By Jane Sutton
MIAMI (Reuters) - The 2012 Atlantic hurricane season will be "below average" with 10 tropical storms, four of which will strengthen into hurricanes, Colorado State University forecasters predicted on Wednesday.
Two of those will become major hurricanes with winds of at least 111 miles per hour, the team founded by forecasting pioneer William Gray said.
In an average year, there are 11 tropical storms, six hurricanes and two major hurricanes in the Atlantic, Caribbean and Gulf of Mexico during the season that runs from June 1 to November 30.
The outlook for a milder 2012 season was based on two main factors. Hurricanes thrive on warm water and the tropical Atlantic has cooled this year, the researchers said. There is also a "fairly high" likelihood that an El Nino effect will develop this summer, they said.
El Nino is a warming of surface waters in the tropical Pacific that occurs every four to 12 years and has far-ranging effects around the globe. It creates wind shear that makes it harder for nascent storms to grow into hurricanes in the Atlantic-Caribbean basin.
But it can produce drought and crop failure in parts of South Asia and wetter than normal conditions in western coastal areas of South America.
Despite the forecast for a moderate number of storms, Phil Klotzbach of the CSU Tropical Meteorology Project said vulnerable coastal residents should take the same hurricane preparations they do every year.
"Regardless of how active or inactive the seasonal forecast is, it takes only one landfall event near you to make this an active season," he said.
There was a 42 percent chance that a major hurricane will hit the U.S. coast this year, compared with a historical average of 52 percent, the researchers said.
The forecasters said there was a 24 percent chance a major hurricane would hit the U.S. East Coast, compared with a historical average of 31 percent, and a 24 percent one would hit the Gulf of Mexico coast, compared with an average of 30 percent.
Weather watchers may notice a few small changes when the National Hurricane Center begins issuing its advisories this year. Forecasters made small tweaks to the Saffir-Simpson scale of hurricane intensity to fix a longstanding problem that arises from rounding.
The scale divides hurricanes into five categories based on wind speed measured in 5-knot increments. For public advisories, knots are converted to miles per hour and kilometers per hour, rounded to the nearest 5 mph or 5 kph.
That created problems when storms neared the threshold dividing the categories. Because of rounding, it was possible for a storm to fall into Category 4 when measured by knots and Category 3 when measured in miles or kilometers per hour.
To fix that, small adjustments in thresholds were made to categories 3 through 5, while categories 1 and 2 were left unchanged. The change takes effect on May 15 and does not affect categories assigned to hurricanes that occurred previously.
The new scale is:
Category 1: 74-95 mph
Category 2: 96-110 mph
Category 3: 111-129 mph
Category 4: 130-156 mph
Category 5: 157 mph or higher (137 knots or higher, 252 kph or higher)
(Reporting By Jane Sutton; Editing by Vicki Allen and Philip Barbara)