Time to Heed the Hurricane Season Forecast?

June 11, 2014 · 1 comment

With this year’s Atlantic hurricane season getting underway, seasonal forecasts are collectively calling for a quieter-than-usual year in the Atlantic basin, which includes the Caribbean Sea and Gulf of Mexico. With busts in these forecasts as recent as last year, however, is this actually reassuring news?

Mark Powell, a NOAA hurricane researcher who is now working with Florida State University’s Center for Ocean-Atmospheric Prediction Studies, was quoted in a recent Palm Beach Daily News article saying such forecasts, which typically are made before the start of the six-month season, “just don’t have any skill this early.”

They practically don’t. The major players of Atlantic hurricane season forecasts—NOAA, Colorado State University (CSU), and the private British forecasting firm Tropical Storm Risk (TSR)—stipulate that there is only a small increase in skill with pre-season forecasts (i.e., how much better such forecasts are) compared to climatology in foretelling the number of named storms that will form in the Atlantic basin from June 1 to November 30. Supporting this, a talk by Eric Blake of the National Hurricane Center presented at the 29th AMS Conference on Hurricanes and Tropical Meteorology stated that NOAA’s May forecasts for anticipated numbers of storms and hurricanes are only slightly better than what climatology showed had occurred in the previous five seasons. Forecast skill does increase as the season nears its peak months: August, September, and October, which is when 70 percent of tropical storms and hurricanes form.

But even these “better” mid-season forecasts can be wrong. In 2013, early predictions for an active season remained high as September neared, yet the actual number of named storms (13) and especially hurricanes (just 2) fell short of most forecasts, which had been collectively predicting a blockbuster season with at least seven hurricanes and three major hurricanes. The long-term average number of named storms and hurricanes is 12 and 6, respectively. There wasn’t a single major hurricane sporting winds greater than 110 mph last year, when climatology said there should have been at least three. In a blog post at the end of last year’s season, Jeff Masters of WeatherUnderground.com detailed the reason the forecasts failed: the large-scale atmospheric circulation, which can’t be predicted more than a week or two out and isn’t part of seasonal forecasts, was not conducive to tropical cyclone formation.

In 2012, however, it was, and the opposite occurred. The number of named storms peaked at 19—not only well above average but also the third highest number of storms on record in a single season. Ten of these went on to become hurricanes, exceeding most seasonal forecasts including NOAA’s, which had called for an average Atlantic hurricane season prior to its start. CSU had projected in June of that year a relatively quiet hurricane season with 13 named storms and only 5 hurricanes. With twice that many hurricanes forming, the season blew the forecast out of the water, making it CSU’s worst bust in decades of predictions. Until 2013.

This year, however, hurricane season forecasters feel the chance of the development of El Niño by the fall is much higher than it was in 2012—70 percent this summer increasing to 80 percent by fall, according to NOAA’s 9 June El Niño status statement—which lends significant weight to the lower hurricane predictions.

Historically, El Niño stifles Atlantic hurricanes. The enormous slug of warmer-than-normal sea surface temperatures along the Eastern Pacific equator, which defines El Niño, imparts a shift in the atmosphere’s circulation that drives unusually strong winds at high altitudes across the tropical Atlantic Ocean during the season. The induced wind shear—a difference in both speed and direction between the surface and aloft—suppresses not only tropical cyclone development but also formation, tearing apart hurricane seedlings before they can organize.

Research in recent years (Journal of Climate: 2009, 2013), however, has shown that a very different effect can result from El Niño’s half-brother – an anomalous warming in the tropical Pacific that pools in the central rather than the eastern part of the ocean basin. When this occurs, as it did in 2004, it can actually amplify the Atlantic hurricane season. That year, four hurricanes—three of them major, including Charley with 150 mph winds—slammed Florida. There were 15 named storms that year.

No forecasters expect a repeat of the devastating 2004 hurricane season. But, already we’ve had a record eight hurricane seasons without landfall of a major hurricane in the United States nor any category hurricane striking Florida. The last one was Wilma in 2005, which hit as a major hurricane.

Whether or not forecasts are accurate this early in the season, the old adage still applies: it only takes one hurricane in your area to create disaster. So be prepared.