Hurricane Hilary brought flooding and record-breaking rainfall to California this week. Mountainous areas of the state received up to 10 inches (25 centimeters) of rain. Hilary was the first tropical storm to hit California since 1939.

Atmospheric scientists have been studying the causes of this remarkable storm. They have concluded that a natural El Niño, human-caused climate change, a persistent heat dome, and unusual wind patterns are the 4 causes of California’s once-in-a-lifetime hurricane.

Hilary’s Formation

The convergence of these elements, accompanied by warm water and hot air, led to the rapid growth and atypical trajectory of Hilary, resulting in an extraordinary weekend of intense rainfall in typically arid regions.

In order to comprehend Hilary’s distinctive path, it’s necessary to investigate its origins in the area south of Baja California and west of Mexico. This region serves as a fertile breeding ground for tropical cyclones, with Hilary’s formation benefiting from abnormally warm surface temperatures and deeper warm water.

Consequently, the storm rapidly intensified, transforming from a nascent system to a powerful Category 4 hurricane. Powerful hurricanes need warm water, deep heat reservoirs, and minimal crosswinds. Hilary ticked all the boxes, resulting in its extraordinary behavior.

Climate scientist Jim Kossin told Time that the phenomenon of rapid intensification has been observed more frequently in recent times.

Why Hurricanes Are Very Rare in California

There are three key reasons why hurricanes don’t typically cause significant damage in Southern California. Firstly, unlike the warm Gulf Stream along the hurricane-prone Atlantic coast, the coast of California and Baja California is cold and brings up cold water from beneath, which impedes hurricane formation.

Secondly, the normal atmospheric conditions in California, which are characterized by dryness and downward motion, are not conducive to the development of hurricanes. (Hurricanes thrive on upward motion.) Despite weakening when it encountered the cold water, Hilary remained sufficiently strong to cause significant impact when it reached California.

The third key reason for California’s lack of hurricanes is that east-to-west prevailing winds typically keeps storms offshore. In Hilary’s case, there was a deviation from these typical wind patterns.

A combination of hot air to the east, a low-pressure system to the west, and a stationary hot air mass over the central United States prevented the storm from turning east. These factors redirected the storm toward California.

The Impact of Climate Change

Jennifer Francis, a scientist from the Woodwell Climate Research Institute, has hypothesized that the unusual and stagnant hot air mass is linked to changes in the Arctic caused by global warming. However, there is disagreement among scientists on this point.

According to hurricane scientist Kerry Emanuel, the likelihood of a storm with rainfall of 15.7 inches hitting downtown Los Angeles has increased from a one-in-108-year chance to a one-in-30-year chance. Research also suggests that tropical cyclones around the world are becoming rainier.

However, it’s important to note that in the case of Hurricane Hilary, the natural El Niño phenomenon also played a huge role. El Niño tends to amplify hurricane activity in California and western Mexico. When storms like Hilary strike, the warmer air holds more moisture, resulting in increased rainfall.

Image Source: PBS NewsHour,