MIAMI, Sept 15 (Reuters) - Tropical Storm Ingrid, the ninth named storm of the 2007 hurricane season, barely clung to storm strength on Saturday as it fought unfavorable atmospheric conditions in the open Atlantic, U.S. forecasters said.

The storm could dissipate because of high wind shear -- the difference in direction and speed of winds at different altitudes -- but some computer models still indicated it could strengthen again if it survives for around three days, the U.S. National Hurricane Center said.

Ingrid had top sustained winds of 40 miles per hour (65 km per hour) as it swirled around 575 miles (930 km) east of the Lesser Antilles islands of the Caribbean at 5 a.m. EDT (0900 GMT), the hurricane center said.

It was moving west-northwest at 10 mph (17 kph), on a track that was likely to keep it over water well north of the islands if it remained a tropical storm.

The 2007 Atlantic hurricane season has been active, though far from as ferocious as record-breaking 2005 when 28 storms spawned 15 hurricanes, including Katrina, which flooded New Orleans and killed 1,500 people on the U.S. Gulf Coast.

On Thursday, Hurricane Humberto slammed into the Texas-Louisiana border area with an unexpectedly powerful punch that killed at least one person.

Before that, two maximum-strength Category 5 hurricanes, Dean and Felix, struck Mexico's Yucatan Peninsula and Central America respectively -- the first time on record that two top-ranked storms on the five-step Saffir-Simpson scale made landfall in the same year.

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