I've killed so few in the rain that I can recollect the exact details of the last time it happened. It was 2007, the first week of April right before sundown. A friend of a friend lost a foal to coyotes and asked me to "thin" the population near their small, 10 acre place (adjacent to a powerline) just outside city limits. Just as I arrived, a downpour started. I sat in the truck waiting for it to let up but light was running out fast. There was no wind; I quickly setup, put the foxpro out about 30 yards in front of me. I was soaked when I put on sound "Luscious Lips" on volume level 2. To my amazement, a coyote came bolting down the powerline within 20 seconds of starting the call. I nailed it at about 50 yards. I kept calling for 10 more minutes but couldn't take the weather anymore. The coyote was an Alpha female that must have been days away from dropping her pups (7 of 'em).
I think the moral of this story is that if you're in very, very close proximity of the coyote, you might stand a chance of calling it no matter the weather. That coyote I killed must have been layed up within 150 yards of me, so I got lucky.
p.s. another point of interest is that over the years, I've killed a bunch of coyotes and one bobcat on this 10 acre plot near town. In fact, the only triple I've ever shot (anywhere) was on this place, and the biggest coyote I've ever taken (43lbs-which is pretty big for western WA). It helps to have the powerline there, but I've learned that coyotes are pretty thick around suburban areas/hobby farms; and nobody is hunting them (at least they're not calling them). Keep in mind that this place I speak of is in King County, WA; the most populated county in the state!