Tropical Storm Rachel triggered rare watches in Southern California and is forecast to sweep into the Southwest, bringing flooding rain and strong wind gusts to cities like San Diego and Las Vegas, as well as enhanced coastal flooding.

That's not all. We'll see heavy snow in parts of the northern Rockies and Major Hurricane Simon is poised to rake southwest Mexico, including Puerto Vallarta.

First, Rachel

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A tropical storm watch has been issued in parts of Southern California ahead of Rachel.

Specifically, that watch includes San Diego County and also extends westward to San Clemente Island off the San Diego County coast.

This means tropical storm force winds (in this case, mainly in gusts) are possible within 48 hours.

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It's the first tropical storm watch issued anywhere in Southern California since the remnant of former Hurricane Hilary moved into the state in 2023. (Note: While a Southern California tropical storm watch was issued in 2023, Hilary was not a tropical storm when its remnant moved over Southern California, according to a National Hurricane Center post-storm analysis.)

Tropical storm warnings have also been issued for parts of Baja California, where the chance of 40 mph winds are much higher.

Rachel is tenaciously hanging on despite few thunderstorms near its center.

Since it is forecasted to both track over record warm ocean water for this time of year and get a boost of thunderstorms from an approaching jet stream, Rachel could maintain tropical storm status until its Saturday night landfall in the northern Baja Peninsula, then possibly even into parts of the lower deserts of southeast California early Sunday.

Potential impacts

Winds, coastal flooding

Rachel and its remnant could produce wind gusts over 40 mph primarily in the Southern California mountains into early Sunday.

These gusty winds could also spread into much of the Desert Southwest Sunday, particularly over higher elevations of Arizona, Nevada and Utah.

These winds could lead to some tree damage, blowing dust and power outages.

Rachel could also enhance coastal flooding Sunday along the Southern California beaches. The National Weather Service is warning beach erosion and damage to roads and structures is possible.

Widespread rain totals of 1 to 5 inches are expected with locally higher amounts possible.

Flash flooding

As the remnant moisture and upper-level spin from Rachel moves into the Southwest, heavy rain could bring flash flooding across the region, including Southern California, lingering into Monday or Tuesday.

This could lead to flash flooding of normally dry creeks, arroyos, slot canyons and urban areas, including parts of the L.A. Basin, San Diego metro, Las Vegas and Salt Lake City.

If you had plans for any of the national parks in the Southwest, we would suggest canceling those plans until Wednesday, given the threat of both flash flooding and thunderstorms.

(MORE: What to do before, during and after a flood)

Wait – is that snow on the map?

Yes, as the moisture from Rachel meets colder air from a front moving in from the Northwest, snow could accumulate in parts of the Sierra Nevada as well as the Rocky Mountains.

Parts of Wyoming, Montana and Idaho could see their first significant snow of the season, with some places forecast to get over a foot.

(MORE: The West's best shot at snowpack recovery is from El Niño)

Much-needed heat relief

As that front collides with the tropical moisture, it will also bring a long-awaited cooldown to parts of the Southwest, including Los Angeles.

After being stuck in the 90s and triple digits for over a week, temperatures will drop 5 to 15 degrees below average for much of the region by Monday.

Highs will be in the upper 60s and 70s for Southern California, while parts of Arizona and New Mexico will drop into the 70s and 80s.

Major Hurricane Simon

As all this is happening, a powerhouse hurricane is taking aim on southwestern Mexico.

Hurricane Simon will slam into Mexico's Jalisco state Sunday at Category 4 intensity.

A Hurricane warning is in affect from Punta San Telmo to El Roblito along with Las Islas Marias.

A tropical storm watch is in effect from East of Punta San Telmo to Lazaro Cardenas.

Simon's core will then track over or near Puerto Vallarta later Sunday.

Damaging winds and flooding rain are both likely along and near the track of Simon in southwest Mexico. Up to 30 inches of rain could lead to dangerous landslides in hilly or mountainous terrain.

This could be the strongest, most damaging hurricane to affect Puerto Vallarta in three years, since Hurricane Lidia downed numerous trees, damaged roofs and triggered landslides in and around the city in October 2023.

Some of Simon's remnant moisture could stream into the central U.S., giving a boost to locally heavy rain and thunderstorms Tuesday into Wednesday.

(MORE: What a historic El Niño means for your fall and winter)

Caitlin Kaiser graduated from the Georgia Institute of Technology with both an undergraduate and graduate degree in Earth and Atmospheric Sciences before starting her career as a digital meteorologist with weather.com.