It's raining; yeah
The only thing about not getting rain often other than the threat of fires is that our dogs become accustom to non rain weather. The biggest complaint in the rainy season is how to get Fido to go out in the rain. I have this problem myself, not with my poodle girl she is from Canada and loves water; whether it's on the ground or coming from the sky but just as long as it's not because of an impending bath. But my California boy and my little Jack Russell are another story, even though Jessie is from Canada. Jessie was always sort of a sissy about the weather and getting her to even walk on wet grass is a chore let alone go out when it is pouring on her. Luke will go out in the pouring rain if its for a walk or a hike, we've done that and it's a blast. But if I want him to go out for a tinkle in the rain? He'd rather not and will hold it until later.
So for dogs who hate the rain the trick is to go back to the very beginning of housetraining and reward your dog with a small treat for going out. You may have to go out there with them in the beginning but they'll catch on really quick. When your dog really dislikes or even hates to do something; giving them a reward for doing it only makes sense. Why should they go out just because you want them to?
Going back to the start will not affect your regular weather visits outside. Sure they'll want a treat but will soon learn they only get one if its raining. My little piglet Jessie would like it to pour everyday just to get that one treat each time she relieved herself outside.
If there is rain in the forecast, break out the cookie jar and add a positive to a negative.