The Eastern Pacific Hurricane Season

Hurricanes Form to the West of the U.S. Every May 15 - November 30

Tracks of all Eastern Pacific tropical cyclones from 1980-2005. Nilfanion/Wiki Commons

Shortly before the start of the Atlantic hurricane season, you may hear mention of another season: the Eastern Pacific hurricane season. 

The Eastern Pacific hurricane season is concerned with tropical cyclones that form to the west of the continental United States, between the Pacific coastline and the International Dateline (140° W). The season runs from May 15 to November 30, with a peak in activity from July through September.

On average, a season will spin up 15 named storms, 8 of which will strengthen into hurricanes, and half of those into major hurricanes. Based on these numbers, the eastern Pacific is considered the second most active hurricane region in the world.

Sound Unfamiliar? It Does to Many U.S. Residents 

Don't know much about this hurricane season? Don't feel too bad. Much of the U.S. public remains unfamiliar with it, despite the close proximity of its storms to the Desert Southwest region of the United States. Sadly, this is likely because it gets much less media attention than the Atlantic season. Unlike Atlantic storms, storms in the eastern Pacific tend to head away from U.S. land areas (for reasons we'll discuss below) which means they're not usually highlighted in news segments.

Yes, You Can Call Them "Hurricanes"

Tropical cyclones in the eastern (and central) Pacific are still referred to as "hurricanes." It isn't until you cross the International Dateline and enter into the Northwest Pacific basin, that they're called "typhoons."

Mexico, Southwestern U.S. Among Most Vulnerable Locations

East Pacific storms typically form very near to the central Mexico coastline and either track westward out into the open Pacific, northwest into Baja California, or northeast across Central America. Storms can also cross into the continental U.S., but this is very rare.

East Pacific Storms A Rarity for West Coast States

Why is it that eastern Pacific hurricanes are such a rarity in the U.S.? One obvious reason is the westward motion of hurricanes and tropical storms. In the Northern Hemisphere, all tropical cyclones are steered to the west, thanks to the upper level Trade Winds, or Easterlies. Whereas this westward-moving global wind aims Atlantic storms directly towards the Atlantic Coast of the United States, it deflects storms away from the U.S. Pacific Coast.

Another reason why storms rarely make landfall along the West Coast? The ocean temperatures found there are very cool--too cool in fact to provide enough heat energy to sustain a hurricane's or tropical storm's strength. Here, sea surface temperatures rarely rise above the lower 70s °F (low 20s °C)--even in summer. And so, not only do tropical cyclones not form there, but those that happen to track back towards the U.S. quickly weaken once they encounter these cooler waters. 

Only 5 tropical cyclones have been documented as having impacted the western U.S. while still a tropical system: the 1858 San Diego Hurricane, an unnamed tropical storm in 1939, Hurricane Joanne (1972), Hurricane Kathleen (1976), and Hurricane Nora (1997). 

  • The 1858 San Diego Hurricane is the only hurricane known to have hit Southern California. While it isn't clear whether or not the storm actually made landfall, wind damage and heavy rains were felt from San Diego north to Long Beach, CA. 
  • The 1939 Tropical Storm is the only tropical storm known to have made a direct landfall in Southern California. On September 25, it came ashore near Long Beach, CA with winds of 50 mph (43 kts) and caused $2 million in property damage to the state.
  • Hurricane Joanne made landfall as a tropical storm over the northern portions of Baja California on October 6. It is believed to have maintained this intensity while crossing into Arizona, thus becoming the first tropical cyclone in recorded history to bring gale-force winds to the state.   
  • Hurricane Kathleen entered California as a tropical storm on September 10. After weakening into a depression over Southern California, it moved across Death Valley. The storm brought record rainfall to California (over 14 inches fell in San Gorgonio, CA) and high winds and severe flooding to Arizona, most notably the city of Yuma. 
  • Hurricane Nora was a powerful Category 4 hurricane that entered the U.S. at the California-Arizona state line on September 25 with tropical storm strength. Nora weakened to a tropical depression hours after, but reached Arizona while still tropical, becoming the third known system to do so. It dissipated near the Arizona-Nevada border the next day.