Hurricane Kirk strengthens, Tropical Depression 13 forms, Gulf still unsettled

Kirk was upgraded to the season's 7th hurricane on Tuesday afternoon, and Invest 91L which is trailing right behind it was upgraded to Tropical Depression 13 on Wednesday morning.  The setup for development in the western Caribbean and Gulf of Mexico is still messy and uncertain.  The next couple of names on the list are Leslie and Milton (Milton is a new name, replacing Michael from the 2018 season).

Kirk is forecast to become a very large and intense hurricane in the coming days, but will remain in the central Atlantic for the next five days or so.  As of Wednesday at 11am EDT, it's still a Category 1 hurricane but could easily reach Category 4 intensity (and possibly 5) over the weekend, far from land.

Something the long-range models are beginning to indicate more consistently is the potential for a significant impact in the British Isles in the middle of next week.  This will have to be watched closely... it's not unheard of to have an ex-hurricane reach that area (it may not technically be classified as a hurricane anymore, but it could be a hurricane-strength extratropical cyclone)... the best analog to Kirk in recent memory is Lorenzo in 2019. 

Also over the course of the coming week, a lot of ocean swell energy emanating from Kirk should begin to reach the U.S. east coast this weekend, resulting in elevated risks of tidal flooding and erosion.  The animation below shows a 8-day forecast of "significant wave height", and you can clearly see Kirk in the middle but its influence on the ocean will span thousands of miles.  By the end of the loop, ominously huge waves approach the British Isles.  Kirk is not a storm we will want to ignore.

Just east of Kirk is Tropical Depression 13, which will undoubtedly soon become the season's 12th named storm, Leslie.  Compared to yesterday (when it was still Invest 91L), the track guidance is showing even less of a threat to the Windward Islands, with nearly all global model ensembles now showing a north turn well before reaching the Windwards.  This example is from the GFS' ensemble.

Finally, the disturbance we've been watching in the western Caribbean continues to fester and challenge models to lock onto anything.  The NHC is still giving it a 40% probability of formation in the coming week... somewhere in the Gulf of Mexico.  The two panels here show the latest GFS (left) and EMCWF (right) track ensembles, and it's a mess.  *IF* a coherent tropical cyclone eventually develops from this, it appears it would generally move eastward toward Florida, but the northern Gulf coast should also be on alert for impacts.

Thankfully, it is not posing a wind or storm surge threat to anyone in the foreseeable future, but heavy rainfall is beginning to become a concern, and it could be directed toward Florida next week.  Again, any slight shifts could also put the northern Gulf coast on alert.

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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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