One corner of California has a summer rainy season

The North American Monsoon reaches the southeastern deserts and nowhere else in the state. On the precipitation charts, those stations break the pattern visibly.

California's defining climate feature is a dry summer. It is what makes the state Mediterranean, it is what builds the fire fuel, and it holds almost everywhere — with one exception, in the southeastern corner.

From July, moisture pushes north out of the Gulf of California and the Gulf of Mexico into the Sonoran Desert. The North American Monsoon is a genuine seasonal reversal, and its northwestern edge reaches the Colorado Desert, the Salton Trough and the eastern Mojave.

What it looks like in the data

On the wet-season concentration chart on the climate page, most California stations render as a solid block of winter months. The desert stations do not: El Centro, Needles, Blythe and Palm Springs all carry visible late-summer colour. At El Centro the wettest month of the year is in the monsoon season, not in winter — a statement true of nowhere else on this site.

Small totals, large consequences

The amounts are tiny by any standard — these stations run between two and five inches a year in total. But desert rain arrives as intense convection onto ground that absorbs almost nothing, so the runoff coefficient is close to one. Flash flooding, debris flows across roads and blowing-dust events ahead of thunderstorm outflow are all monsoon phenomena.

The monsoon also reaches the eastern crest of the Peninsular Ranges and the southern Sierra, which is where a good deal of California's summer lightning comes from — and therefore a good deal of its summer fire ignition.