storm, so this year is only its 6th time being used.

Then, the easterly wave I've been mentioning for several days is very close to becoming a tropical depression or tropical storm.  It's centered about 1600 miles east of the Lesser Antilles and is forecast by models to strengthen in the coming days.  However, models also indicate it will turn north way before reaching the Lesser Antilles, so it should not be a threat to land.  The next name on the list is Joyce.

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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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Watching western Caribbean for activity next week

There is a growing signal among long-range global model ensembles that the next storm could start brewing in the western Caribbean in the coming days.  There is nothing to look at yet, but a broad low pressure system could take shape on Friday-Saturday east of Honduras then consolidate and track north from there.  The next name on the list is Helene.

This animation below shows trackable low pressure systems evolving from today through next Thursday from the most recent run of the American global model (GFS) ensemble.  Definitely don't pay attention to details, but you get the general idea of the formation area, and the approximate spread of possibilities as it heads north.  Based on this, the spread is roughly centered on the Florida peninsula with some members out over the Bahamas and out over the Gulf of Mexico by the middle of next week.  The European model's ensemble is similar but everything is shifted slightly west.

The key at this point is to monitor the various ensembles and look for inter-run and inter-model consistency and trends.  But for Cuba, Bahamas, Florida, and the northern Gulf coast, it's time to pay attention every day.

The water temperatures in the western Caribbean are extremely warm... record-breaking.  The figure below shows four panels: the left side is sea surface temperature (actual value on top, anomaly on bottom) and the right side is ocean heat content (actual value on top, anomaly on bottom).  All of this means that should anything form in the western Caribbean soon, the ocean will provide endless high-octane fuel for it.

The deep tropics continue to be much less active than normal, and certainly less active than anticipated this season.  Although the typical large-scale environmental factors are conducive for development, there is something odd happening over Africa.

From mid-August through early October, Africa tends to be a major source of tropical cyclone seedlings.  African Easterly Waves come off the continent every few days, and a small percentage of them usually become hurricanes.  This summer, the area of convergence that runs east-to-west across the continent has been displaced quite far north.  Instead of those waves coming off at the normal 10-15°N, they've been coming off at 15-20°N and immediately encountering cool water and dry air.  This also places frequent thunderstorms and heavy rain over the Sahara Desert resulting in tremendous flooding in central and western desert countries.

As such, the season's Accumulated Cyclone Energy (ACE) is down to about 83% of average for the date and slipping roughly 2% more behind each day without an active named storm.  That means if nothing forms through September 25, this season's ACE will be down to 70% of average.

The next chart shows how 2024 stacks up against previous years (again, through September 17).  It's nothing too notable... some recent years were much higher and some were much lower by now.

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Francine will be third hurricane landfall on the US Gulf coast this year

Shortly after my post on Monday, Francine did indeed form, and then was upgraded to the season's fourth hurricane on Tuesday night.  It is nearing Category 2 hurricane intensity, and will make landfall in Louisiana tonight.  Elsewhere, a strong easterly wave out near Cabo Verde could become Gordon by this weekend.

The NHC's experimental version of the "cone of uncertainty" graphic shows inland wind-related watches and warnings, removes the border around the cone, and eliminates the dotted treatment of the Day 4 & 5 part of the forecast... all-around some really nice improvements.

Of course, in addition to the wind-related watches and warnings, there are storm surge watches and warnings too, and as of the Wednesday morning forecast, a peak storm surge of 5-10 feet is possible from Intracoastal City over to Port Fourchon.  Tide schedules vary within this region, but water levels will be maximized if the peak storm surge arrives at or near high tide.

As mentioned in the title of the post, Francine will be the third hurricane to hit the northern Gulf coast already this year (Beryl and Debby were the others).  That's a lot... though there were five in 1886, four in 2020, 2005, and 1985... and three in 1934, 1915, 1888, and 1860.  So it's not unprecedented, but it's unusual and certainly not welcome! Oh, and the season's not over yet.

Once Francine makes landfall it will continue to move inland and weaken, dumping some hefty rain totals along the southern Mississippi Valley before rather quickly dissipating.  You can find the rainfall forecast maps (and other hazards) on NHC's website at https://www.nhc.noaa.gov/graphics_at1.shtml?start#contents

NHC has four other areas of interest highlighted for potential development in the coming week, but the only one I'll discuss here is the one presently near Cabo Verde... the others have very low probabilities of becoming tropical cyclones.

That wave, tagged as Invest 93L, exited the African coast on Monday and has a lot of model support for development very soon.  Persistent thunderstorm activity is located in a relatively small portion of the broad circulation, and it appears very close to becoming the season's 7th tropical cyclone.

As of now, global model ensembles indicate a west-northwest track for the next 5-6 days followed by a turn to the north somewhere near 50°W longitude (well before the Lesser Antilles).  They also suggest it will struggle to intensify too much until that north turn, after which it could become the season's 5th hurricane. At any rate, as of right now, it does not appear that future-Gordon will be a threat to land.

Catching up on the progress of the season's activity as measured by ACE, even with Hurricane Francine out there, 2024 is struggling to keep up with climatology... right at 98% of average for September 11.  Of course, there's still half of hurricane season ahead of us, and all indicators continue to point to it being above-average.

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Soon-to-be-Francine threatens northern Gulf coast this week

It has been an eerily quiet three weeks since advisories ended for Ernesto on August 20, but at the flick of a switch, the tropical Atlantic is lightning back up with activity.  There is a disturbance brewing in the southern Gulf of Mexico and a couple easterly waves east of the Lesser Antilles that I'll discuss here.  The next three names on the list are Francine, Gordon, and Helene.

The system in the Gulf of Mexico, tagged as "Potential Tropical Cyclone Six" is close to becoming the season's 6th named storm, Francine, and poses a threat to the northern Gulf coast later this week.  It's currently a bit disorganized, but model guidance is confidently showing a quick increase in organization today.  The latest NHC forecast for this disturbance (a tropical cyclone has not formed yet) tracks it northward along the far western Gulf, reaching the coast sometime on Wednesday afternoon, likely as a hurricane. Remember: the cone itself does not show impacts, only a historic 2/3 likelihood of where the center of the storm will track.

On that trajectory, it will pass over a patch of extremely anomalously high ocean heat content tomorrow.  In addition, sea surface temperatures along its path are 30-31°C, so the ocean under future-Francine is primed to fuel and sustain anything.  The ocean heat content averaged over the entire Gulf is the highest it's been on record for the date. 

NHC forecasts it to become the season's 4th hurricane on Wednesday, and although its window of favorable environmental conditions is relatively brief, it appears there will be an opportunity for rapid intensification from midday Tuesday through midday Wednesday.

Regardless of exactly how much it's able to intensify in the next couple of days, the potential for significant flash flooding from heavy rainfall is there over parts of the southern US.

Now for a quick briefing of the two easterly waves in the deep tropics between the Caribbean and Africa.  Both of these have been given a 60% probability of forming within the coming week by NHC, and both are expected to continue tracking toward the west-northwest.  Since neither of these pose a threat to land through early next week (slim chance of Leeward Islands this weekend), we'll just keep an eye on them as see how they evolve this week. The long-range model guidance is not too bullish on either of them. But given the time of year, every disturbance in the deep tropics warrants some attention.

This week is the climatological peak of hurricane season, and although we have no named storms right now, that's likely going to change quickly.

Looking at the season's overall activity in terms of ACE (Accumulated Cyclone Energy), even if Francine does form today, 2024's ACE will slip a hair below the climatological value for the first time since June 29. But based on NHC's forecast, that will be rectified by tomorrow and 2024 will be back above average.  This time of year it takes a decent amount of activity just to keep pace with the average.

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