With this year’s Atlantic hurricane season right around the corner (June 1), I want to offer some updates and refreshers since most people don’t think too much about hurricanes during the winter. I say “most” because there’s a big community of researchers and forecasters that eat, breathe, and sleep hurricanes all year long!
While this season is expected to have above-normal activity, it is important to realize that major hurricanes can still form and make landfall even in inactive seasons; the odds just increase when there are more storms.
New for 2013
What’s in a name…?
Since 1950, tropical cyclones in the Atlantic Ocean have been given names to help quickly and uniquely identify current and past storms (see “The reasoning for hurricane names and their history”).
From 1950-1952, storms were given names from the Royal Air Force’s phonetic alphabet. For the following 26 years, all storm names were female names. Then in 1979, the current system of alternating male and female names was introduced, and the same six lists are rotated through in six years. So unless a name is retired, it will be used again six years later. For example, Ana has been used in 1979, 1985, 1991, 1997, 2003, 2009, and will be used again in 2015. But occasionally, the first time a name is used is also the last. The most recent example is Igor; Igor was introduced to the list when Ivan was retired in 2004. But, Igor was then retired after its one and only use in 2010.
According to the National Hurricane Center, names get retired “if a storm is so deadly or costly that the future use of its name on a different storm would be inappropriate for reasons of sensitivity”. This year, we have three new names in the list. In 2007, Dean, Felix, and Noel were retired; they are replaced by Dorian, Fernand, and Nestor. The full list of names for the 2013 season is shown to the right.
A shrinking cone…
Another noteworthy update for 2013 is a smaller “forecast cone” (also referred to as the “cone of uncertainty”) for tropical systems. The National Hurricane Center (NHC) uses a cone on its forecast track graphics to convey typical forecast errors. While a center line is drawn for clarity, forecasters stress not to focus too much on it because there are likely to be errors in the forecast, and the storm will affect areas far away from the center.
The cone is designed to enclose two-thirds of the recent forecast errors meaning, that on average, there is still a one-third probability that the center of the storm could track outside of the cone.
Of course, the center of the storm is just a small part of the story. Since any given storm expands well beyond its center, the impacts of a storm can be felt a substantial distance away. Thus, the forecast cone is NOT an impacts cone.
Presently, the same cone size is used for an entire season and for all storms. The 2013 cone size is determined by NHC’s track errors from the 2008-2012 seasons. As track forecasts improve, the cone gets smaller!
The figure below compares the cone used in 2008 to the cone that will be used in 2013 for an identical [hypothetical] storm and forecast. This is great progress, as reducing the uncertainty in a forecast can reduce the time and resources spent on preparations and evacuations.
New tropical weather impacts-based graphics
Also in the spirit of improved visual communication of forecasts, there is an experimental web-based product that has been in development for over ten years, and has undergone several iterations of public feedback and inter-agency collaboration.
When an active storm threatens the coast, the Tropical Cyclone Impact Graphics (TCIG) website lets users view detailed maps showing potential impacts from wind, storm surge, inland flooding, and tornadoes. Impacts in each of those categories are ranked and color coded from “None” up through “Extreme”.
This impacts communication product is expected to transition to fully operational in the 2014 or possibly 2015 hurricane season, but it is being supported on an experimental basis for this season.
Its development has been further motivated by the recommendations of both the Hurricane Irene and the Hurricane Sandy service assessments. Given the best efforts of forecasters, emergency managers as well as the general public need a reliable, consistent, and clear picture of where the impacts will be felt and how severe they’ll likely be, this product should be a big step in the right direction.
Update to policy for watches and warnings
Finally, last October’s Hurricane Sandy was a challenge when it came to issuing watches and warnings along the northeast U.S. coastline. It was clearly a hurricane when it made landfall on Jamaica, Cuba, and the Bahamas, but its future identity was uncertain as it passed east of Florida and the Carolinas.
It was expected to transition to an extratropical cyclone (think Nor’easter) prior to landfall on New Jersey, so high wind warnings and flood warnings were issued instead of hurricane warnings.
Some felt hurricane warnings should have been hoisted and criticized the move.
In response to the criticism, NHC has added the ability to maintain tropical storm and hurricane watches and warnings even if the storm loses is tropical characteristics. So in a case like Sandy, a hurricane warning could be issued 36 hours prior to landfall, then the storm could become a subtropical or post-tropical cyclone, and a hurricane warning could still be valid as the storm comes ashore… keeping the stream of products and the message to the public consistent.
Early season climatology and outlook
Hurricane season officially extends from June 1 through November 30, but activity can occur outside of those arbitrary dates. The six-month season climatologically encompasses about 97 percent of all hurricane activity, and the core three month period of Aug-Sep-Oct encompasses about 78 percent of all hurricane activity. Last year, Alberto and Beryl both formed in May, but did not become hurricanes. The last time a hurricane formed in May was Alma in 1970, so it is indeed a rare event.
In late May and June, the few storms that do form have their beginnings in the western Caribbean Sea, over the Bahamas, or in the Gulf of Mexico. Gradually, as we get into late June and July, the favored formation areas expand into the deep tropical Atlantic (just east of the Lesser Antilles) and off the U.S. East Coast.
This spring, conditions across the Atlantic basin indicate that activity throughout the season should be above normal (see “Four groups predict very active 2013 Atlantic hurricane season”). In terms of seasonal Accumulated Cyclone Energy (a cumulative metric for storm activity), forecast groups are expecting 2013 to be about 180 percent above normal, including elevated probabilities of U.S. hurricane landfalls. Some key factors in that forecast are the state of El Nino, ocean temperatures across the deep tropical Atlantic, surface pressures in the central basin, among many others.
Most global models agree that ocean temperatures in the equatorial eastern and central Pacific will remain close to normal… meaning that there likely won’t be an El Nino or La Nina this summer. During strong El Nino events, hurricane activity in the Atlantic is suppressed, while during strong La Nina events, it’s enhanced. Models are forecasting a 60-70 percent chance of neutral conditions to persist through the season.
However, two simple ingredients that point to increased chances of above normal activity are the surface pressure and sea surface temperature. With 30 years of seasonal prediction expertise, the Colorado State University forecast team utilizes a published statistical approach. While the relationships between predictive factors are complex, a couple things to look for are below-normal surface pressures in the central part of the basin (dark purple area in the graphic below), and above-normal SSTs in the far eastern basin (reds and oranges in the graphic below). So far, in tropical cyclone development regions, pressures are low and sea surface temperatures are high - a bullish signal for storm activity.
Within the satellite era, according to CSU’s Bill Gray and Phil Klotzbach, the season thus far is most analogous to 1966, 1996, and 2004 – which were active years.
The 2004 hurricane season is recent enough that people still remember it, and not very fondly (that year had some notable landfalls: Charley, Frances, Ivan, and Jeanne). But seasonal forecasts are statistical in nature; they do not include forecasts of specific storms hitting specific places.
So, without knowing if and when a hurricane might come to visit your area, what can you do now? If you live near a coastline, there are action items that you can address anytime to maximize your preparedness and minimize your stress. 1) exterior: have trees trimmed away from your house and power lines, remove debris and trash, locate and inspect your window protection. 2) interior: buy non-perishable staples like water and canned goods, check your insurance policies for coverages and accuracy, think about evacuation plans (where would you go, what are your criteria).
* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.