New research indicates that cool water just below the surface of the Gulf of Mexico cause the strongest hurricanes to almost always lose intensity before they hit that part of the United States coastline. Most of the strongest hurricanes have decreased in intensity just before hitting the Gulf Coast, where two-thirds of all hurricanes to hit land in the United States have struck in the past 30 years. For example, Katrina was a Category 5 hurricane with winds of 175 mph in the Gulf but made landfall in 2005 as a Category 3 with a wind speed of 125 mph. When hurricanes move over the northern Gulf of Mexico, high surface winds tend to mix cooler water up to the surface, which can lessen a storm's intensity. Researchers also found that strong upper-level winds that storms encounter while approaching the Gulf Coast.