Since, IIRC, most of the marijuana in California is grown in the northern part of the state (Mendocino and Humboldt Counties), and Northern California gets more rain than the deserts of southern and central California, not much.
"Northern California is enjoying a healthy wet winter so far, with rainfall levels at 100 percent of their historic average or above in nearly every city, and reservoirs, while still not back to normal, steadily filling. But rainfall totals in the south are anemic, and falling further behind as each major storm only drenches the northern part of the state, leaving the south dry."
- San Jose Mercury News, 2/9/15