04 April 2022

2022 "Cone of Uncertainty" Update & Refresher

Anyone who lives on a hurricane-prone coast or even watches television is familiar with the "cone of uncertainty" produced by the National Hurricane Center.  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 coming five days. It is 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 almost always shrinks each year too.  Hurricane track forecasts are gradually improving, meaning that in general, there is less uncertainty where a storm will track now than there was a decade ago.  In fact, a two-day forecast now is as accurate as a one-day forecast was a decade ago, and a five-day forecast now is more accurate than a three-day forecast was two decades ago!

The map below shows a sample satellite image with the new 2022 cone overlaid on the 2015, 2009, and 2003 cones for comparison.  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.

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 2022 cone size is thus determined from track errors during the 2017-2021 seasons.]

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.  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.
  • 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.  Several years ago, NHC added the latest observed size of the wind field to their 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.
  • 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!
With that in mind, let's imagine what a cone of uncertainty for intensity might look like. Consider this: averaged over the past five years (2017-2021), the mean error in a 1-day forecast is +/- 9 mph, the error in a 3-day forecast is +/- 14 mph, and the error in a 5-day forecast is +/- 18 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 2022 are based on the NHC's intensity errors during the 2017-2021 seasons (and 2021 used 2016-2020, and so on).  This five-year averaging helps to mimic the smooth trends of the cone of uncertainty, but it is not a 2/3 probability like the track cone.

Now, using a hypothetical intensity forecast and the average error values for 2022, 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.

Some important terms:

Potential Tropical Cyclone: a disturbance (or "Invest") that is not yet a tropical cyclone, but is expected to become a tropical cyclone within the next 48 hours and produce tropical storm or even hurricane conditions on land. Watches and warnings such as those outlined below can be issued for a Potential Tropical Cyclone because of its anticipated impacts.

Storm Surge Watch: life-threatening inundation from rising water moving inland from the shoreline is possible generally within 48 hours.

Storm Surge Warning: life-threatening inundation from rising water moving inland from the shoreline is expected generally within 36 hours.

Hurricane Watch: sustained winds of 74 mph (64 knots or 119 km/hr) or higher are possible. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical-storm-force winds.

Hurricane Warning: sustained winds of 74 mph (64 knots or 119 km/hr) or higher are expected. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.

Tropical Storm Watch: sustained winds of 39-73 mph (34-63 knots or 63-118 km/hr) are possible within the specified area within 48 hours.

Tropical Storm Warning: sustained winds of 39-73 mph (34-63 knots or 63-118 km/hr) are expected somewhere within the specified area within 36 hours.

You can read about typical damage caused by hurricanes of all categories at Saffir-Simpson Hurricane Wind Scale.

Strong winds and thunderstorms arrive well before the center of the storm (sometimes a couple of days!), so when the time comes, be sure to plan and finalize your preparations prior to the expected arrival of tropical storm force winds, not the expected arrival of the storm's center.

This year will be my 27th year of writing these blog posts.  That's actually impossible to believe, but I hope the information and updates have been useful and educational.  Thank you for your continued interest!

29 November 2021

An incredible sixth consecutive active hurricane season ends

This post marks the end of my 26th year writing these updates on tropical Atlantic activity.  During that time, I have written approximately 1350 posts spanning 444 tropical cyclones, 197 hurricanes, 92 major hurricanes, and 50 retired storm names. I was honored to have been invited to write for the New York Times' hurricane blog from 2007-2010, and then for the Washington Post's Capital Weather Gang since 2012.  I truly appreciate your continued interest!

The 2021 Atlantic hurricane season is the sixth consecutive season with above-average activity: there were 21 named storms, 7 of which became hurricanes, and 4 of those became major hurricanes (Category 3+).  The average values of those quantities are 14, 7, and 3.

2021 was only the third season to ever exhaust the regular list of 21 names... the other two times being 2005 and 2020.  It was also the seventh consecutive season with a named storm formation prior to the official start of hurricane season on June 1, helping to pull the trendline earlier when we look back on the past fifty years of first named storm formation date. 
Timeline of the date of first named storm formation over the past fifty years. The cyan line marks the official start of hurricane season, the magenta line is the median date of first storm formation, and the dashed gray line is the linear trend.  I do not count Hurricane Alex in January 2016 here since that was meteorologically a late addition to the 2015 season.

The Accumulated Cyclone Energy (ACE) was above-average all season long, and blew past a full average season's total on September 28th -- it ended up at about 141% of average (using 1971-2020 as the baseline climatology).  Rather than counting the number of storms, ACE is a metric that accounts for the overall intensity and duration of whatever storms there are.

The ACE was above 129 units for the sixth consecutive year -- this has never happened before, not during the satellite era, not since records begin in 1851. This sustained level of tropical cyclone activity in the Atlantic is unprecedented even for four years, let alone six!

Hurricane Larry and Hurricane Sam were the heavy hitters of the year by this measure... Larry contributed 33% of the season's total and Sam contributed 37% of the total. The storm in third place is way down at 7%, and that was Ida.  It is worth mentioning that 9 of the 21 named storms were only around for two days or less, and they contributed just 4% of the season's ACE, combined.

As you can see on the chart above, the burst of activity in late September came to an abrupt end on October 5th.  The lone straggler was Tropical Storm Wanda which was around from October 31st to November 7th, and formed from a former Nor'easter.  It's extremely rare and peculiar to have an active season essentially shut down in the first week of October!

Although still preliminary, the 2021 season is the 4th costliest Atlantic hurricane season, behind 2017 (1st), 2005 (2nd), and 2012 (3rd).  The economic losses are expected to exceed $70 billion, bumping the fresh-in-our-memory 2020 season down to 6th place.

Drilling down to individual storms, Ana formed prior to the official start of the season, on May 22, as referenced above.  Bill formed off the southeast U.S. coast and headed out over the open ocean. Claudette, Danny, Elsa, and Fred all made landfall on the U.S. mainland.

Grace was the season's first major hurricane, reaching Category 3 intensity right as it made landfall in Mexico near Veracruz.

Enhanced infrared satellite animation of Category 3 Hurricane Grace making landfall on August 21st.

Hurricane Henri was quite impactful in the northeast U.S. when it made landfall in New England on August 22nd as a tropical storm.  It caused widespread power outages and produced record-breaking rainfall in New York City and flash flooding across several states.  This same area would be impacted with more heavy rain by Ida in just over a week.

A full-resolution version is available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

The most intense of the landfalling storms was Hurricane Ida, which made landfall near New Orleans on August 29th, on the 16th anniversary of Hurricane Katrina's landfall in the same location.  It was a Category 4 hurricane, reaching peak intensity right as it made landfall.  Its strong winds and storm surge caused extensive catastrophic damage across southern Louisiana.

A full-resolution version is available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

As usual, Ida's trail of destruction didn't end at the coastline. Three days after landfall, post-tropical cyclone Ida interacted with a mid-latitude trough and a very focused band of extreme rainfall was the result.  Although the event was remarkably well-forecast days in advance, rainfall totals in PA, NJ, and NY and then into southern New England were incredible, devastating, and deadly.

Ida was responsible for 115 fatalities and over $65 billion in damages from Venezuela and Colombia, then Jamaica and Cuba, and finally the United States.  It's always tempting to "write off" a hurricane after landfall, but in fact, there are often several days of severe impacts remaining as it moves inland... quite far from where it made landfall.  Ida is tied with 2012's Sandy as the 4th costliest hurricane in U.S. history.

Moving ahead, Larry was a long-track major hurricane, and a named storm for 10.5 days.  It clipped Newfoundland at the end of its journey on September 11th, causing fairly significant damage, and rip currents associated with it killed two people in Florida and South Carolina.

Like Grace and Ida, Nicholas reached its peak intensity right at landfall -- near Galveston TX on September 14th as a Category 1 hurricane.

The strongest storm of the season was Sam, which fortunately remained over water in the central Atlantic. It was a named storm for 12 days, nearly 8 of which were spent above Category 3 intensity! It also produced the fifth most ACE of any Atlantic hurricane on record (https://twitter.com/BMcNoldy/status/1445428509220892673)! 

Hurricane Sam on September 26th, when it was just a click away from reaching Category 5 intensity.

Keep in mind that all statistics presented here are preliminary; the National Hurricane Center will have the final post-season reanalysis of all storms complete in a few months.  We will also find out which names will be retired early next year.  But we can reasonably anticipate that Ida will be retired, so I will offer these following updated charts with that in mind (and not knowing if any other names will be retired from this season).  Ida's all-but-certain retirement puts "I" storms even more in the lead, as well as retired storms that peaked at Category 4 intensity.  (The colors on the bars are only scaled by value for the sake of visual interest.)

Next year's name list starts off with Alex, Bonnie, and Colin.  Two new names appear on the 2022 list:  Martin and Owen, which replace Matthew and Otto from 2016.

31 October 2021

Trick or Treat? October ends with Wanda

What is now Subtropical Storm Wanda had its origins over Georgia six days ago, then was a potent Nor'easter a few days later, and then transitioned from an extratropical cyclone to a subtropical cyclone, earning a name in the process.  Wanda is the season's 21st named storm, and the last on the regular list of names.  Victor was the last named storm to form in the Atlantic (until now), and that was named way back on September 29th!

Wanda won't be affecting land in its future, but has peak sustained winds of 50 mph and is located about 1800 miles east of North Carolina and 1900 miles west of southern Portugal -- truly the middle of Atlantic!  It is not forecast to reach hurricane intensity, and should transition back to an extratropical cyclone by the end of the week.

The cumulative ACE (Accumulated Cyclone Energy) is at about 143% of average for the date, and Wanda won't contribute too much more to the overall tally.

With one month remaining in the official Atlantic hurricane season, there's a chance that Wanda won't be the last.  If anything should form, we'll switch over to the auxiliary list, shown below.  This list was chosen to replace the use of the Greek alphabet.  Only 2005 and 2020 ever exhausted the regular list of 21 names before, so to happen in two consecutive years now is extraordinary!

There is actually an easterly wave that just left the African coast, and NHC is giving it a 30% probability of becoming a tropical cyclone in the next five days.  It has been tagged as Invest 95L.  This would be a very late-season Cabo Verde storm should it form.