Hurricane Hilary triggers first-ever tropical storm warning in Southern California

Hurricane Hilary was heading for Mexico’s Baja Peninsula Category 4 strong storm on August 18, 2023 and was forecast to be moving into Southern California at or near tropical storm strength already on August 20th. For the The first timethe National Hurricane Center issued a tropical storm warning for much of Southern California.

Hurricane Scientist Nick Grondin explains how Hurricane Hilary could wreak havoc over much of the country with the help of El Niño and a heat dome dangerous flash floodWind damage and mudslides in the US Southwest.

How rare are tropical storms in the Southwest?

California has had only one confirmed tropical storm to make landfall. It was September 1939 and it called the Long Beach tropical storm. It caused about $2 million Damage in the Los Angeles area would be approximately $44 million today. A hurricane in 1858 came close but did not make landfall, although its winds caused significant damage to San Diego.

What the Southwest sees fairly regularly are tropical cyclone remnants, storms that persist after a tropical cyclone has lost its surface circulation. These residual storms are more often in the region than people might think.

just last year Hurricane Kay was similar to Hurricane Hilary and brought significant rainfall to Southern California and Arizona. known, Hurricane Nora in 1997 made landfall in Baja California, Mexico, and continued moving north, bringing tropical storm winds to California and causing widespread flooding Hundreds of millions of dollars in damageespecially on fruit trees and agriculture.

The National Hurricane Center’s five-day precipitation forecast for August 18, 2023 shows precipitation amounts well above what some areas typically receive in a year. National Hurricane Center

A study led by an atmospheric scientist Elizabeth Richie found in 2011 that on average approx. 3.1 remainder systems Tropical cyclones hit the US Southwest every year from 1992 to 2005. That’s a short record, but it gives you an idea of ​​the frequency.

Typically, tropical cyclone remnants do not extend beyond California, Nevada, and Arizona. although it would not be unprecedented. In this case, meteorologists assume that the effects will reach far to the north. The National Hurricane Center forecast at least one on August 18 moderate risk of flooding much of southern California, southern Nevada and far western Arizona, and regions east of San Diego are at high risk of flooding.

What makes this storm so unusual?

One influence is that El Niño climate patterns this year showing signs of strengthening in the Pacific. Another, possibly less intuitive, is that Heat dome over large parts of the USA

During El Niño, the tropical Pacific is warmer than normal, and both the eastern and central Pacific experience more frequent storms, as we saw in 2015 and 1997. In general, hurricanes need at least 80 degrees Fahrenheit (27 degrees Celsius) to maintain their intensity. Usually the waters off Southern California are way cooler. But given Hurricane Hilary’s high initial intensity over warmer waters to the south and the fact that the storm is moving quickly, forecasters believe it could survive the cooler waters.

The influence of the heat dome is interesting. meteorology researcher Kimberley Wood posted a fantastic thread on X formerly known as Twitter describing this large-scale pattern around similar storms that have affected the Southwest United States. A common denominator in these storms is the presence of a ridge or high pressure system in the central US. When you have a high-pressure system like the heat dome that covers large parts of the country, the air is pushed down and heats up significantly. The air around this ridge moves in a clockwise direction. There is now a low-pressure system over the Pacific Ocean with anti-clockwise winds. The result is that this Winds will likely accelerate Hilary northward to California.

Despite the rarity of tropical cyclones reaching California, since the storm’s formation, numerical weather forecast models have generally indicated that Hilary is likely to accelerate along the west coast of Baja California and advance into southern California.

What are the risks?

The threat of tropical-gale-force winds prompted the National Hurricane Center to record tropical storms for Southern California for the first time. However, water is almost always the main problem in tropical storms. In Florida, that means storm surges in low-lying areas and heavy rainfall. Flash flooding can occur in California from extreme rainfall, aggravated by mountains.

When a tropical storm sweeps over a mountain, it can cause more lift, more condensation in the air, and more precipitation than might otherwise be expected. It happened to Hurricane Lane in Hawaii in 2018 and may also occur in other tropical cyclone locations with significant orographic or mountainous impacts, such as the west coast of Mexico.

This can lead to dangerous flash flooding down the drain. There may also be a secondary hazard – mudslides, also in areas recovering from wildfires.

In dry areas, heavy rainfall can also trigger flash floods. Forecasts as of August 18 indicated that Death Valley is likely to come about 10 cm of rain over a three day period since the storm – that’s about twice the average a whole year long. Death Valley National Park posted a Flood warning for August 20th to 25th.

As Hurricane Hilary makes landfall in Baja California, forecasters are forecasting dangerous flooding, storm surges and wind damage in Mexico before the storm reaches southern California.

Keep in mind that this is still an evolving situation. Forecasts can change, and it is enough if a rain wave forms in the right place, leading to significant flooding. Those heading to Hilary should check with their local weather offices for more information. This would also include local National Weather Service Offices in the United States and Servicio Meteorologico Nacional in Mexico.

This article was updated on August 19, 2023 when the National Hurricane Center upgraded the tropical storm watch to a tropical storm warning.

Nicholas GrondinAssistant Professor of Environmental Studies, University of Tampa