How Eastern Pacific Hurricanes are a Threat to the United States

While the Atlantic hurricane season receives much of the attention and publicity in the United States, the hurricanes and tropical storms that form in the Eastern Pacific can be a huge concern for the southwestern United States and Hawaii.

In fact, you only have to go back to last year to see examples of how Eastern Pacific storms can cause significant impacts in the United States. In August 2014, former Hurricane Iselle made landfall in Hawaii as a tropical storm. More than a month later, the remnants of Hurricane Odile soaked the Desert Southwest with flooding rainfall.

Below we examine the historical impacts from Eastern Pacific hurricanes and tropical storms in the Desert Southwest, Hawaii and even California.

Southwest Tropical Flood Threat

Although the majority of the hurricanes and tropical storms that form in the Eastern Pacific move away from land and are only a concern to shipping interests, the weather patterns at the beginning and end of the season are more conducive to these systems potentially affecting Mexico and the southwestern United States.

Arizona is the southwestern state with the history of the most tropical storm encounters. According to the National Weather Service in Tucson, Arizona, a total of eight tropical storms or depressions have remained intact and affected Arizona directly since 1965. Five of these actually made it as tropical storms with sustained winds of 39 mph or higher.

Flooding rains are typically the greatest concern from any tropical systems that survive the journey into the southwestern United States. This means that the actual number of systems that have affected the Southwest throughout history is much higher since many of them dissipate before actually reaching the United States. Why is this?

While the storm may no longer be intact, the remnant tropical moisture can fuel drenching rains as it moves into the region. The most recent example of this happened in September 2014.

Hurricane Odile made landfall on the southern end of Mexico's Baja Peninsula on Sept. 14, 2014 where it caused serious damage. After Odile's wind circulation dissipated, the remnant moisture from Odile spawned heavy rains across parts of southern Arizona, southern New Mexico and Texas, resulting in localized flooding. Some of the most serious flooding was reported in Carlsbad, New Mexico.

(RECAP: Hurricane Odile)

Another example of major impacts from the remnants of an Eastern Pacific storm was Norma in 1970.  Even though the system dissipated near central Baja Mexico, the moisture from the storm was drawn northward by a storm system moving into the western United States around Labor Day. The resulting heavy rains caused serious flash flooding and 23 fatalities across central and southern Arizona, making it the state's deadliest natural disaster.

From the Eastern Pacific to Hawaii

Hurricanes, tropical storms and tropical depressions that form in the Eastern Pacific can make the long journey into the central Pacific Ocean basin and eventually affect Hawaii. For reference, the dividing line between the central Pacific and eastern Pacific Ocean basins is 140W longitude. 

In 2014, Hurricane Iselle formed in the Eastern Pacific and crossed into the central Pacific Ocean basin as a hurricane. Iselle would then go on to make landfall on the Big Island of Hawaii as a tropical storm on Aug. 8.

(RECAP: Hurricane Iselle)

A more serious example of the impacts Hawaii can see from tropical systems that originate in the eastern Pacific was Hurricane Iniki in 1992.

Iniki, the costliest hurricane in Hawaii's history, originated in the Eastern Pacific as Tropical Depression Eighteen-E on September 5, 1992. The depression then crossed over into the central Pacific Ocean basin to the west of 140W longitude where it became a tropical storm and eventually a powerful hurricane.

The island of Kauai was especially hard hit by damaging winds. Along the southern coast, many structures were wiped out by storm surge flooding and large, battering waves.

On average, four to five tropical cyclones (tropical depressions, tropical storms, hurricanes) occur in the central Pacific Ocean basin each year, according to Dr. Rick Knabb, the National Hurricane Center Director and former hurricane expert at The Weather Channel. About two-thirds of these systems originate from the eastern Pacific.

A few other times in history we've seen hurricanes take a path similar to Iniki that starts south of Hawaii and then turns northward towards the islands due to a dip in the jet stream. The other hurricanes that have had significant direct impacts on Hawaii in the last six decades were Nina (1957), Dot (1959) and Iwa (1982). Iniki is the only one of the four that officially originated as a tropical cyclone in the eastern Pacific Ocean basin. It's possible that Dot also originated in the eastern Pacific, but because of poor data over the oceans in 1959, this cannot be confirmed.

Hawaii is usually protected from tropical storms and hurricanes approaching from the east that are aimed directly at the islands due to cooler waters and strong winds aloft, which contribute to weakening. 

Hurricane Flossie in 2007 is an example of a tropical cyclone that originated in the Eastern Pacific and was aimed at the Big Island of Hawaii before it weakened dramatically due to wind shear. Hurricane watches and tropical storm warnings were issued since Flossie was forecast maintain hurricane intensity as it arrived in the vicinity of the Big Island. Thankfully, Flossie weakened quicker than anticipated, resulting in minimal impacts outside of high surf and winds to 39 mph.

Many other tropical storms and hurricanes have passed safely to the south of Hawaii thanks high pressure north of the islands. It's the rare case illustrated by Iniki that poses a threat to our nation's fiftieth state.

A California Rarity

Southern California is the only portion of the Pacific coastline in the Lower 48 that could see a tropical storm or hurricane landfall from the eastern Pacific. Although very rare, we have some examples from past history which show us that this can happen.

On October 2, 1858, the only known hurricane to hit Southern California slammed into San Diego. Sustained hurricane-force winds resulted in extensive property damage.

Since 1858, a tropical storm has made landfall in Southern California. In September of 1939, a tropical storm with winds of 50 mph hit Long Beach. According to the National Weather Service in Oxnard, flooding caused moderate crop and structural damage. High winds surprised the shipping industry in the area, which resulted 45 deaths.

Why are tropical storm and hurricane landfalls such a rare event in Southern California? The primary reason is the very cool ocean temperatures in the path of a tropical system trying to make the journey to the Pacific coast of the United States. As in 1858, the hurricane must be moving fast enough, over waters just warm enough, to maintain its intensity on the way north to California.

PHOTOS: Hurricane Iniki (1992)