30 August 2023

Idalia rapidly intensifies and makes landfall, Franklin weakening

I unfortunately did not have time to write a post on Tuesday, but a lot happened.  On Tuesday morning, Idalia had just become a Category 1 hurricane.  By Tuesday night it was a Category 2 hurricane and by Wednesday morning it was a Category 4 hurricane.  The peak wind speed increased by 58 mph in 24 hours: rapid intensification.  The storm took full advantage of the hyper-hot water temperatures in the eastern Gulf and minimal wind shear.

It made landfall in the Big Bend region of Florida, and the last time that part of the state experienced anything like this was 1896.  The peak winds dropped slightly as it was making landfall so it was technically a Category 3, but at that point it was too little too late.  That part of the state is extremely surge-prone because of the concave shape of the coastline, though it will be a while until we know the official peak storm surge values.

It's not over of course, as the storm is tracking over Georgia, then South Carolina and North Carolina, creating rainfall flooding and coastal flooding issues along the way.  The full suite of NHC products is available at https://www.nhc.noaa.gov/graphics_at5.shtml?start#contents, but this map shows the probability and timing of tropical storm winds.

Franklin has passed west of Bermuda and is now weakening as it heads into the north-central Atlantic. It will become a strong extratropical cyclone this weekend over the cold ocean there.

But wait, there's more!  Tropical Depression 11 formed on Tuesday morning, though it's not expected to be around long.  Ex-Gert is actually still lingering out there and also has a dim future.  Then there's a new easterly wave that just left Africa yesterday, Invest 94L.  That is strongly favored to develop by the models, but almost certainly turn north into the open ocean.  I won't go into any detail now, but the wave behind that one looks like potential trouble in model-land.  That should leave the continent on Sunday or so...

Not surprisingly, the ACE has spiked due to these two major hurricanes and is now at 138% of average for the date.  If the activity magically switched off right now, the ACE would slip back to the average value in just one week: September 5.

As far as named storm counts go, here's the latest status.  Any way you slice it, this season is well ahead of average for the date.

28 August 2023

Idalia to bring significant impacts to Florida, Franklin becomes Category 4 hurricane

The two features of interest in the Atlantic, Idalia and Franklin, continue to strengthen...

Idalia is just a sneeze away from becoming the season's third hurricane as it nears the western tip of Cuba.  It is finally moving slowly to the north and will enter the Gulf of Mexico tonight.  It has been affected by some moderate northerly vertical wind shear which has kept the thunderstorm activity displaced to the south of the surface circulation, but as the day goes on, that displacement is decreasing and it appears that Idalia is close to beginning a period of rapid intensification.  We've been fortunate to have continuous radar coverage of the storm since Friday from Cancun in Mexico and from La Bajada in Cuba: https://bmcnoldy.earth.miami.edu/tropics/radar/

This set of Hurricane Threats and Impacts (HTI) graphics is from Monday morning but in the few hours since then, these maps have not changed much.  These graphics have been an operational NWS product for every single tropical cyclone near the U.S. since 2015, and I've been a big proponent of them because they put the focus on the hazards rather than the track.  There are four threat levels for each of four hurricane hazards: wind, storm surge, rainfall, and tornadoes.

Idalia is forecast to make landfall in Florida's Big Bend region as a Category 3 hurricane on Wednesday morning, with tropical storm conditions beginning Tuesday evening. There are too many evolving watches and warnings to put in here, but visit https://www.nhc.noaa.gov/graphics_at5.shtml?start#contents for the latest.  And remember, being outside of "the cone" does not mean you are safe... the cone has nothing to do with impacts -- that's what watches and warnings are for.

Meanwhile, south of Bermuda, Franklin was upgraded to the season's first major hurricane today, and subsequently strengthened even more to reach Category 4 status. As of early Monday afternoon, Franklin had 145 mph peak sustained winds and is still forecast to strengthen... perhaps briefly reaching Category 5 status later today.

Bermuda is under a tropical storm watch, and tropical storm winds could reach the island on Tuesday evening, with the storm's closest approach on Wednesday morning.  But Franklin's winds are also driving some hefty swells and elevated water levels along the US east coast, aggravating what are already-high tides this week around the full moon.

The ACE has just popped back above 100% of average for the date for the first time since August 18 (now at 107%).  The next 2-3 days will produce a big spike in the tally.

27 August 2023

Tropical Storm Idalia has eyes on Florida as Franklin strengthens south of Bermuda

On Sunday morning, Tropical Depression 10 was upgraded to Tropical Storm Idalia, the 10th named storm of the season.  Climatologically, the 10th named storm forms on September 22, so this is definitely ahead of that pace.  The storm is stalling between the Yucatan peninsula and Cuba as expected, and should start moving toward the Gulf of Mexico on Monday morning.  Hurricane watches are in effect for parts of Florida.

Sunday afternoon's model runs had the benefit of aircraft reconnaissance data.  Unfortunately, the guidance has trended stronger and it's now looking very likely that somewhere in the northern Gulf coast of Florida will experience a hurricane landfall, perhaps a major hurricane, on Wednesday.  Tropical storm force winds could begin Tuesday morning in parts of southwest Florida and then Tuesday evening in northwest Florida.  That means just two days of preparation time as of Sunday evening there.

The global model ensembles show a track toward Florida's Big Bend area and landfall centered around Wednesday morning.  However, it's important to note the spread to see the current realm of potential outcomes based on these models.  And then remember that a hurricane is not a point or a line, it's hundreds of miles across.  So as always, impacts such as flooding rain, storm surge, and strong winds occur well away from the center. 

We can also look at the latest suite of deterministic model tracks, and they include a couple that are on the far southern periphery of the ensemble spread shown above.

Then we have the official NHC forecast and the track forecast cone.  For this, remember that it's a deterministic forecast and the accompanying cone represents a 2/3 likelihood of the path of the center of the storm (using the error statistics from the previous five years).  So 1) there's still a 1/3 chance the center of the storm tracks outside of the cone and 2) the cone tells you nothing about where impacts will be experienced.  (see 2023 "Cone of Uncertainty" Update & Refresher).  Keep yourself updated on the latest forecast, watches, and warnings related to Idalia at https://www.nhc.noaa.gov/refresh/graphics_at5+shtml/174602.shtml?cone#contents.

After its encounter with Florida, Georgia, South Carolina, and North Carolina will have to contend with it, and one factor this coming week is an exaggerated risk of coastal flooding because of abnormally high tides around the Full Moon.

Franklin is still in the open ocean between Hispaniola and Bermuda.  It should avoid a direct hit on Bermuda, but the island now has a rather high chance of experiencing tropical storm conditions on Tuesday night.  It is forecast to become the season's first major hurricane tomorrow, which is just four days ahead of climatology.

With a tropical storm and a hurricane churning simultaneously, the Accumulated Cyclone Energy (ACE) is accruing at a pace just in line with climatology.  The ACE is at 93% of average for the date.

26 August 2023

Threat increasing for Florida, and Franklin becomes 2nd hurricane of the season

Since my update on Thursday, Franklin did eventually become the second hurricane of the season on Saturday morning and the disturbance that was over central America trekked north and is now Tropical Depression 10 near the northeast tip of the Yucatan Peninsula.

Franklin should manage to avoid land (west of Bermuda, south of Nova Scotia and Newfoundland) but is forecast to intensify further, perhaps becoming a major hurricane (Category 3+) on Monday-Tuesday.  After that, wind shear will increase dramatically and the ocean temperature under it will become rather chilly for hurricanes.  If Bermuda does get scraped with tropical storm conditions, it would be on Tuesday.

The disturbance we've been watching for several days in the western part of the basin was just upgraded to Tropical Depression 10 on Saturday afternoon, and it has certainly developed an ominous satellite presentation during the day.  When this reaches tropical storm intensity, the next name on the list is Idalia (replacing Irma six years ago).

We also have the benefit of excellent radar coverage between Cancun in Mexico and La Bajada in Cuba (latest animations can be found at https://bmcnoldy.earth.miami.edu/tropics/radar/):

The NHC forecast brings this to hurricane intensity on Tuesday as it makes landfall somewhere in the northern Gulf coast of Florida.  Model guidance has a pretty big spread though, with some models bringing it to Category 3 intensity at landfall.  In terms of track, this map below shows the track density from four global model ensembles combined.  Depending on the track it takes, notice that after Florida, areas to the north such as South Carolina and Georgia will also experience some impacts (the map doesn't explicitly show impacts, just track density).

Keep in mind the water temperatures in the western Caribbean and eastern Gulf of Mexico are extremely warm -- even warmer than average for this time of year which is saying a lot.  This will increase the odds of rapid intensification any time the center is over water.  Rapid intensification is notoriously challenging to reliably forecast.

Based on the initial official forecast, here are the probabilities and most likely arrival times of tropical storm force winds.  Remember, it's not a tropical storm yet, so the chances are relatively low near its present location.  A key part of the forecast is an anticipated stall near the tip of the Yucatan peninsula... it should start moving again on Monday.  A lot rests on where the inner core stalls (land vs water).

Given the uncertainty, anyone in Florida should be watching this extremely closely and be mentally preparing to start taking action on Sunday-Monday. A strong hurricane landfall on Tuesday-Wednesday is not out of the question.

Since this will inevitably come up, "I" storms are indeed the most frequently retired, and it's not even close.  This is mostly just luck, because if it was related to the time of year, then "H" and "J" storms should be pretty high too (or "G" and "K" for that matter), and they're not.  Of the 13 "I" storms that have been retired over the past 70 years, 7 of them have been just in the last 14 years (Ike, Igor, Irene, Ingrid, Irma, Ida, Ian).  Let's hope we don't have a reason to add Idalia to the list.

24 August 2023

Franklin expected to become a hurricane, potential Gulf activity early next week

Since my previous update on Monday, Emily and Gert dissipated, Harold formed and quickly raced into Texas and dissipated, and Franklin crossed Hispaniola.  Today, we are watching Franklin, a disturbance that has good chances of developing in the Gulf of Mexico early next week, the remnants of Emily, and a disturbance west of Cabo Verde.

I labeled the features on the satellite image above, and among the five of them, only Gert is not a consideration in the coming days.  Emily could make a comeback, Franklin will likely become the second hurricane of the season, 92L and what I'm calling future-93L have medium chances of development in the next few days.  For the sake of brevity, I will limit today's update to just the two westernmost systems: the one over central America and Franklin.

Franklin passed directly over Hispaniola on Wednesday, never losing tropical storm status.  Now the forecast brings it northward toward Bermuda, but it should pass safely west of the island on Monday into Tuesday.  Only the easternmost model outliers bring Franklin close enough to cause notable impacts.  However, since it's forecast to be nearly a Category 3 hurricane at the time, Bermuda needs to be paying close attention to any eastward shifts in the forecast track.  If tropical storm conditions do reach the island, they would begin Monday morning. (Bermuda is the black speck in the top-center of the satellite animation below)

Now on to the system over central America.  As of early Thursday afternoon, it has not been tagged as an Invest yet, but the next in line is 93L.  That also means we don't yet have the full suite of model guidance available for it, but we can use the global models and their ensembles for now.

This has plenty of model support for crossing into the western Caribbean on Friday, then slowly oozing its way north into the southern Gulf of Mexico on Monday morning.  Then there's a bit of spread in track forecasts... from south Florida up to the Florida panhandle.  As of now, there's no guidance to indicate a hurricane threat, but given the super-warm water temperatures in the eastern Gulf, it's too soon to discount that possibility.

Between the European and American deterministic and ensemble models, the European is decidedly more bullish with this... the ensemble spread is shown below, where each L represents the low pressure center from an ensemble member.  You definitely don't want to extract any details from this, but it's useful to see the spread in track and intensity, then importantly, to compare this to future cycles to see the trends.

Should this become a tropical storm, the next name on the list is Idalia, which replaces Irma (retired after 2017).  Last year's "I" storm (Ian) also came from the western Caribbean and headed for the west coast of the Florida peninsula, but mercifully as of now there is zero indication of anything like that happening this time around.

The recent flurry of activity has been so minimal that it actually generated less ACE (Accumulated Cyclone Energy) than what happens climatologically! It's at about 91% of average for the date, and slipping further behind the average each day.  That could start reversing if Franklin becomes a strong hurricane for a few days.

21 August 2023

Emily, Franklin, Gert, and soon-to-be Harold bring the Atlantic to life

In a surge of activity, nearly every area of interest over the past few days has become a tropical cyclone or is about to.  Since Saturday's post, TD6 became TS Gert, Invest 98L became TS Emily, Invest 90L became TS Franklin, Invest 91L became TD 9, and the easterly wave near Cabo Verde is now Invest 92L.  You have every right to be confused.

While that is a long list, it will be easy to knock a few out right away.  TS Emily dissipated on Monday morning, and Gert is barely clinging to tropical depression status and will dissipate shortly.  The wave near Cabo Verde is not too close to development today, so for this update I'll just mention it but not go into any depth.

Moving on to Tropical Storm Franklin... on Monday afternoon it is located in the Caribbean Sea roughly 300 miles south of Hispaniola and about to make a turn toward the north.  That will bring it to Hispaniola on Wednesday; tropical storm warnings are in place and as always in mountainous areas, heavy rainfall is a significant concern (this includes Puerto Rico).

Models are in good agreement on a track across Hispaniola then into the open Atlantic where it has a decent chance of becoming the season's second hurricane.  The plot below shows the track density from an ensemble of four global model ensembles (LOTS of members!).

Tropical Depression 9 is in the central Gulf of Mexico and racing west toward the south Texas coast.  Although the water temperature in the Gulf is extremely/anomalously warm, this storm thankfully will not have enough time to get its act together.  It is expected to become a tropical storm soon (the next name is Harold), and reach the Texas coast midday Tuesday.  Wind and storm surge will be minimal threats, and it's moving fast enough that it won't be able to dump too much rain either.  You can view the latest suite of hazard products at https://www.nhc.noaa.gov/graphics_at4.shtml?start#contents

Gert was the season's 8th named storm, and from the 1991-2020 climatology, the average date of 8th named storm formation is September 9.  And if Harold forms on Tuesday from TD 9, it would be the 9th named storm and be 3.5 weeks ahead of average.

The Accumulated Cyclone Energy (ACE) is trickling in from the weak named storms, but not even fast enough to keep pace with climatology this time of year!  As of the 21st, it's at about 93% of average for the date.

19 August 2023

Tropical Depression 6 forms, plus four areas of interest scattered across the tropical Atlantic

Right on the climatological clock, the tropical Atlantic has awakened.  There are FIVE features of interest peppered from the Bahamas to Africa.

In the map above, the current position of TD6 is the red circle, and the current positions of the four disturbances are the colored Xs (yellow for low probability of formation within a week, orange for medium, and red for high).  The hatched blobs indicate the areas of potential formation... they are not track forecast cones. Remember, those things have not become tropical cyclones yet and perhaps never will.

I'll begin with Tropical Depression 6 which is located about 700 miles east of the Leeward Islands.  This African easterly wave developed in a brief window of opportunity, and due to some strong vertical wind shear and dry air, it's not expected to intensify to a tropical storm, and NHC actually forecasts its dissipation by Monday, well before reaching the Leeward Islands.

Tropical Depression Six

Now let's move west to east across the basin.  The first disturbance is not an Invest yet, but will be 91L.  It's a disorganized mass of thunderstorms spawling over the western Bahamas, south Florida, and Cuba, slowly making its way westward.  Model guidance suggests it could acquire a closed surface circulation and become a tropical depression once it's in the Gulf of Mexico... but no models develop it into anything troublesome.

Given the all-time record warm water temperatures in the Gulf of Mexico though, we obviously can't tune it out completely.  Widespread areas of 32-33°C water is unheard of in the Gulf, but it's what we've got right now.  This system should reach southern Texas or northern Mexico on Tuesday.

The next feature is another African easterly wave, tagged as Invest 90L.  It's just crossing the Windward Islands on Saturday afternoon and should continue moving west through the Caribbean for another couple of days.  Then, there's agreement among the models that it will turn north toward Hispaniola.  After exiting the Caribbean on Wednesday-ish, it actually has a better chance of intensification, and will likely remain over the open ocean.

1600 miles east of Invest 90L is Invest 98L.  Similar to TD 6, this has a brief window in which environmental conditions will be favorable for development.  After the next few days, wind shear should increase substantially and be the demise of whatever this becomes by then.  The track forecasts in model guidance generally turn it north by the time it reaches 50-55°W.

And finally, 1200 miles east-southeast of Invest 98L is a new African easterly wave.  It's also battling a hostile environment and has pretty minimal chances of development in the coming week, but we'll want to watch it.  So, even though the map looks extremely active overall, only a couple of the disturbances could end up as named tropical storms.

It's too soon to know if any of these will become tropical storms, or in what order, but the next few names on this year's list are Emily, Franklin, Gert, Harold, and Idalia.  Harold and Idalia are new names on this list, replacing Harvey and Irma from six years ago.  Relative to the 1991-2020 climatology, this year's Accumulated Cyclone Energy (ACE) just crossed below the average today for the first time all season.