22 May 2021

Ana kicks off 7th consecutive year with pre-season activity

A lingering low pressure system northeast of Bermuda was just upgraded to Subtropical Storm Ana on Saturday morning, which makes 2021 the seventh consecutive year with a named storm prior to June 1, and Ana is the ninth pre-season named storm in that same period.  [A bit of history: Ana is still an original name from when it was introduced in 1979.]

Ana has peak winds of 45 mph and is nearly stationary but will begin to drift away from Bermuda.  While it could bring stormy and gusty conditions to the island, it is not forecast to strengthen much as it will encounter a more hostile environment by the end of the weekend. A tropical storm watch is in effect for the island.

This early activity barely seems strange anymore.  There have been named storms forming prior to June 1 every year since 2015 now!  The list includes Ana (2015), Alex (2016*), Bonnie (2016), Arlene (2017), Alberto (2018), Andrea (2019), Arthur (2020), and Bertha (2020).  Although there's no trend in the date of first hurricane formation, there is a clear trend in the date of first named storm formation over the past 50 years. In this chart, I do not include Alex (2016) because it occurred in January and was meteorologically a remnant of the 2015 season.

The season officially begins on Tuesday, June 1.  Last year, the season's THIRD named storm formed on that date!  The next name on this year's list is Bill.

If you missed the first two posts of the season which serve as introductions and refreshers, you can find them at:

20 May 2021

2021 Hurricane Season Intro

Five-day forecast of hurricane wind speed probabilities for Hurricane Irma (2017). (NHC)

Hurricane Basics

Hurricanes are large low pressure systems that generally require warm tropical water to form and strengthen.  They come in a range of sizes, but are classified solely by their wind speed. Since the early 1970s, the Saffir-Simpson Scale has been used to rate hurricanes by the peak sustained wind speed found somewhere in the storm. There will always be higher gusts, but those are not used to categorize hurricane intensity.

Before becoming a hurricane, there are two stages. When a big cluster of thunderstorms becomes persistent and organized enough to have a "closed" circulation in the surface wind, it is designated a tropical depression and given a number.  While these can produce tremendous amounts of rain, they are quite weak in terms of wind speed.  If the sustained winds (not gusts) reach 40 mph, it is upgraded to a tropical storm and given a name. Tropical storms also have the potential for heavy rain, but winds are now strong enough to do serious damage to vegetation (and falling trees or tree limbs can take out power lines and roofs/buildings).  It is unsafe to be outdoors during tropical storm force winds.

If a tropical storm continues to strengthen and sustained winds reach 74 mph, it is upgraded to a hurricane.  From there, the stronger it gets, the higher the category rating it gets.  The scale tops out at Category 5 (157+ mph) because at that point, catastrophic failure of structures and trees has already happened.  Only 2% of named storms in the Atlantic ever reach Category 5 intensity. The most intense Atlantic hurricane on record is Allen in 1980 with 190 mph sustained winds, and the most intense landfalling hurricane in the United States was the 1935 Labor Day Hurricane with 185 mph sustained winds. Note that the categories are not evenly-distributed by wind speed, but instead are binned by destructive potential (what breaks).

The term "tropical cyclone" collectively refers to any of these stages: tropical depression, tropical storm, and hurricane.  The term "major hurricane" refers to any hurricane that is Category 3 or higher on the Saffir-Simpson scale.  It has nothing to do with impacts, damage, or fatalities it may cause. A hurricane of any category can be destructive and deadly; or in other words there's more to the story than the category!

To illustrate a point about size, the satellite images below show two major hurricanes in the eastern Gulf of Mexico at approximately the same scale and intensity: Charley (2004, left) and Rita (2005, right). While the peak wind speed was roughly the same, the larger a storm is, the more areas and people it will impact if it hits land.

In terms of structure, a well-organized hurricane is characterized by a calm, clear central eye which is surrounded by a thin ring of intense wind called the eyewall. The eyewall is the "business end" of the beast, and contains the highest wind speed on which the category rating is based.  Wind speeds drop rapidly (exponentially) outside of the eyewall, but rainbands spiral outward from there for hundreds of miles and contain violent winds and tornado-producing thunderstorms.  Impacts such as storm surge, tornadoes, and flash flooding can all extend hundreds of miles from the center of the storm.

Radar image of Hurricane Irma (2017).  Although radar images depict rainfall intensity, it correlates well with wind intensity too in many cases. The eye is the central "hole", the intense eyewall is the red-orange ring surrounding the eye, and some rainbands can be seen spiraling out from the eyewall.

Seasonal Climatology

The official Atlantic hurricane season spans June 1 through November 30, and by design, the vast majority of activity is typically confined to those dates (not necessarily all of it).  Within the season, about 85% of the activity normally falls within the core months of August, September, and October.

The rest of the content has a more south-Florida-centric theme, but each region has its own unique history, climatology, and evacuation rules so be sure to look them up if you're in a hurricane-prone area of the coastline. Although the climatological peak of the season is in early-mid September, the month with the most hurricane impacts in south Florida is October.  June, July, and November have typically been relatively quiet here, while August and September are when the strongest storms have hit.  Also notice that about half of the storms that impact us come from the east (generally Jun-Sep) and half come from the west (generally Oct-Nov).  Hurricanes are big, so we can experience significant impacts even without a direct landfall.

Tracks of hurricanes from 1851-2020 that passed within 100 miles of Miami, grouped into each month of hurricane season. This works out to an average of one nearby hurricane every three years.

Miami-Dade County has only been affected five times in the past fifty years, including one major hurricane.  Compare that to the previous fifty years, when the county was hit seventeen times, eleven of which were major hurricanes!  The chart below shows a timeline of hurricane impacts in the county with major hurricanes marked in red, and overlaid in the orange line is the county's blossoming population.

Although 2017's Hurricane Irma counts because it produced hurricane-force winds in far western parts of the county, the metropolitan eastern parts of the county experienced "only" tropical storm conditions.

There is no law of nature that says we cannot get hit twice in the same season... it has happened six times since records began in 1851. In 1948, we actually got hit twice within two weeks. Then more recently in 2005, Katrina and Wilma produced hurricane conditions in the county two months apart. Though rare, it doesn't hurt to be aware of the possibility.

Property Preparation

As we saw in the abnormally lengthy span between Wilma and Irma, an extended break allowed a messy situation to arise: twelve years of untested tree growth. The prolonged tropical storm force winds associated with Irma were good at finding weak, rotten, top-heavy, or diseased trees and breaking or uprooting them. If another Irma were to occur this year, the debris piles would be noticeably smaller.  But if something stronger comes along (a hurricane of any category), branches and trees that withstood Irma could go.

This is the perfect time to look around your property to see what you can do to minimize and mitigate damage long before a storm threatens.  That includes random unused outdoor items, and especially trees.  Having your trees properly pruned, thinned, and/or topped (or removed if they're dead or rotten) now can save you and your neighbors damage and power loss during a storm.  If the tree is growing into or over power lines, it's best to report it to FPL and they'll take care of it -- it saves them time to do preventative work rather than repair work.  (If you have a mango, avocado or other tree with fruit on it now, it's best to wait until right after fruiting season to trim it.)


If you have a generator, try it out before the start of each hurricane season just to be sure it works. If you have shutters for your windows and doors, make sure they are all accounted for and in good working order.  These would be issues you don't want to discover two days before a hurricane landfall.

Review your insurance policies and coverage (auto, home, windstorm, flood, renters), and make sure you know a number to call after a storm if you need to file a claim.  Keep important documents (including insurance policies) in a safe and dry place or bring them with you if you leave.  A quick call to your agent is easy and can give you peace of mind that you have the coverage you thought you had.  Having a photo/video inventory of your possessions is also a good idea. Many insurance company phone apps make this really easy.

Evacuation Zone and Plan

In the event of an intense hurricane approaching, storm surge increasingly becomes a concern.  Storm surge is the rise of water levels due to strong onshore winds "bulldozing" the ocean onto land, and is the #1 killer during hurricanes in the U.S. by far.  Storm surge also does not correlate perfectly with a storm's category rating... it  depends on the size of the storm, forward speed of the storm, coastal topography, offshore bathymetry, and more.  A large slow-moving Category 1 hurricane could generate a larger storm surge than a small Category 3 hurricane, for example.  Remember: "There's more to the story than the category!"

In Miami-Dade County, evacuation zones are defined primarily by storm surge risk (not wind risk), and the zones are not limited to the immediate coast.  The surge of water can travel up canals and rivers.  The zones are crude in shape, and the entire county would never experience the peak surge or evacuation orders.  But knowing your zone letter gives you a general sense of your potential risk from storm surge and evacuation priority... here is a map centered on the northeast portion of the county:

and here is the interactive website if you want to peruse more on your own:
In Broward County, a two-tier evacuation zone system is used and the map is available at http://www.broward.org/Hurricane/Documents/EvacuationMapADACompliant.pdf

If you decide to evacuate, it is a good idea to have a place and plan in mind before the stress of an approaching storm sets in.  Secure your house, shut off all power/gas, bring valuables and important documents with you, and always let friends and neighbors know where you're going and how to contact you.

If you decide to stay and shelter in place, be prepared to be self-sufficient for at least three days, longer if possible. This means enough food, clean water, and medication for every person and pet in the house. Also expect to be without electricity for an extended time (possibly over a week), as well as internet access and even phone service. Communication will likely be a challenge in the immediate aftermath. If you have a portable generator, don't forget to stock up on gas for it.  Spare batteries and basic first aid supplies should also be part of your standard hurricane season checklist.

If you haven't read my intro and refresher post about the National Hurricane Center's "cone of uncertainty", you can find that HERE

This will be my 26th year of writing these updates for the blog, though it has taken a few forms over the years. I really appreciate everyone's continued interest!

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!