As the Atlantic Basin remains very quiet, today’s blog update looks at tropical cyclones which impacted the southwestern U.S. It was prepared by Dante Diaz, a member of my hurricane team.

When it comes to tropical storms and hurricanes, the headlines are often focused on some community between Brownsville, TX and Eastport, ME or a Caribbean vacation destination. You almost never hear about hurricanes and tropical storms impacting California. However, there are a few rare ones that manage to find their way to California and the Southwest, at least at tropical storm strength. There are two good reasons why such systems are so rare.

First, the sea surface temperatures off the coast of California are much too cold to support a tropical cyclone. Second, the vast majority of systems forming in the East Pacific are driven westward by a large area of high pressure to their north. To make it to California or the Southwest as a tropical storm, several factors must come together. There needs to be a break in the ridge of high pressure to the north. In addition, the tropical system needs to have both the speed and strength to overcome the cold waters off California or the high and dry terrain of the Desert Southwest.

Fig. 1. Typically, tropical cyclones need sea surface temperatures above 26C to maintain their intensity. Cold water in the vicinity of California causes many tropical cyclones to dissipate before they can reach the coast.

The list of those that have made it to the southwestern United States is short to say the least. Four systems are known to have produced sustained winds to tropical storm force winds here (Chenoweth and Landsea 2004).  They are a September 1939 tropical storm that hit California, Joanne, 1972 that impacted Arizona, Kathleen that struck California and Arizona in 1976, and Nora that reached Arizona in 1997. In addition, Chenoweth and Landsea (2004) have done a study of a system in 1858. Their findings indicate that this system brought winds to hurricane force over coastal San Diego on October 2, 1858. Although there are a few other systems indicated to be at tropical storm strength over the Southwest in the National Hurricane Center best track database (such as Lester, 1992), only the aforementioned five systems were observed to have produced winds to tropical storm force in the United States. Numerous other systems reached the southwestern United States as weakening tropical depressions. Only the September 1939 tropical storm made landfall.  Joanne, Kathleen and Nora made landfall in Mexico before entering the United States. The San Diego Hurricane of 1858 remained just offshore.

Fig. 2. Tracks of Hurricanes Joanne (1972), Kathleen (1976), and Nora (1997) that brought tropical storm force winds to parts Arizona and California.

Fig. 3. A weather map at 1400 LT 2 October 1858 depicting when the hurricane was making its closest approach to the San Diego area. From Chenoweth and Landsea 2004.

The main threat from any tropical cyclone in the Southwest is the potential for destructive flash floods. If the 1939 tropical storm was to strike again today, Chenoweth and Landsea (2004) estimate it would cause as much as $200 million (in 2004 dollars) in damages. Although unique in the historical record, the San Diego Hurricane shows that even California is not immune to the effects of hurricanes. The high tides, heavy rains, and winds to hurricane force would bring even greater damage totals, on the order of several hundred million dollars today in coastal southern California.

Weakening tropical cyclones may bring heavy rain and flash floods to the Southwest, but there is still the potential for a greater hazard on the California coast. Once in a while, the weather patterns manage to bring a hurricane out of the Tropical Pacific to the shores of southern California, inflicting the kind of damage usually seen over the Gulf and Atlantic Coasts of the United States.

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