At a Glance

The eastern Pacific hurricane season begins May 15.Hurricanes and tropical storms in this basin can affect the southwestern U.S. and Hawaii.2018 featured several examples of how eastern Pacific storms can be a threat to the U.S.A tropical storm or hurricane landfall in Southern California is extremely rare.

Hurricane season in the eastern Pacific Ocean begins Wednesday, May 15, just over two weeks earlier than the official beginning of the Atlantic season.

The Pacific Ocean's earlier start date is due to warmer waters and typically weaker wind shear earlier in the season compared to the Atlantic.

Hurricanes and tropical storms that form in the eastern Pacific can occasionally be a concern for the southwestern United States and Hawaii, and last year was a reminder of this threat.

From the Eastern Pacific to Hawaii

Hurricane Lane's Hawaii approach was the 2018 eastern Pacific tropics' biggest story for the United States.

Lane developed over the eastern Pacific in August of last year and crossed into the central Pacific Ocean. For reference, the dividing line between the central and eastern Pacific Ocean basins is 140 degrees west longitude.

After intensifying to a Category 5 southeast of Hawaii on Aug. 21, 2018, Lane then weakened significantly as it passed more than 100 miles south of the island chain. Parts of the islands were still swamped by flooding rainfall even though the weakened storm eased nerves and spared Hawaii from extreme winds and destructive storm surge.

Infrared satellite imagery showing the demise of Lane from a Category 5 hurricane to a tropical storm Aug. 22-25, 2018.

The Big Island and Maui received the biggest soaking from Lane, triggering flooding and landslides. Mountainous terrain in combination with Lane's moisture helped one location on the Big Island to record more than 50 inches of rain.

Tropical Storm Olivia should also not be forgotten from last year. The storm a few weeks after Lane. Olivia got its start in the eastern Pacific where it was once a major hurricane. It then crossed into central Pacific and weakened before making an .

Olivia was the first tropical storm on record to make landfall over Lanai and Maui.

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.

There were reports of structural damage, trees downed, power outages, and some flooding on the Big Island. Roofs were reportedly removed from homes just southeast of Hilo.

1992's Hurricane Iniki might be the most serious example of the impacts eastern Pacific tropical systems can generate in Hawaii.

Iniki, , originated in the eastern Pacific as Tropical Depression Eighteen-E on Sept. 5, 1992. The depression then crossed over into the central Pacific Ocean where it became a tropical storm and eventually a powerful hurricane.

The island of Kauai was especially hard hit by damaging winds, as Iniki made landfall there as a Category 4 storm. Many structures were wiped out by storm surge flooding and large, battering waves along the southern coast.

Satellite image of Iniki at landfall.

( Central Pacific Hurricane Center)

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

Many other tropical storms and hurricanes have passed safely to the south of Hawaii thankstohigh pressure north of the islands. It's the rare cases illustrated by Iniki and Lane – when a hurricane first moves south of the island, then arcs north – that pose a greater threat to our nation's fiftieth state.

Southwest Tropical Flood Threat

Most eastern Pacific tropical storms and hurricanes move away from land and are only a concern to shipping interests. But weather patterns at the beginning and end of the season are more conducive to tropical systems that affect Mexico and the southwestUnited States.

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

Tropical storms and hurricanes in the eastern Pacific typically weaken as they move north due to colder water and increased upper-level winds. Depending on the weather pattern across North America, moisture from the storm can get pulled north into the Southwest U.S.

Flooding rainfallistypically the greatest concern from any tropical systems that survive the journey into the southwestern United States. Even systems that are no longer tropical cycles when they reach the United States can still produce flooding rains.

While such a storm may no longer be intact, the remnant tropical moisture can fuel drenching rains as it moves into the region.

The 2018 eastern Pacific season had at least four former hurricanes that met their demise in Mexico but sent remnant moisture anywhere from the Desert Southwest into Texas: Bud in June, along with Sergio, Rosa and Willa in October.

Rosa's moisture , prompting water rescues, according to the National Weather Service. Moisture from and helped to soak parts of Texas.

The tracks of four former hurricanes that enhanced moisture in the Southwest and/or Texas in 2018.

The 2015 hurricane season had two former hurricanes whose moisture enhanced rainfall in the Southwest and resulted in deadly and/or damaging flooding.

The interaction between moisture from the remnants of in July 2015 and a disturbance moving into California resulted in historic July rains in Southern California. Both Los Angeles and San Diego broke all-time July rainfall records at a time of year when rainfall is almost always scant in Southern California. Because of the heavy rain, the Tex Wash Bridge near Desert Center, California, was undermined by floodwaters and collapsed in mid-July, closing Interstate 10.

in September 2015 injected moisture into the middle and upper atmosphere across the Southwest, resulting in heavy rainfall and flooding.Moisture partially associated with Linda led to flooding that killed 20 people in Utah on Sept. 4, .

A California Rarity

Map showing the position of the 1858 San Diego hurricane on October 2.

(Michael Chenoweth and Christopher Landsea/NOAA)

Very strong hurricanes have threatened the west coast of Mexico from the eastern Pacific. In 2015, 215 MPH Hurricane Patricia became the strongest east Pacific hurricane on record before making landfall in Mexico as a Category 4 storm.

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. Althoughextremely rare, history showsthat it can happen.

On Oct.2, 1858, theslammed into. Sustained hurricane-force winds resulted in extensive property damage.

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

While technically not a tropical storm at the time, what had been Hurricane raced north into the desert of southeast California in September 1976, producing wind gusts to 76 mph in Yuma, Arizona, and unleashing a 4 to 6 feet deep wall of water that destroyed 70 percent of homes in Ocotillo, California.

Cool ocean temperatures in the path of tropical systems trying to reach the Pacific coast are the primary reason landfalls are so rare in Southern California. Coastal waters get dramatically cooler as you move north along the coast from Mexico.

As in 1858, any hurricane would have to be moving fast enough, over waters just warm enough, to maintain its intensity on the way north in order to have a California landfall.