• Hilary has rapidly intensified into a major hurricane off Mexico’s Pacific coast.
  • Hilary will weaken as it parallels the Baja Peninsula.
  • It will pose a potentially catastrophic flash flood risk to parts of California and the Desert Southwest.
  • It also prompted a first ever tropical storm warning in Southern California.

Tropical storm warnings are now in effect for Southern California as H?urricane Hilary threatens to bring gusty winds and potentially catastrophic flash flooding this weekend. Hilary will first bring hurricane conditions to Baja California.

H?ere is what you need to know about this storm, its potential impacts and why it’s taking this track.

W?here Hilary is right now: Hilary is currently located several hundred miles southwest of Cabo San Lucas, Mexico, and is moving north-northwest. Hilary is a very large storm and is beginning to bring bands of rain to Baja California.

A? tropical storm warning has been issued for parts of Southern California, including L.A., Orange and San Diego Counties, as well as portions of Riverside, San Bernardino and Ventura Counties. This is the first time the National Hurricane Center and National Weather Service have issued such a warning for Southern California.

A tropical storm warning is in effect for the Baja California Peninsula, including Cabo San Lucas.

Hurricane watches and warnings extend up the Pacific coast of the Baja Peninsula, as shown in the map below.

I?n addition, flood watches are in effect across a wide swath of the Southwest, including Los Angeles, San Diego and Las Vegas.

W?here Hilary is headed and how strong it will become: Hilary rapidly intensified from a tropical storm to a Category 4 hurricane Wednesday into Friday. Fortunately, it will weaken this weekend.

Hilary has made its turn toward the north-northwest, and that will take it close to at least part of Mexico’s Baja Peninsula this weekend. Hilary will likely bring hurricane conditions to parts of Baja California, in the areas covered by warnings in the map above by late Saturday.

In a much weaker state, Hilary is forecast to track over parts of California late Sunday into Monday, then dissipate soon after that.

California and Southwest impacts and when may they arrive: Regardless of what meteorologists call it, Hilary and its leftovers will have several impacts in parts of the Southwest.

1?. Biggest Concern: Flash flooding will become a dangerous, potentially catastrophic threat in Southern California into southern Nevada lasting into Monday.

H?eavy rainfall has already begun to impact parts of the Desert Southwest.

S?ome areas will see multiple rounds of heavy downpours that can dump half an inch to an inch of rain or more in an hour, leading to flooding of normally dry creeks and arroyos. Landslides and mudslides are likely in some areas, especially where at least moderate rainfall happens over areas recently burned by wildfires.

The National Hurricane Center (NHC) expects rainfall amounts of 3 to 6 inches, with isolated totals up to 10 inches. This would likely lead to significant and rare impacts, according to the NHC.

NOAA’s Weather Prediction Center has issued a high or level four of four risk of excessive rainfall in parts of Southern California for Sunday into early Monday, including America’s driest location, Death Valley. These rare high risk flood days often result in flood fatalities and damage.

2?. High surf, coastal flooding and rip currents will also be a significant threat. The most dangerous flooding, including storm surge, is expected along parts of the Baja Peninsula on the Pacific side, near the center of Hilary.

H?owever, swells will also arrive along the coast of Southern California this weekend, and could last into Monday. These will likely impact south- and southeast-facing beaches the most, leading to dangerous high surf and perhaps some coastal flooding, especially at high tide.


3?. Even if it’s just the remnants of Hilary, some stronger wind gusts are possible in Southern California later Sunday, possibly into early Monday that could lead to power outages and tree damage. In some areas, these winds could mimic a Santa Ana wind, except could be accompanied by rain.

A track as a tropical storm over California would be extremely rare: If Hilary is still categorized as a tropical storm over California, that would be the first time that happened in almost 26 years, since the former Hurricane Nora tracked near the Colorado River in California as a tropical storm in Sept. 1997.

T?here were only two other times that happened prior to 1950, including a Sept. 1939 tropical storm in the L.A. Basin and an Oct. 2, 1858 hurricane, which tracked from San Diego to Long Beach.

(?MORE: California’s Tropical Storm And Hurricane History)

W?hy Hilary will weaken as it tracks farther north: Eventually, Hilary is expected to lose steam quickly.

A primary reason for this is that it will move over much cooler water once it moves past roughly the latitude of the southern tip of the Baja Peninsula.

Another reason is that Hilary’s center could move over parts of the Baja Peninsula.

G?iven those two factors, Hilary will likely be in a much weaker state by the time it passes over parts of Southern California.

W?hy Hilary will move so far north: Most Eastern Pacific tropical storms and hurricanes form off the Mexican Pacific coast, then track over open water to the west and northwest and fizzle.

I?n this case, Hilary will be drawn northward because of the wind flow around a very strong “heat dome” of high pressure in the central U.S., together with a U-shaped trough of low pressure aloft near the West Coast.

I?n fact, some computer models suggest Hilary’s remnant spin could be pulled as far north as Idaho or western Montana.

Hilary will also send a pulse of deep tropical moisture northward into not just the Desert Southwest, but potentially as far north as the Northern Rockies.

(?MORE: Hurricane Season Forecast Update)

W?hy does this sound so familiar? Last week, the remnant of what was Tropical Storm Eugene dumped up to 1 inch of rain in parts of Southern California, a type of more widespread rain not common during the state’s dry season.

A?lmost one year ago, Hurricane Kay also crawled northward along the southern Baja Peninsula before eventually fizzling south of the U.S.-Mexico border offshore.

W?hile it never tracked into Southern California or the Southwest, Kay’s remnants dumped up to 6 inches of rain in parts of the desert and high country of Southern California, leading to localized flash flooding, mudslides and debris flows.

Kay’s remnants also produced wind gusts over 80 mph in the San Diego County mountains, which downed some trees and power lines in a few areas.

Jonathan Erdman is a senior meteorologist at and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Twitter and Facebook.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.