A common phrase among even the most devoted of bird watchers is "I don't do gulls", and with good reason! Properly identifying seagull species is a tricky task and entire books are dedicated to the subject. My favorite of which is this.
Often times what is best to help guide an identification is not knowing the various plumage patterns for possible rarities, but instead simpily the knowlege of the types of gulls you are likey to find in your area at given point during the year.
This app provides a summary of observational citizen science data to compute a likelihood for the species of gulls you are likey to see in a california county during each month.
Link: https://california-seagulls.shinyapps.io/seagull_app/
Author: Gordon Blasco