In most of the U.S., the rainy season comes in spring. Not California

Los Angeles Times

13-02-2025

As the first major atmospheric river of the winter arrives in Los Angeles, it brings with it the hope that the fire risk has finally receded, the danger that severe landslides could occur in the fire-scarred hills around the city, and the possibility that Southern California's rainy season is, at long last, going to begin in earnest.
Many accounts of the Palisades and Eaton fires have attributed their intensity in part to a delay in the winter rains, and that framing is not wrong. However, it doesn't answer a basic question: why does Los Angeles receive virtually all its rain during the winter?
It seems like merely a fact of life on the West Coast that summers are dry — in a typical year, LAX receives about 2½ inches of rain in the eight months between April and November, which represents less than 20% of the annual average. But compared to most of the rest of the country (and the world), this is an unusual pattern.
Go northeast over the Sierra Nevada to Salt Lake City — which on average receives about the same annual precipitation as L.A. — and the wettest months are March, April, and May. Go east to Santa Fe instead — also about the same annual precipitation — and the wettest months are July and August. Keep going east, getting steadily wetter as you get farther from the Rockies, and most of them have either a flat precipitation cycle (for example, no single month in Boston contains more than 10% of its average annual precipitation) or a spike in the spring or summer.
It is not a coincidence that the mountain ranges of the West — the Cascades, the Rockies, and even the relatively more modest San Jacintos — divide the region up into different precipitation patterns. In Joshua Tree National Park, torrential summer downpours will sometimes sweep in from the Gulf of California, but the Transverse Ranges form a wall that prevents that moisture from reaching Los Angeles.
Similarly, the so-called 'Pineapple Express' that carries warm, wet air from Hawaii to the Pacific Coast typically spends itself on the Sierras — as the winds are forced up over the mountains, they drop most of their water as rain or snow. The phenomenon of dry conditions on the leeward side of a mountain range (that is, the side sheltered from the winds) is known as a 'rain shadow,' and it can produce some of the driest conditions on earth — some weather stations in the Atacama Desert in Chile, downwind of the Andes, have never recorded any rain at all.
But just as important as the literal ridges that crisscross the land are the meteorological ridges that exist in the atmosphere. Atmospheric ridges are long regions of high pressure, typically associated with hot, dry air. Not unlike mountain ridges, they force air to flow around them, creating their own versions of rain shadows. One of these ridges — known as the 'subtropical ridge' — typically circles the globe at around 30 degrees latitude and gives rise to the Arabian, Saharan, and Sonoran deserts.
During the summer months when the sun warms the North Pacific, the subtropical ridge bends north between Hawaii and California, and in doing so blocks moist air from flowing off the ocean and onto the coast. During the winter the ocean cools and the so-called 'Aleutian Low' expands south from Alaska, freeing a path for atmospheric rivers to bring rain to the West Coast. A similar phenomenon occurs over the Atlantic, creating dry summers and wet winters in Lisbon, Rome and Athens. In fact, a location where less than 10% of annual rain falls during the summer is said to have a 'Mediterranean climate' due to the prevalence of this pattern in Southern Europe.
Given that the movements of these atmospheric ridges are driven by temperature changes, it should come as little surprise that climate change could have a profound effect on them. In recent history, the deep drought that gripped California between 2011 and 2017 was driven by a phenomenon known as the 'Ridiculously Resilient Ridge' — a period when, due to unusually high sea surface temperatures in the Pacific, a persistent high-pressure system prevented storms from reaching the West Coast even during the winter months.
Some studies suggest that persistent ridges off the coast — and therefore prolonged droughts — will become many times more frequent due to climate change. Changes in these ridges could also contribute to a dangerous phenomenon called 'hydroclimate whiplash': in one year, warmer air (which can hold more moisture) could bring torrential rains that spur vegetation growth, in the following year warmer ocean waters could produce ridges that suppress rain, leading the vegetation to dry out and provide fuel for wildfires.
As the climate warms and weather extremes grow ever more extreme, the important question may shift from 'Why does it rain so much in the winter?' to 'What will happen to L.A. if it doesn't?'