I'm going down on the 3rd of May with a High School group.
I've been there several time in late April and found the mosquito's to be very annoying at the put in. They get unbearable in the Summer and could be a real problem at the end of May. I'm going prepared for the little bastards, as this is one of the latest put in's I've done. Last year I put in around the first of April and saw not a one. Every year I've gone before that we were there in mid to late April and they were just coming out for a bite to eat.
End of May...........they could be out in full force. Don't be afraid to take head gear.
I always drink a cap full of vinagar in Vernal before heading down to the put in and repeat every two days as needed. I like using this method over the DEET spray method, but have used both at once in bad bug conditions. I'm taking both for this trip.
The weather should be plenty warm during the day...........but plan for cool nights......at the end of May. Enjoy!!