December and January compete for the title of the Bay Area's rainiest month, and the race is close enough that the winner changes by place and by year. In San Francisco, January edges out December on average by a small margin, and February is close behind. In the Santa Cruz Mountains and the Marin hills, where the terrain squeezes extra rain from storms, the totals are much higher but the same months lead. Why the peak does not come in November, when the rainy season starts, or in March, says a lot about how Bay Area storms work.
December and January: The Peak Months
San Francisco's long-term averages put December and January at roughly 4 to 4.5 inches each, the highest of the year. Which of the two comes out ahead in a given year depends largely on when the big atmospheric river storms happen to arrive. December wins in some years and January in others, and over many years the difference is small.
December and January are wetter than November because the Pacific storm track takes time to move south after summer. In November the wet season is starting, but the North Pacific High has not fully weakened and pulled away. By December and January, the jet stream is usually strongest and best placed to steer storms into Northern California. In February and March it begins to drift north again, and storms gradually become less frequent.
Location Matters: Orographic Amplification
The biggest monthly totals come not from San Francisco but from the Bay Area's hills. In a wet December or January, the Mount Tamalpais watershed can get 15 to 20 inches. So can the Santa Cruz Mountains around La Honda and Skylonda. Those totals dwarf anything in the flatlands, because the terrain wrings extra rain out of atmospheric river storms.
In the inland valleys, December and January are still the wettest months, but the totals are smaller. San Jose, in the sheltered Santa Clara Valley, averages around 3 inches in January, roughly two-thirds of San Francisco's total. The Santa Cruz Mountains catch much of the storm moisture before it reaches the valley floor, leaving the South Bay in a partial rain shadow.
Year-to-Year Variability in Wet Months
Monthly averages describe the general pattern, but any single December or January can stray far from it. In a drought year, December may bring half an inch when the average is over 4. During an El Niño winter or a run of atmospheric river storms, one month can bring San Francisco 10 inches or more. That is close to half a normal year's rain. The extreme case was January 1862, during the Great Flood of 1862, when San Francisco recorded more than 24 inches in a single month. Bay Area rain comes mostly in a few big storms rather than steady moderate rain, so each month's total depends on how many of those storms hit.
That variability is a defining feature of a Mediterranean climate. An “average” December or January rarely happens in practice, because most months turn out either wet or dry. The averages blend a few very wet months with many dry ones. In any given year, the month that feels rainiest may be November, January or February, depending on when the atmospheric rivers arrive.
