Of course it all depends, living in AZ and NM it is apparent the Southwest is very dry. It can rain anywhere in the four corner states and an hour later you wouldn't even know it. Sure we do get a torrent of rainfall from time to time, with our 4wd truck it has never lead to being stuck.
Since rain is scarce in most of Cali, I would think off road boondocking is quite similar when rain is in the forecast. We have a week of RVing planned for Utah in November, other than cooler temps we have not thought about getting enough rain to worry about traversing the off roads.
