I live in a mediterranean zone near a huge lake and during summer mosquitos are your every night companions (specially if you're working during late night hours). But when a summer storm brews the mosquitos disappear for two or three days. Why? This has been for me a recurrent question, and the answer has been always obvious: few of them survive being hit by raindrops.
You can make 1000 theories about how our tiny vampire friends deal with raindrops, but it's pretty clear that intensive rain (>3hours) wipe out mosquitos population for several days...