(Reuters) -Forecasters at Colorado State University for a second time raised their estimate for storms during this year's Atlantic hurricane season, citing "record" sea surface temperatures.

The group had last month raised its outlook to a near-normal season and number of storms. On Thursday, it boosted its forecast to 18 named storms, producing nine hurricanes, four of which could become major storms with winds of at least 111 miles per hour (179 kph).

"The continued anomalous warming of the tropical and subtropical Atlantic is the primary reason for the increase in our forecast numbers," wrote Phil Klotzbach, who leads Colorado State's Tropical Meteorology and Climate Research group.

The effect of El Nino, a weather phenomenon that suppresses Atlantic hurricane activity, this year has been offset by very hot ocean waters. These fuel more energetic storms by putting more vapor into the air, which can produce more intense precipitation.

June average sea surface temperatures across the north Atlantic were 0.91 degrees Celsius higher than the average for 1991-2020, and half a degree greater than the previous warmest June, the European Union's Copernicus Climate Change Service, which tracks ocean and air temperatures, said on Thursday.

The Atlantic storm season began on June 1 and runs through Nov. 30.

CSU's raised outlook is well above that of the U.S. National Oceanic and Atmospheric Administration, which predicts a near-normal season with between 12 and 17 named storms, five to nine hurricanes, and one to four major hurricanes.

(Reporting by Gary McWilliams; Editing by Marguerita Choy and David Holmes)