The official NOAA forecast was released Thursday, May 19 and it falls right in line with what Universities and private meteorological companies have been forecasting for the last few months: another above-average Hurricane Season is predicted in the Atlantic for 2011.
Compared to last year, 2011 is forecasted to be less active, but still above normal with early predictions anywhere from 23-18 named storms forming in the Atlantic and Gulf of Mexico. On average, 10 named storms actually form.
Estimates range from 6-10 of these storms becoming hurricanes, with 3-6 of these becoming major storms; those reaching a Category 3 status on the Saffir-Simpson scale carrying winds of 111 mph.
Why the active season? First off, surface water temperatures must be at least 80 degrees Fahrenheit for tropical systems to form. Waters in key tropical development areas are warm, but still not quite as warm as the record Atlantic temperatures we saw last year.
Take a look at the image below. This represents the sea surface temperatures as of May 17, still two weeks shy of the official start of hurricane season, which begins on June 1.
Where we see the dark orange and red colors, the water is already warm enough for tropical development. That includes parts of the Gulf of Mexico. If we see activity early in the season (June and July) this is where we generally see formation.
Next up are the wind patterns over the formation areas. These are actually related to water temperatures in the Pacific Ocean near the Equator. During La Niñas, the waters in the Eastern Equatorial Pacific are cool. The opposite is true of El Niños.
The blue hues in the Central and Eastern Pacific indicate a La Niña.
El Niño and La Niña periods affect the vertical wind shear profiles over the formation areas. Vertical wind shear is a change in wind speed and direction as you go higher up in the atmosphere. During El Niños, the vertical wind shear is generally very strong, which will tear storms apart as they try and get going. In these years, things are usually fairly quiet. On the other hand, the chances for the continental U.S. and the Caribbean Islands to experience hurricane activity increases substantially during La Niña periods due to less wind shear.
While we are currently in a La Niña period, it is forecast to lessen over the summer, becoming neutral during the peak of hurricane season. Keep in mind that neutral does not necessarily mean calm, no does La Niña guarantee a busy season. These are just overall trends that we tend to observe.
Another reason that things may be active this year is that they have been for the past several years. The Atlantic multi-decadal cycle has been in a high-activity era since 1995. Since then, 12 years have been "above-normal." Current climate conditions are also similar to other active years in recent memory, including 2008 when 16 named storms formed including Hurricane Ike, which made landfall near Galveston in September of that year.
Hurricane Season runs from June 1 through November 30, with peak activity occurring in August through early October.