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Is John a threat to the U.S.?
Eastern Pacific hurricanes are most likely to impact the U.S. in El Nino years, where the ocean along the Mexican coast heats up to much above normal temperatures and can fuel very intense hurricanes. This was the case in 1997, when Category 5 Hurricane Linda, the most intense hurricane ever observed in the Eastern Pacific, moved parallel to the coast and threatened California. The National Hurricane Center issued several advisories for Linda alerting San Diego to the possibility of receiving tropical storm force winds from Linda. The storm turned out to sea before reaching California, however.

This is not an El Nino year, but recent warming of the waters in the Eastern Pacific has led some El Nino experts wondering if a late-arriving El Nino might be on the way. Water temperatures along the Pacific coast of Mexico are 1-2 degrees C above normal all the way to the California coast, giving 2006 the possibility of allowing a tropical storm to reach California. It is very rare for an Eastern Pacific storm to move far enough north to affect the Arizona or California. Since 1900, only four tropical cyclones have brought tropical storm force winds to the Southwestern United States: an unnamed tropical storm that made landfall near Long Beach, CA, in 1939 (52 mph winds south of L.A.); the remnants of Hurricane Joanne in 1972; the remnants of Hurricane Kathleen in 1976 (76 mph wind gust at Yuma, AZ); and the remnants of Hurricane Nora in 1997. In addition, a hurricane just missed making landfall in October 1858 and brought hurricane force winds to San Diego and tropical storm force winds all the way to Los Angeles.

In order to affect California, a tropical cyclone would have to be moving quickly, so the the cold waters off the coast would not weaken it too fast. The alternative would be for the storm to barrel up the narrow Gulf of California, where water temperatures remain warm all the way to the end. To my knowledge, no such storm has ever been able to shoot more than half way up the narrow Gulf of California before dashing itself to pieces on the rugged terrain on either side. I'd be surprised if John manages to bring tropical storm force winds to the U.S.

Dr. Jeff Master's Blog on weatherunderground.com