Hilary was upgraded to a Category 1 hurricane Thursday and experts have forecast the storm to reach Category 4 status by the time it makes landfall, packing winds of at least 130 miles per hour (209 kilometers per hour).
Weather experts believe Hilary will make landfall Friday with its track going northward in the Pacific Ocean through the Baja, Mexico peninsula and into the state of California and portions of the southwest US.
"The biggest threat from this system is going to be the heavy rainfall potential," National Hurricane Center (NHC) Deputy Director Jamie Rhome said in a video statement. "This is going to be a big concern for us here in the southwest United States from a heavy rainfall threat, heavy rainfall potential."
While forecasts of three to six inches of rain are predicted in many areas, Rhome said a potential of 10 to 15 inches poses a flash flood risk for Los Angeles, San Diego and Palm Springs, California as well as Las Vegas, Nevada.
Forecasters said it is equivalent to getting several months' worth of rain in a few days, which is anomalous in that region, considering August is the driest month of the year with Los Angeles averaging 0.0 inches of rain and San Diego averaging just 0.1.
"This is a lot of rain, this is a lot of rain for this part of the country ... doesn't typically get that kind of rainfall, and if we see this rainfall materialize, we're definitely gonna have some flooding problems," said Rhome.
In addition to the rain and flash flood threats, the NHC said heavy winds from Hilary could also cause plenty of destruction.
As Hilary is downgraded from a hurricane once it makes landfall in Mexico, if the system enters California as a tropical storm, it will be the first in that state in 84 years.
"Even as it's weakening, though, potentially spreading tropical storm-force winds, especially at higher elevations into the southwest United States," said Rhome.
The only time a tropical storm made landfall in California this century was in Long Beach in 1939, according to the National Weather Service, which also said it is unusual for the first tropical system of the season to form on the West Coast versus the East Coast or the Gulf of Mexico.