Hurricane Hilary intensifies to Category 3 strength, threatening significant flooding in Southern California, Southwest

By Mary Gilbert, CNN Meteorologist

(CNN) — Hurricane Hilary is rapidly intensifying in the Pacific Ocean southwest of Mexico on Thursday and is on track to deliver potentially significant rain and flooding to parts of the Southwest as a weaker system starting this weekend.

Hilary strengthened into a major Category 3 hurricane Thursday evening with sustained winds of 120 mph and even stronger gusts, according to the National Hurricane Center. The storm will likely reach Category 4 hurricane strength on Friday with winds of at least 130 mph, the center warned.

The hurricane was about 445 miles south of Cabo San Lucas, Mexico, Thursday evening.

There remains a wide range of outcomes for the heaviest rain and strongest winds in the US as the storm moves north over the next couple of days along Mexico’s Baja Peninsula. Small deviations in the hurricane’s track could change the forecast for the most intense rain and wind.

Hilary’s rainfall could arrive as early as Saturday in parts of the Southwest, with the worst of its impacts set to arrive in California early Monday.

Shifts in the forecast track will also affect which areas of northwestern Mexico will face the worst of Hilary’s winds, which will be strong enough to snap trees, down power lines and cause significant damage to property closest to the storm’s center.

Mudslides and flash flooding are possible from a general 3 to 6 inches of rain across Mexico’s Baja Peninsula from Thursday to early Monday, with greater amounts possible in the higher terrain.

Significant flood potential

Hilary is expected to weaken significantly before it reaches Southern California and parts of the Southwest, but there’s an increasing chance of significant impacts to these areas in the form of heavy rain and flooding.

Southern California could receive some of Hilary’s heaviest rainfall. Widespread rainfall amounts of 2 to 4 inches may fall there and in southern Nevada from Saturday through Monday. The heaviest rainfall is expected mainly Sunday and Monday. Locally higher amounts up to 6 inches are possible in areas impacted by the heaviest deluges.

Rainfall amounts of 1 to 2 inches are possible for parts of Arizona, Central California and northern Nevada.

Multiple days of heavy rainfall will give the ground little opportunity to absorb moisture and can progressively worsen the flood threat.

Daniel Swain, a climate scientist at the University of California at Los Angeles, said Wednesday that “multiple years’ worth of precipitation” could potentially fall in some of California’s driest areas.

One of those places is Death Valley, California, the hottest place on Earth. Death Valley typically receives about 2 inches of rain across an entire year, according to NWS data. Moisture from Hilary could unleash enough rain to give Death Valley at least a year’s worth of rainfall in a single day.

Rainfall this exceptional proved destructive in Death Valley last year. Around 1,000 people became stranded in Death Valley National Park last August when 1.46 inches of rain fell in 24 hours and unleashed flash flooding that washed away roads and entombed cars in floodwater-swept debris.

Dramatic weather changes for suffering Southwest

Despite the flooding danger, the rainfall would help combat drought and recharge groundwater across parched portions of the Southwest. Drought conditions expanded in New Mexico and remained steady in California and Arizona this week, the US Drought Monitor reported Thursday.

The seasonal monsoon that supplies the region with a large percentage of its yearly rainfall has been missing for much of the summer, and cities like Phoenix are still waiting for measurable rainfall.

The combination of rainfall and increased cloud cover across the Southwest is expected to bring a significant cooldown over the weekend. Temperatures that have been in the upper 90s to 110s could drop by as much as 20 degrees.

Phoenix may not reach a high temperature in the triple digits over the weekend for the first time since the middle of June.

Hilary may accomplish rare feat

Hilary is more likely to make landfall in Mexico and cross into California. But if the storm makes landfall in California as a tropical storm, it would be the first to do so in nearly 84 years, and only the third tropical storm or stronger to do so on record, according to data from the National Oceanic and Atmospheric Administration.

The most recent was an unnamed tropical storm in 1939, NOAA records show.

Before that, the San Diego Hurricane made landfall in October 1858 – California’s only hurricane landfall on record, research shows.

1997’s Nora was the last and only other tropical storm to maintain its status after crossing into California.

The current forecast also calls for Hilary to maintain its tropical status as it heads into Nevada, something that has never happened on record.

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CNN’s Taylor Ward, Aya Elamroussi and Eric Zerkel contributed to this report.