Ropical Storm Hilary made landfall on Mexico’s Baja peninsula and moved into Southern California with damaging wind and heavy rainfall on Sunday, Aug. 20, 2023. For the first time ever, the National Hurricane Center had issued a tropical storm watch for large parts of Southern California. Forecasters warned of a “potentially historic amount of rainfall,” and the governors of California and Nevada declared states of emergency.

Hurricane scientist Nick Grondin explained ahead of landfall why the storm, with help from El Niño and a heat dome over much of the country, could bring flash flooding, wind damage and mudslides to the region.

How rare are tropical storms in the Southwest?

California had only one confirmed tropical storm landfall in the past. It was in September 1939 and called the Long Beach Australia WhatsApp Number List Tropical Storm. It caused about US$2 million dollars in damage in the Los Angeles area – that would be about $44 million today. A hurricane in 1858 came close but didn’t make landfall, though its winds did significant damage to San Diego.

What the Southwest does see fairly regularly are the remnants of tropical cyclones, storms that continue on after a tropical cyclone loses its surface circulation. These remnant storms are  in the region than people might think.

Just last year,took a similar track to the one Hurricane Hilary is on and brought significant rainfall to Southern California and Arizona. Famously, made landfall in Mexico’s Baja California and kept moving north, bringing tropical storm-force winds to California and widespread flooding that caused.

What’s making this storm so unusual?

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One influence is the El Niño climate pattern this year, which is showing signs of strengthening in the Pacific. Another, which might be less intuitive, is the heat dome over much of the U.S.

During El Niño, the tropical Pacific cphonenumber is warmer than normal, and both the eastern and central Pacific tend to be more active with storms, as we saw in 2015 and 1997. Generally, hurricanes need at least 80 degrees Fahrenheit (27 degrees Celsius) to maintain their intensity. Normally, the waters off Southern California are much cooler. But with the high initial intensity of Hurricane Hilary over warm water to the south, and the fact that the storm is moving fast, forecasters think it might be able to survive the cooler water.

The influence of the heat dome is interesting. Meteorology researcher Kimberly Wood published a fantastic thread on X, formerly known  as Twitter, describing the large-scale pattern around similar storms that have affected the southwestern United States. A common thread with these storms is the presence of a ridge, or high-pressure system, in the central U.S. When you have a high-pressure system like the heat dome covering much of the country, air is pushed down and warms significantly. Air around this ridge is moving clockwise. Meanwhile, a low-pressure system is over the Pacific Ocean with winds rotating counterclockwise. The result is that these winds are likely to accelerate Hilary northward into California.

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