Eastern Pacific Hurricane Season Begins: Here's How the Storms Can Threaten the U.S.

This year's season got off to a record-early start, however, with the formation of Tropical Storm Adrian near the coast of Central America on May 9. No other storm in the northeast Pacific basin has formed earlier in the season than Adrian in the satellite era.

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 occasionally be a concern for the southwestern United States and Hawaii. The past few years have provided us a very strong reminder of what kind of impacts the eastern Pacific season can have on the Lower 48.

(MORE: 2017 Atlantic Hurricane Season Forecast)

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.

(MORE: Hurricane Central)

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.

Just last year, Tropical Storm Newton nearly made it into southern Arizona after first striking Baja California as a hurricane. Newton then weakened and eventually degenerated into a remnant low in northwest Mexico just south of the Arizona border. The system still brought tropical-storm force winds to higher terrain locations of southeast Arizona along with soaking rainfall on Sept. 7, 2016.

2015 provided several more examples of how moisture from eastern Pacific hurricanes can affect the United States.

• Hurricane Blanca (June 2015): Moisture from Blanca's remnants brought mainly light rains to the southwestern states. Tucson, Arizona, picked up 0.21 inches of rainfall from Blanca's moisture. That exceeds their average rainfall for the entire month of June which is 0.20 inches (1981-2010). Measurable rain fell in Yuma, Arizona, for just the seventeenth time in June since records began in 1876.
• Hurricane Dolores (July 2015): The interaction between moisture from the remnants of Dolores and a disturbance moving into California resulted in historic July rains in Southern California. Both Los Angeles and San Diego broke their all-time July rainfall records during a time of year when rainfall is scant in Southern California. Because of the heavy rain, the Tex Wash Bridge near Desert Center, California, was undermined by flood waters and collapsed in mid-July, prompting a closure of Interstate 10. 
• Hurricane Linda (September 2015): Linda injected moisture into the middle and upper atmosphere across the Southwest, resulting in heavy rainfall that caused flooding in some areas. According to the National Hurricane Center, moisture partially associated with Linda led to deadly flooding that killed a combined 20 people in Utah on September 14. On September 15, the remnants of Linda in combination with a disturbance aloft brought heavy rain to Southern California. Los Angeles (2.39 inches) and San Diego (1.15 inches) both saw their second wettest September day on record.
• Hurricane Patricia: With maximum sustained winds of 215 mph on Oct. 23, 2015, Patricia was the strongest hurricane on record in either the eastern Pacific or Atlantic basins. Patricia then made landfall along the Pacific coast of Mexico as a Category 4 and weakened rapidly while moving inland across the mountains of Mexico. Moisture and remnant upper-level energy from Patricia was then pulled northward across northern Mexico and into the south-central United States. This helped to add more fuel to a weather pattern that was already conducive to heavy rain in that region.

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)

In 2015, five named storms from the eastern Pacific basin crossed into the central Pacific, including Guillermo, Hilda, Ignacio, Jimena and Nora. None of them brought major impacts to Hawaii other than high surf.

A 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.

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 to 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 in 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)