2025 "Cone of Uncertainty" Update & Refresher

Anyone who lives on or near a hurricane-prone coast is undoubtedly familiar with the track forecast cone or "cone of uncertainty" which has been produced by the National Hurricane Center since 2002.  It begins as a point at the current position of a tropical cyclone and expands to show the potential position of the storm's center in the next five days. It is often called the "cone of uncertainty" because the further out in time you go, the more uncertain the forecast becomes... and it tends to look like a cone!

(By the way, "tropical cyclone" is a blanket term that refers to tropical depressions, tropical storms, and hurricanes.)

A "cone of uncertainty" for Hurricane Irma (left) and Hurricane Harvey (right). Both cones are from 2017 and are therefore identical to each other in their construction. 

The size of the cone is fixed for every forecast of every storm during an entire hurricane season, but the size slowly evolves from year to year. If the storm is moving quickly, the cone will appear more elongated and if the storm is moving slowly, the cone will appear more compact... but it's the exact same cone.  The examples shown above are from Irma (left) and Harvey (right); both storms occurred in 2017, so both cones are identical in their construction.

The cone is updated each year prior to the start of hurricane season, and it usually shrinks each year.  Overall, hurricane track forecasts are gradually improving, meaning that in general, there is less uncertainty where a storm will track now than there was a couple decades ago.

The map below shows a sample satellite image with the new 2025 cone overlaid on the 2015, and 2005 cones for comparison.  This year, the cone is slightly smaller at most lead times compared to 2024.

Improvements are getting increasingly challenging to achieve because there can never be a perfect forecast of a chaotic system like the atmosphere. We call this a "limit of predictability", and there will come a time when we reach it and meaningful improvements can no longer be made.  Some would argue we are very close to -- if not already at -- that limit.  It's critical to understand that forecasts evolve and there is always some amount of uncertainty in them.

So just how is the size updated each year?  The National Hurricane Center uses its own track forecast errors over the previous five years to calculate a circle at each "lead time" (1 day, 2 days, ... 5 days).  The size of that circle is designed to enclose the position of the storm's center with 2/3 probability, meaning that there's historically a 1/3 chance the storm will track outside the circle at that time.  Lines connecting the various circles complete the shape of the cone. [Note that the 2025 cone size is thus determined from track errors during the 2020-2024 seasons.]

This next chart shows the evolution of the cone's size at each forecast day -- it's clear that progress is slowing and perhaps has reached a limit at some (most?) forecast lead times.

Since the cone is so widely used yet sometimes misunderstood, here are some key refreshers:

• The cone does not tell you anything about where impacts will be experienced.  It is NOT a "cone of concern"! Even for a perfect down-the-middle track forecast, impacts such as strong wind, heavy rain, storm surge, and tornadoes will extend beyond the cone. Cone graphics on the NHC website include some of the relevant watches and warnings, as shown in the examples at the top of the post.  Starting experimentally this year, tropical storm and hurricane watches and warnings will be displayed inland too, not just on the coast. 

• The cone does not tell you anything about the size of the storm.  Regardless of how strong they are, hurricanes come in a wide range of sizes.  Back in 2017, NHC added the latest observed size of the wind field to its cone graphics to help illustrate this (see the Irma and Harvey examples above... the orange and red shading indicates the extent of tropical storm and hurricane-force winds at the time the forecast was issued).

• The cone does not tell you anything about the actual uncertainty associated with the forecast. Since the size of the cone is fixed, it cannot become more narrow or broad to accommodate a more or less predictable environment.

• Nothing magically happens at the edge of the cone. If a hurricane is approaching and you are scrutinizing each new forecast to see if you are inside the cone or not, you are missing the point of it.  It is arbitrarily chosen to be the 67% historical probability threshold... a 75% probability cone would be larger, and a 50% probability cone would be smaller.  Again, it is NOT a "cone of concern".

• If you use the cone graphics from NHC, there is some information about intensity provided. At each forecast point, there is a letter written inside the black dot corresponding to a general intensity range: D (tropical depression), S (tropical storm), H (hurricane (Category 1-2)), and M (major hurricane (Category 3+)).  But keep in mind that there is uncertainty associated with the intensity forecasts too!

So, let's imagine what a cone of uncertainty for intensity might look like. Consider this: averaged over the past five years (2020-2024), the mean error in a 1-day forecast is +/- 8.1 mph, the error in a 3-day forecast is +/- 12.6 mph, and the error in a 5-day forecast is +/- 16.5 mph.  But there is also a wide range of values that go into those averages, meaning that there is a small probability of a very large error and a small probability of near-zero error.

To create the next figure, I simply averaged five years of intensity errors together such that the values listed for 2025 are based on the NHC's intensity errors during the 2020-2024 seasons (and 2024 used 2019-2023, and so on).  This five-year averaging helps to mimic the smooth trends of the track forecast cone, but it is not a 2/3 probability like the track cone.

Next, using a made-up five-day intensity forecast and the average error values for 2025, I created the following chart.  This is what a cone of uncertainty for intensity could look like; a hypothetical intensity forecast is shown with the red line and the cone of uncertainty is the light red shading surrounding the forecast.

Summary of the Hyperactive 2024 Hurricane Season

The 2024 Atlantic Hurricane Season will finally come to an end on November 30, and by any measure, it was an extremely active and impactful season.  

This post marks the end of my 29th year writing these updates on tropical Atlantic activity.  During that time, I have written approximately 1450 posts spanning 502 tropical cyclones, 224 hurricanes, 103 major hurricanes, and 53 retired storm names. I know some of you reading this have been following along the entire time, but whether you've been reading these posts for 29 days or 29 years, I truly appreciate your continued interest!

Track map of tropical cyclones during the 2024 hurricane season. Storm tracks are color-coded by intensity, and each storm's peak intensity, minimum central pressure, and Accumulated Cyclone Energy (ACE) is listed on the right.

Due to record-warm ocean temperatures and the forecast development of La Niña, we went into this hurricane season expecting it to be extremely active, perhaps among the most active on record.  Those record warm ocean temperatures persisted throughout the season, but La Niña did not quite develop yet. As such, the season did not produce as much activity as most seasonal forecasts were projecting, but it still ended far above average.

There were a total of 18 named storms, 11 of which became hurricanes, and 5 of those became major hurricanes (Category 3+ on the Saffir-Simpson Scale).  The climatological averages of those numbers are 14, 7, and 3.

Five of the 11 hurricanes made landfall along the U.S. Gulf coast, making it an especially devastating year there.  That concentration of landfall activity in the Gulf is rare, but not unprecedented.  Looking back to 1851, one year had six hurricane landfalls along the U.S. Gulf coast (1886), and there were two other years with five (2005 and 2020).

Of course, hurricane hazards don't end at landfall.  This was most dramatically illustrated by the terrific inland flooding event we saw before and during Helene's trek through the southeast U.S., and the significant tornado outbreaks spawned by Beryl over TX/LA/AR and by Milton over FL.

48-hour composite radar animation over the southeast U.S. covering Hurricane Helene's landfall.  This shows the tremendous rainfall that was occurring over western North Carolina (for example) long before Helene even reached the area.  The green polygons indicate flash flood warnings.

Two storms reached Category 5 intensity (Beryl and Milton), which is quite rare.  First, over the past century, only 2% of Atlantic tropical cyclones reach Category 5 intensity, so that by itself is a high bar. Second, there have been just seven other years with two or more recorded Category 5 hurricanes: 2019, 2017, 2007, 2005, 1961, 1933, and 1932.

Enhanced infrared satellite images of the two Category 5 hurricanes near their peak intensity this season.  Left: Hurricane Beryl (165 mph on the morning of July 2, ~0700 UTC) in the eastern Caribbean Sea and right: Hurricane Milton (180 mph on the evening of October 7, ~0200 UTC) in the southern Gulf of Mexico.

Another interesting statistic that comes out of the storm counts is the higher-than-normal ratio of storms that became hurricanes.  11 out of 18 is 61%... climatologically, that number is 50% (216 out of 433 from 1991-2020).  Perhaps the exceptionally warm ocean temperatures across the tropics helped more storms than normal intensify into hurricanes, and/or perhaps the subtropics were more hostile than normal resulting in fewer relatively weak storms.  In other words, one could arrive at that climatological 50% ratio by having 18 named storms and 9 hurricanes OR by having 22 named storms and 11 hurricanes.  There's interannual variability in that ratio, and I suspect it's often those two factors that determine the outcome.  This year, I think it was both.

Shifting away from storm counts, we can look at the Accumulated Cyclone Energy, or ACE.  This is a commonly-used metric that accounts for the overall intensity and duration of whatever storms there were.  One very intense long-track hurricane can rack up more ACE than several other storms.  For example, Beryl's total ACE was higher than Alberto, Chris, Debby, Ernesto, Francine, Gordon, and Helene combined!  It was a very strange season with enhanced activity at the beginning and toward the end, with very little in the usual peak part of the season.  Not every year follows climatology!

That mid-season dearth of activity is quite unusual. That is the time of year when easterly waves regularly exit the African continent and can become the classic long-track "Cabo Verde" storms.  However, this year, those waves came off of Africa several degrees further north than normal and encountered a very different -- and more hostile-- environment than normal.  So, that entire mode of storm genesis was effectively shut down for weeks.

The chart above shows the daily ACE accrued during 2024 (yellow bars) and during the 1991-2020 climatological period (blue curve).  The explosive beginning to the season provided by Beryl dominated the activity until October when we had the unprecedented trio of Hurricanes Kirk, Leslie, and Milton all active simultaneously in early October.  The same data are shown below, but as a seasonal accumulation.  The season ended at 132% of average, and is in the top 12% of all seasons going back to 1851.

Now let's take a little closer look at a few individual storms...

Alberto was the first storm, and it formed on June 19 in the far western Gulf of Mexico.  While that may sound typical, it was the latest first named storm in a season since 2014 (that was Arthur on July 1)!  Over the past fifty years, the median date is June 15. 

Beryl was a storm for the history books, and with several entries.  It was the highest ACE-producer of the season by far.  It formed from an African easterly wave on June 28, which by itself is unusual... we don't typically see the "Cabo Verde season" spring into action until mid-August or so. Six hours after being declared a tropical depression, it became a tropical storm, and then a Category 1 hurricane a day after that.  Twelve hours later and it was a Category 3 (major) hurricane, and 36 hours later it was a Category 5 hurricane -- that adds up to 3.5 days to go from a tropical depression to a Category 5 hurricane, spanning June 28 through July 2.  On July 8 it made its final landfall in Texas as a Category 1 hurricane, then spawned an amazing 68 confirmed tornadoes across several states.  This would be the first of five landfalling hurricanes along the U.S. Gulf coast this season. Here are some intensity/intensification stats and records, largely pulled from my posts on June 29, June 30, July 1, and July 2:

- On June 29, it became the earliest hurricane to form so far east (~49°W)... that's about 10° or 675 miles further east than the previous record

- On June 30, it became the earliest Category 3 (major) hurricane, beating the previous record by 9 days

- On June 30, it became the earliest Category 4+ hurricane east of the Caribbean in the "Main Development Region", beating the previous record by 39 days

- The earliest we ever saw the intensification rate that Beryl achieved on June 30 (55 kt or 63 mph in a day) was September 1, an unbelievable 63-day margin

- On July 1, it became the first Category 4+ hurricane to pass through the Windward Islands anywhere south of Saint Vincent

- On July 2, it became the earliest Category 5 hurricane on record in the Atlantic, beating the previous date by fifteen days (Emily on July 17, 2005)

- With 165 mph peak winds, it became the strongest hurricane ever observed during July, beating the previous strongest by 5 mph (also Emily on July 17, 2005)

Beryl's approximate surface wind swath produced by a parametric model.

Debby and Francine were the second and third hurricanes to make landfall along the U.S. Gulf coast, on August 5 and September 11, respectively.

Helene was one of those storms that we saw coming for a long time.  I wrote a post on September 17 highlighting it, then it eventually formed on September 24 in the western Caribbean Sea.  A day later it became a Category 1 hurricane and due to very favorable environmental conditions in the eastern Gulf, rapid intensification was forecast by NHC and was likely to occur.  It did, and Helene made landfall on the evening of September 26 as a Category 4 hurricane in the Big Bend area of Florida. Helene was the strongest hurricane landfall in recorded history in Florida's Big Bend.  My post early on September 25 pointed out some notable hazards that ended up coming to fruition:

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.

Indeed, all of that happened, and the radar loop toward the beginning of this post shows what made the rainfall-related impacts so devastating in western North Carolina and surrounding areas.  Well ahead of Helene, a mid-latitude trough/front was already producing huge rainfall amounts over several states, with moisture funneling northward from the Gulf of Mexico and Helene (in science lingo, this was classified as a PRE, or "predecessor rain event").  Then Helene made landfall in Florida and eventually made its way into that same already-flooded area, unleashing several more inches of rain.  It was too much for anywhere to handle, and catastrophic floods ensued, crippling the area for months.  Helene is responsible for at least 234 fatalities across seven states.  It also emphasizes the point that water, not wind, is the biggest killer with hurricanes... and it's not even close.

I'll mention Kirk and Leslie here, in the context that they were hurricanes from October 1 through October 7 and from October 5 through October 10, respectively.  The next storm, Milton, was a hurricane from October 6 through October 10.  The point of this is that for 18 hours spanning October 6-7, there were THREE hurricanes active simultaneously in October for the first time in recorded history.  That has happened several times earlier in past seasons, but never so late in the season.

Milton is yet another storm this year with several entries in the record books. Like Helene, it was a slow formation... I first wrote about it on September 27, and it later formed on October 5 in the southwestern Gulf of Mexico.  Conditions in the Gulf still favored rapid intensification, and that was reflected in forecasts from the beginning.  Rapid intensification was an understatement for what it did.  From my post on October 7:

"In one of the most explosive rounds of intensification in recorded history, Milton's peak winds increased from 90 mph at 2am EDT on Monday to 175 mph at 2pm then 180 mph at 5pm.  It went from a Category 1 to a Category 5 hurricane in just nine hours."

- With peak winds of 180 mph, Milton is the strongest tropical cyclone to occur anywhere on the planet so far this year.

- With a minimum central pressure of 897 mb, Milton is the 5th strongest hurricane in Atlantic history, behind Rita 2005 (895 mb), Labor Day 1935 (892 mb), Gilbert 1988 (888 mb), and Wilma 2005 (882 mb).  Allen 1980 is the only other sub-900 mb hurricane in Atlantic history at 899 mb.

- The intensification rate of 84 mb in 24 hours and 90 mph in 24 hours was the third highest in the Atlantic (behind Wilma 2005 and Felix 2007) but the fastest in the Gulf of Mexico.

It continued a trek eastward across the Gulf, only slightly weakening to a Category 3 hurricane when it made landfall near Tampa, Florida on October 10, becoming the fifth and final hurricane landfall along the U.S. Gulf coast this season.  

Rafael formed in the western Caribbean on November 4, then intensified quickly before making landfall in western Cuba as a Category 3 hurricane. It weakened a bit after that landfall, but briefly regained Category 3 intensity in the central Gulf of Mexico on November 8 before dissipating.  In the Gulf, its intensity peaked at 120 mph, meaning Rafael is tied for the strongest hurricane in the Gulf during November (the tie is with Kate on November 20, 1985).

Nine of this year's storms, from Beryl through Sara, have long land-based radar loops available in my archive at https://bmcnoldy.earth.miami.edu/tropics/radar/ if you wish to find any of those.

Finally, here is a look at the preliminary track and intensity verification statistics of the forecasts made by the National Hurricane Center throughout the season.  I also plotted their own average errors over the past five seasons for reference (black points/line).  Track forecast errors were well below average at all lead times, but intensity forecast errors were more of a wash with fairly average errors at 1- and 4-day lead times, slightly above average at 2 and 3 days, and below average at 5 days.

Hurricane Season 2025 begins on June 1, and the first few names are Andrea, Barry, and Chantal.  There is one new name on the list, Dexter, which replaces Dorian after its retirement in 2019.  As you may know, we use a set of six rotating lists for storm names, and the modern naming convention began in 1979.  The 2025 list was therefore first used in 1983, then 1989, 1995, etc.  For a little storm name trivia, this list has had an impressive 14 names retired from it over those seven years: Alicia (1983), Hugo (1989), Luis (1995), Marilyn (1995), Opal (1995), Roxanne (1995), Allison (2001), Iris (2001), Michelle (2001), Dean (2007), Felix (2007), Noel (2007), Ingrid (2013), and Dorian (2019).

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TD 19 forms near Honduras, could be potential threat for Florida next week

On Thursday morning, a late-season African easterly wave just intensified to become Tropical Depression 19.  This was located way back by Cabo Verde on November 1 and then made a slow trek across the deep tropics... it's currently centered off the eastern tip of Honduras.  It is very close to becoming the season's 18th named storm, Sara.

This is a very precarious location for a developing tropical cyclone.  On one hand, it's over extremely warm water and in low vertical wind shear, but on the other hand, it's really close to land.  Heavy rainfall will be a big hazard regardless, but how much can the winds intensify given these competing factors?  The model guidance and the NHC forecast indicate a balance, where it can reach tropical storm intensity but not much beyond that for the next 3-4 days before running into the Yucatan peninsula on Sunday.  Once over the Yucatan, it will weaken.

After it emerges from the Yucatan, there's a large spread of model scenarios.  Many of them more or less dissipate it and that's the end of the story. The rest re-intensify it to some degree over the Gulf and it gets ushered to the northeast toward Florida by a potent mid-latitude trough and cold front.  So, Florida could get some impacts from this around Wednesday, but as of right now, it appears those impacts would be relatively minor.  A hurricane landfall appears very unlikely, but it's worth keeping a close eye on this one.

These two maps show the latest storm tracks from the American model ensemble (left) and the European (right).  To help provide some context of probabilities, the left one has 30 members and the right one has 50 members.

Although unlikely, IF this should end up making landfall in Florida at hurricane intensity, it would join a very short list of November hurricane landfalls in that state.  The only previous ones in the history books are:

        Unnamed - Nov 4, 1935   - Category 2

        Kate         - Nov 21, 1985 - Category 2

        Nicole      - Nov 10, 2022 - Category 1

but none of those originated in the Caribbean Sea or the Gulf of Mexico.

And, if that happens, 2024 would tie the record set in 1886 of SIX landfalling hurricanes on the U.S. Gulf coast (the current five are Beryl, Debby, Francine, Helene, and Milton.  

And finally, for those keeping close tabs on the season stats... through today there have been 17 named storms, 11 hurricanes, and 5 major hurricanes. The average counts through this date are 13, 6, and 3.  In terms of Accumulated Cyclone Energy (ACE), that's up to about 134% of average for the date.  And if the season ended today, it would be in the top 11% of all seasons back to 1851.

As of now, there's no other activity on the foreseeable horizon, but if we do get another storm this season, the next name after Sara is Tony.

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