Brace for Hurricane Hilary: Flooding in the Desert, Tropical Storm Watch in San Diego

If you had California as the first contiguous U.S. state to be included in the forecast path for the Atlantic hurricane season, come collect your prize.

In a summer chock full of freak weather events, Mother Nature may be saving the freakiest for last: a hurricane or tropical storm impacting the driest and hottest part of the country.

Hurricane Hilary, churning as a Category 4 storm with 145 mph winds as of midday Friday, is forging a path that will bring it over southern California and the southwestern deserts of Arizona and Nevada by the end of the weekend, though likely as a much weaker system.

San Diego County is under a tropical storm watch, the first time any part of California has ever been under such an advisory. The inland deserts and mountains are looking at the potential of a major rain event that forecasters worry could bring severe flash flooding to an area not accustomed to heavy, sustained tropical rains.

Palm Springs could receive a year's worth of rain over the course of just 72 hours.

In a Friday press conference, meteorologists with the National Hurricane Center (NHC) said there's the potential for between three and six inches of rainfall in the southern California deserts and mountains. In some areas, that could be as high as 10 inches.

"To put that into context, that's as much rain as some of these areas get in an entire year, and much of it could fall in a matter of days," the NHC said.

If Hilary makes landfall, whether along the northern Baja peninsula or the southern tip of California near San Diego, it would likely be as a tropical storm or depression. But coastal flooding -- including storm surge and destructive waves -- would still be a major concern for a region not used to it.

Should Hilary come ashore in California as a tropical storm, it would mark the first time that's happened since September 1939. 

That year, El Cordonado lashed the coast of Long Beach, according to records from the National Weather Service, causing what was then a whopping $2 million in structural damage. Dozens of people died in floods and dozens more died at sea. 

At the time El Cordonado hit, “Californians were generally unprepared,” the NWS records note. 

This time, Californians are being told to take action, though officials have not yet issued a state of emergency.

Officials are, however, offering residents of San Diego County free sand and sandbags to use to line up their property in an effort to ward off flooding and water damage. A county instructional video offers residents tips on how to best use sandbags to protect property.

“Sandbags are best used to divert, not dam water. So don’t build a wall across an anticipated flow,” the video says. “Rather, build to it, keeping the flow running away from your house and down to the street.”

Hurricanes usually form when ocean temperatures are above 79 degrees Fahrenheit. Historically, the eastern Pacific tends to be below that threshold, according to data compiled by the National Oceanic and Atmospheric Administration.

But a severe marine heat wave, combined with the wild card of a growing El Nino pattern, has pushed Pacific temperatures higher by 5 degrees Fahrenheit, adding to the conditions that have made Hilary into a significant hurricane.

The unusual Pacific heat comes on top of other this summer's other marine heat waves that have led to hot tub-like temperatures off Florida and concerns about mass coral reef die-offs.