"Everyone is urged to plan ahead and expect not to be able to enter or leave the Peninsula if flooding occurs. "
Poster Comment:
I used to live there or I did when I visited my grandparents. It is 110 miles south of SF on the coast.
An atmospheric river is caused by the weakening polar magnetism which allows the jet stream to wander about. If two fronts get to pushing against a front between them, then all that water which was meant to fall over a very wide area falls on a very small one causing severe floods. If the flood is bad enough it can destroy crops. This area is home to wealthy people and to productive farms.