Weathercatch: Why Hurricane Hilary targeted California, yet its remnants ended up in the Inland Northwest

Until last weekend, many of us in the Inland Northwest had never heard of a hurricane charging toward Southern California – much less feeling its impacts in our region. After all, it’s big news when hurricanes hit the East and Gulf coasts, which happens almost yearly. But we never hear a peep about one advancing on the West Coast.

As the idiom goes, never say never.

Hurricanes and tropical storms typically don’t make direct strikes on the West Coast, but once a century or so, one barges over the Pacific Ocean to make landfall. Until Hurricane Hilary bore down on California Sunday, only two others were known to have hit the Golden State. The first was a hurricane that slammed San Diego back in 1858, the other a tropical storm that struck Los Angeles County in 1939, according to the American Meteorological Society.

A big reason these weather events rarely muscle their way to the West Coast is because the offshore water is too cold for them to form and build in strength. With currents flowing south from the Gulf of Alaska, ocean surface temperatures rise at the most to the low 70s, according to the National Weather Service. Meanwhile, water temperatures in the Atlantic, warmed by the Gulf Stream, average about 80 degrees.

Wind direction is another reason hurricanes and tropical storms rarely strike the West Coast. Hurricanes that form in the Atlantic are driven westward toward the mainland by easterly trade winds. In the Pacific, however, prevailing winds tend to steer tropical storms away from land and back to sea.

So what changed to make Hurricane Hilary possible? Although Hilary weakened to a tropical storm by the time it made landfall, it still packed a punch, generating enough energy to propel remnants as far north as southeastern Washington and parts of Idaho. Many of us saw rain showers for the first time in weeks on Monday and Tuesday of this week.

An unusual confluence of factors put the storm on a collision course with Northern Mexico and Southern California. North America had just experienced its second hottest July on record, leaving Pacific waters off the coast of Mexico abnormally warm, according to the National Oceanic and Atmospheric Administration. And a growing El Niño – a naturally occurring climate cycle that forms in the southeastern Pacific Ocean – likely contributed to warmer-than-normal seawater as well. The warming waters enabled Hilary to acquire more energy as it formed over the ocean and helped it to maintain its strength.

Another factor was the wind direction. Two opposing atmospheric pressure systems redirected the winds from eastward to northward, steering Hilary toward the mainland instead of out to sea. The storm’s impact was brief but intense. The desert areas of Southern California, Arizona and Nevada were hardest hit with heavy rain, flooding, debris flows and strong winds.

Hilary’s remnants cut a swath due north, bringing rain to parts of the Inland Northwest Monday through Tuesday evening. Tropical monsoonal moisture sometimes makes it way to our region. But the tail end of a hurricane that formed in the eastern Pacific Ocean is exceedingly rare.

Nic Loyd is a meteorologist in Washington state. Linda Weiford is a writer in Moscow, Idaho, who’s also a weather geek.