21 June 2024

Two disturbances to watch in the coming days

As a brief follow-up since my previous post on Monday, the season's first named storm, Alberto, did indeed form in the southwest Gulf of Mexico on Wednesday, then drifted westward into Mexico... making landfall near Tampico as a tropical storm.


Today, there are two more systems of interest, one headed for Florida/Georgia and one expected to form in the same area and fashion as Alberto did.  The one headed toward Florida is tagged as Invest 92L.  Fortunately, neither one is expected to intensify much, but another round of heavy rainfall will be a huge concern with the one following in Alberto's footsteps.


A radar animation of Invest 92L from the Jacksonville NWS radar is shown below, spanning the morning hours from 4am-9am.


This disturbance is getting more organized by the hour, but is rapidly running out of time before it's over land.  However, model guidance shows that it will get picked up by an approaching trough and *could* be back over the ocean (including the warm Gulf Stream) early next week where it has an even better chance of becoming a tropical cyclone.  That is, if there's anything left of it after 2-3 days over land.


Shifting our attention 1000 miles to the southwest, another disturbance is brewing over the Yucatan peninsula and the National Hurricane Center is giving it a 60% probability of becoming a tropical cyclone in the next seven days.  As I mentioned in the beginning, the evolution and track of this system will very closely mimic what we just saw with Alberto.

The next couple of names on the list are Beryl and Chris. Tidbit: the first five names on this year's list are all original from the 1982 list when they were first used (there are six lists of names that get reused).  So, Alberto, Beryl, Chris, Debby, and Ernesto were all used in 1982, 1988, 1994, ..., and 2024.  This year, we have two new names on the list: Francine and Milton.  They replace Florence and Michael which were retired after the 2018 season. 

17 June 2024

Season's first named storm could form this week

Atlantic hurricane season officially began on June 1, though it has been fairly quiet so far.  There are a couple of areas of interest now: one headed toward the southeast U.S. coast with a low chance of formation (30%), and one in the southern Gulf of Mexico with a high chance of formation (70%).  If the Gulf of Mexico system reaches tropical storm status, it would take the first name on this year's list: Alberto.


Evident in the infrared satellite animation below is the very disorganized appearance of the eastern one north of Puerto Rico and the huge envelope of moisture and thunderstorms slowly coalescing around a deepening low pressure in the Bay of Campeche.


The models are not in great agreement over what happens, but they do agree on the big picture.  The western system in the Gulf of Mexico will slowly ooze northward, though may track westward enough to run into Mexico before making it as far north as Texas.

There's minimal support for development of the eastern system, and among the ensemble members that do anything with it, it heads northwest toward somewhere between northern Florida and North Carolina.  It remains weak and as of now does not appear to have a big rainfall footprint associated with it.

Regardless of development into a tropical cyclone, heavy rain from the system in the Gulf will find its way to Texas from Tuesday morning through Thursday morning... this map shows the forecast rainfall totals over the coming week:


Over the past five decades, the median date of first named storm formation is June 15, so we're a tad behind that, and this is already the latest a first named storm has formed since 2014 (that was on July 1).

08 April 2024

2024 "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 2024 cone overlaid on the 2014, and 2004 cones for comparison.  This year, the cone is actually larger at all lead times compared to recent years: at a 1 day lead time it's the biggest it's been since 2020, at 2 days it's the biggest since 2018, and at 5 days, it's the biggest since 2016.

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 2024 cone size is thus determined from track errors during the 2019-2023 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 (2019-2023), the mean error in a 1-day forecast is +/- 8.0 mph, the error in a 3-day forecast is +/- 12.3 mph, and the error in a 5-day forecast is +/- 19.8 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 2024 are based on the NHC's intensity errors during the 2019-2023 seasons (and 2023 used 2018-2022, 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 2024, 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.