What Do Seasonal Hurricane Forecasts Mean to the Residents of Florida?

Copyright © 2002 James B. Elsner. All rights reserved.

Every year sounds a warnings of the approaching hurricane season. ``This year it will be very active," or ``Fewer storms are expected due to El Niño." Who issues these forecasts? How accurate are they? Should I use them to cancel my Caribbean vacation? Are they useful in making my preparations? Here we provide a brief look into this active area of research.

Two pioneers in the field of seasonal hurricane forecasting are Professor William Gray and Dr. Neville Nicholls. During the 1970's, Dr. Nicholls discovered that El Niño1 meant fewer tropical cyclones near Australia. Since an El Niño conditions change slowly from one month to the next, Dr. Nicholls could anticipate Australian storminess a few months in advance. Professor Gray from Colorado State University used a similar idea to predict hurricane activity over the Atlantic beginning in 1984. Gray, who has been watching the tropics for nearly a half century, noticed that jet-level winds during an El Niño can hamper the development of hurricanes. The business of seasonal hurricane forecasting grew from there. Currently the National Oceanic and Atmospheric Administration (NOAA), the parent organization of the National Weather Service, issues an ``official" forecast for the season.

Most seasonal forecasts are based on hurricane records from the past. For instance, 11 hurricanes formed in 1995, 9 in 1996 and 3 in 1997. The variation is not entirely random. Fewer hurricanes in 1997 resulted from unfavorable winds of El Niño. To make sense of this information, climatologists use statistics. Meteorology helps answer the question why fewer hurricanes form during El Niño, but statistics provides the mathematics to make forecast models. Models predict the number of hurricanes expected over the North Atlantic, which includes the Gulf of Mexico and Caribbean Sea.

The average year has between 5 and 7 hurricanes. A below average year has fewer than 5 hurricanes and an above average year has more than 7. Forecasts are more accurate than simple guesswork. However, since most hurricanes remain out at sea, the number has little relationship with your chance of experiencing high winds or surge. A case in point is 1992, when a below average year (4 hurricanes) resulted in a devastating blow to Florida in the form of Hurricane Andrew. By comparison 2001 was an above average year (9 hurricanes) but no direct hits to the United States.

More precisely, suppose the forecast calls for an average hurricane season (5, 6 or 7 hurricanes). How many of these will affect Florida? This is the billion dollar question. Among the last 52 years (1950-2001) 21 of them were average, 15 were above average, and 15 were below average. The average number of Florida hurricanes during the 21 average seasons is 1.38, this compares with 1.25 during above average seasons and 0.80 during below average seasons. Surprisingly the probability of a Florida hurricane is highest during average seasons. The figure shows probabilities of Florida hurricanes conditioned on the abundance of hurricanes that form. During a below average season, the probability of a Florida hurricane is low (30% compared to a average value of 40%). Yet during a normal season the probability of seeing at least two Florida hurricanes increases from 10% to 15%. Assuming the seasonal forecast is perfectly reliable and the present climate is no different than the past, a forecast of an average season is the most ominous to Florida residents.

 
Figure: Probabilities of a hurricane making a direct hit on Florida. The cyan bars indicate climatological probabilities and the magenta bars indicate conditional probabilities based on overall activity being (a) below normal, (b) normal, and (c) above normal. Probabilities indicate the chance of seeing no hurricane (0), exactly one (1) hurricane, exactly two (2) hurricanes, etc. Number of years over the 52-year period (1950-2001) with precisely 2, 3, etc hurricanes over the North Atlantic (d).

Recently scientists have been working to make seasonal forecasts more relevant by focusing directly on landfalls. Professor James Elsner at Florida State University has identified climate signals that indicate where storms will track. A forecast that the storms will track through the Caribbean portends an active year along the Gulf coast. Although this information is useful to coastal residents, the accuracy of these predictions remains limited. The science and technology of hurricane climate is new. Every year we learn more. Seasonal forecasts will get better and more specific. Meanwhile the advice is to be prepared regardless of what the prognosticators say.

Discussions with Matt Green on this topic are greatly appreciated.

Footnote:

1. El Niño refers to the periodic warming of the ocean waters off the coast of Peru, typically commencing around Christmas time.