* Colorado State cuts tropical storm forecast to 10

* Team predicts four of those will become hurricanes

* CSU expects El Nino to help squelch storm development

* But London's TSR bucks trend, raises its forecast

(Adds CSU detail, TSR forecast)

By Jane Sutton

MIAMI, Aug 4 (Reuters) - The Colorado State University storm research team cut its 2009 Atlantic hurricane season forecast on Tuesday, predicting that El Nino would trim the tally to 10 tropical storms, with four becoming hurricanes.

The noted forecasting team founded by pioneering storm researcher William Gray had predicted on June 2 that the season would see 11 tropical storms, including five hurricanes.

Tuesday's forecast said two of the hurricanes would reach "major" status of Category 3 or higher, with sustained winds of more than 110 miles per hour (177 km per hour).

It marked the third time Colorado State's forecast has been reduced in the tropical Atlantic Ocean, due mainly to the development of El Nino conditions in the eastern Pacific.

El Nino is a periodic warming of sea waters that can dampen Atlantic hurricane activity by increasing wind shear, a difference in wind speeds at different altitudes that can tear apart nascent cyclones.

The National Oceanic and Atmospheric Administration, the U.S. government climate agency, said in early July that the eastern Pacific had demonstrated El Nino conditions.

The Colorado State team said those conditions, combined with other factors, "will likely lead to a fairly quiet season."

While several well-known forecasters have recently cut their predictions for this season, London-based Tropical Storm Risk bucked the trend on Tuesday and raised its forecast to 12.6 tropical storms, with 6.5 strengthening into hurricanes and 2.8 of those becoming major hurricanes.

That was up from TSR's July 6 forecast of 11.4 tropical storms, 5.6 hurricanes and 2.4 major hurricanes.

TSR said sea surface temperatures in the Atlantic region where storms most often develop are expected to be warmer than previously thought, making this a near-average season.

Its forecasters gave greater weight to the storm-nurturing effects of warmer sea surface temperatures than to the suppressing effects of the wind shear.

Meteorologists have had varied success in predicting seasonal hurricane activity, coming close some years but falling widely off the mark in others. None had predicted the record-setting 2005 season would bring 28 storms.

Forecasters compare a broad range of atmospheric and oceanic conditions with those of past years and try to draw correlations with busy or quiet seasons.

"The Atlantic basin has the largest year-to-year variability of any of the global tropical cyclone basins," the Colorado State researchers said. "These seasonal forecasts are based on statistical schemes which, owing to their intrinsically probabilistic nature, will fail in some years."

The first two months of the Atlantic season, June and July, did not produce any tropical storms or hurricanes. The season runs through Nov. 30.

Forecasters warned not to read much into the slow start, since the busiest part of the season is typically from late August to mid-October and is still ahead.

In 2005, Hurricanes Katrina and Rita temporarily knocked out a substantial part of U.S. crude and fuel production, toppling offshore platforms, wrecking undersea pipelines, flooding coastal refineries and sending energy prices soaring.

Storms can also damage crops in the Gulf of Mexico and Caribbean regions, affecting the prices of such commodities as orange juice, sugar, coffee and cotton. (Editing by Jim Loney and Eric Walsh)