01 April 2021

2021 "Cone of Uncertainty" Update & Refresher

Anyone who lives on a hurricane-prone coast or even watches television is familiar with the infamous "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 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 the 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 were 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 2021 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.

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 2021 cone size is thus determined from all track errors during the 2016-2020 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.  Recently, NHC has added the 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), and M (major hurricane (Category 3+)).  But keep in mind that there is uncertainty associated with the intensity forecasts too!
To think about a cone of uncertainty for intensity, consider this: averaged over the past five years (2016-2020), 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 +/- 19 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.

Other important terms:

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.

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 center.

This year will be my 26th 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!

30 November 2020

The record-smashing 2020 Hurricane Season ends today

The 2020 Atlantic hurricane season officially ends today, and it has certainly been one for the record books.  As of today, there were 30 named storms, 13 of which became hurricanes, and 6 of those became major hurricanes (Category 3+ on the Saffir-Simpson Scale).  For context, the average of those same quantities over the past fifty years is 12.2, 6.3, and 2.5.

This also marks the end of my 25th year writing these updates on tropical Atlantic activity.  During that time, I have written approximately 1305 posts spanning 412 tropical cyclones, 190 hurricanes, 86 major hurricanes, and close to 50 retired storm names (2019 and 2020 TBD). I was honored to have been invited to write for the New York Times hurricane blog for four years, and then for the Washington Post's Capital Weather Gang from to 2012 to present.

The previous record number of named storms was 28, set in 2005.  And not only did 2020 beat that record, it did so quickly.  With the exception of the 1st, 2nd, and 4th storms, the remaining 27 were all the earliest formation dates on record! 

A common metric used to describe overall activity is the Accumulated Cyclone Energy, or ACE.  It is independent of the number of storms and their tracks, but essentially measures a cumulative intensity and duration of all storms.  14 of the 30 named storms were around for three days or less, and coincidentally 14 of the 30 never got stronger than a mid-range (50-knot) tropical storm.  So despite the large number of named storms, 17 of them were fairly short-lived and/or weak -- a handicap for generating a lot of ACE.  The 2020 season is in 10th place in terms of ACE, and is at 178% of an average season.

For the sixth year in a row, there was pre-season activity. 2020 came out of the gate strong, with Arthur and Bertha forming during May. Then Cristobal formed on the first day of the official season and it never let up after that.  A record ten named storms formed during September.  The regular list of 21 names was exhausted by mid-September, and the Greek alphabet was utilized for only the second time ever... beginning with Alpha on September 18 (which strangely enough was Portugal's first named storm landfall).  As of today, nine letters of the Greek alphabet have been used.

Two things stand out to me in that track map at the beginning of the post. The first is that only one hurricane (Teddy) existed in the tropics between the central Caribbean and Africa. That is not a trait we tend to think of with very active seasons.  The second is the high concentration of tracks in the western Caribbean and Gulf of Mexico.  I'm not exactly sure what caused this pattern, but some large-scale atmospheric/oceanic forcing was at work.  I don't think the weak La Nina alone explains this. Unfortunately, storms in the western Caribbean Sea and Gulf of Mexico almost always make landfall... there's no way out.

A record-breaking twelve named storms made landfall in the contiguous U.S., easily surpassing the previous record of nine set in 1916. Five of those twelve hit Louisiana alone, and three of those five were hurricanes (Laura, Delta, and Zeta). The Yucatan peninsula had three landfalls, including two hurricanes (Delta and Zeta), and then there's Nicaragua. Two Category 4 hurricanes made landfall at the same location (technically seven miles apart) just two weeks apart: Eta and Iota. Iota, a mid-November storm, became the season's strongest storm, rapidly intensifying to reach Category 5 status.  Not only was it the season's only Category 5 hurricane, it made 2020 the fifth consecutive year to have a Category 5 hurricane. November 2020 was the only November to have two major hurricanes.

Tracks of the landfalling tropical storms and hurricanes in the western Caribbean Sea and Gulf of Mexico during 2020.

Although the final reanalysis of the season won't be completed for a few more months, the preliminary track and intensity verification of forecasts made by the National Hurricane Center are shown below.  To arrive at these, all 24-hour forecasts are evaluated against all observed values at that time, and so on out to 120 hours.  The average error over the prior five seasons is included for reference. Overall, track forecasts were extremely close to the average out to four days, then slightly higher at five days (Paulette was a big reason for that drift at five days). Intensity forecasts were near-average out to two days then better than average for three, four, and five-day forecasts.

Of course, the bounds of the official hurricane season (June 1 through November 30) are arbitrary, and nature could still throw in more storms during December.  Years with post-season storms include 2013, 2007, 2005, 2003, 1984, 1975, etc. so it's certainly not unheard of; in fact, there have been sixteen known named storms to form during December since 1851, five of which became hurricanes.  The next two names would be Kappa and Lambda. Otherwise, the 2021 regular list will kick off with Ana and Bill.

Tracks of the 16 known storms that became tropical storms or hurricanes during December since 1851.

17 November 2020

Iota becomes Category 5 hurricane before landfall in Nicaragua

Enhanced satellite image of Hurricane Iota as it made landfall in Nicaragua on 17 November 0400 UTC.

From bad to worse... Category 4 Hurricane Eta made landfall in Nicaragua two weeks ago today, spreading catastrophic wind, flooding, and mudslides far inland through Central America. On Monday, Iota rapidly intensified to become the season's strongest hurricane, reaching Category 5 intensity just prior to making landfall in the same location as Eta. It "weakened" only slightly to a top-end Category 4 hurricane as the eye crossed the coastline early Tuesday morning. It's impossible to imagine two such hurricanes in two weeks at the same location. Iota will now dissipate over the mountainous terrain of Central America, dumping huge amounts of rain along the way. 

Iota was the only Category 5 hurricane of the 2020 season (so far), but it also claimed the title of being the latest Category 5 hurricane on record.  The one and only other candidate was in early November of 1932.  It also marks an unprecedented string of five consecutive years with Category 5 hurricanes in the Atlantic. This map below shows the tracks of the seven Category 5 hurricanes that have occurred over the past five years: Matthew (2016), Irma (2017), Maria (2017), Michael (2018), Dorian (2019), Lorenzo (2019), and Iota (2020).  Then there were ZERO from 2008-2015, then eight from 2003-2007. They definitely come in surges.

Unfortunately, there's a hint of another tropical wave right behind Iota, and there is some support for its development in the models -- the National Hurricane Center is giving it a 40% probability of forming later this week as it approaches Central America. This map shows a forecast of minimum surface pressures of trackable lows from the American GFS model ensemble on Friday, and that clustering of ensemble members near Costa Rica and Nicaragua is very troubling. Should this become a tropical storm, it would be the season's 31st and be named Kappa.

While the season continues to obliterate records for the number of named storms, the earliest formation date of the Nth named storm, and also for number of landfalls, it just now snuck into 10th place in terms of ACE (Accumulated Cyclone Energy).  ACE is a common metric used which basically accounts for overall intensity and duration, not the number of storms or where they go. The nine years that had more ACE are all very familiar to tropical cyclone enthusiasts: 1933, 1926, 2005, 1893, 1995, 2004, 2017, 1950, and 1961. It's fitting that 2020 now joins the top ten list.