With Hurricane Hilary headed toward the Southwest U.S., I thought I’d share a bit about another hurricane/tropical storm that hit the Southwest in 1976.

First of all, hurricanes and tropical storms are much more common on the East Coast than on the West Coast The main reason is that the ocean water is much warmer off the East Coast than off the West Coast. While at roughly the same latitude, the average surface water temperature in September is 15 degrees warmer at Charleston SC (82°) than at San Diego (67°).

The map above shows global ocean currents. Generally, you will find warm currents on the east side of continents and cold currents on the west side of continents. The Gulf Stream takes warm water from off the coast of Florida up the East Coast, then northeast to northwest Europe. In January, the average high temperature in Dublin, Ireland is much warmer (46°) than in Boston MA (37°), despite the fact that Dublin is 785 miles further north than Boston.

Hurricanes that form off the west coast of Michigan are more apt to move west-northwest away from the coast, eventually dissipating as they reach cooler water. However, some hurricanes do move more north-northwest along the coast toward the Southwest U.S. These storms weaken as they move north, but once in a while a tropical depression will spread clouds and showers up into Southwest U.S.

Several times, these storms have produced heavy rain and strong winds in the Southwest U.S. See the thread here on storms that have caused significant damage in the Southwest.

This only seems to happen when we have El Nino and we have El Nino conditions right now. The above map is sea surface temperature anomaly or difference from average. This was derived from satellite data from August 16. Note the warmer than average water along the Equator in the Pacific Ocean, west of South America. You can also see the water is significantly warmer than average near the Baja Peninsula of northwest Mexico. This warm water will delay the weakening of Hurricane Kathleen and any further tropical storms this year.

Another characteristic of storms that bring showers (and possible thunderstorms) to the Southwest is rapid movement. The storms race up the Baja Coast and don’t have as much time to weaken. Tropical Storm Kathleen in 1976 moved as fast as 35 mph.

Hurricane Kathleen became a tropical depression on Sept. 6, 1976, roughly 300 miles southwest of Acapulco. It reached tropical storm strength (winds of 40 mph) on the 7th and hurricane status on the 9th. It made landfall on the 10th in the northern Baja near Ensenada. In Mexico, the highest rainfall total was 6.52″ near San Antonio.

Heavy rain spread north into S. California, S. Nevada and far W. Arizona. Here the storm interacted with higher mountains and produced locally catastrophic rain and flooding. The highest rainfall total was 14.76″ at Mt. San Gorgonio, east of Los Angeles.

Hardest hit was the town of Ocotillo. Half the town was destroyed as a four-foot wall of water swept away homes and vehicles. A 1/4 mile stretch of Interstate 8 was washed away, as well as a 60-foot bridge. Railroad tracks were covered by mudslides. Yuma AZ had a wind gust of 76 mph and 2.87″ of rain fell at one weather station along he AZ/NV border. Golfball-sized hail fell from a thunderstorm associated with Kathleen west of Tucson AZ.

Palm Desert got more than a year’s worth of rain in less than 3 days. Showers spread north all he way to Montana, where a few mountain locations had over an inch of rain.

Agricultural crops were hard hit, especially the raisin and lettuce crops. Power and water was knocked out to thousands of customers. Total damage estimates were as high as 160 million dollars.

Storms like Kathleen are quite rare, but are nothing new. A tropical rain event like Kathleen (or Hilary) is part of the climate of the Southwest.