Breaker blows during the rain.
Sounds like there may have been a loose or corroded connection out there connected to a standard breaker, causing arcing that finally blew it out. From time to time, with the power OFF, re-tighten all connections. Heat passing through electrical wiring can cause expansion and contraction, and can loosen a connection, causing arcing.
I have my boathouse on a 20 amp GFCI breaker, and lightning in the general area will trip it every time. It doesn't even need a close strike, just more electricity flashing around in the air. (It's an inconvenience from tripping once in a while, but that's better than getting hit with 240v through an aluminum pontoon boat hanging on steel boatlift cables in the water.)
If you had a standard 20 amp breaker before, replace the GFCI with a standard. Even heavy fog or a drop of water can trip a GFCI breaker connected to an outdoor service.
All of my outdoor outlets are on another GFCI breaker (code). High moisture in the air can cause condensation build-up, and also trip out an outdoor GFCI circuit. I use a hair dryer on high heat to dry them out, and they start working fine (you can actually see steam coming out of the receptacle when I use the hair dryer).
The GFCI breaker is your problem now.
(I just replaced my 10-year-old boathouse GFCI breaker. The 1995 breaker cost me about $135 and the 2005 breaker cost me about $172. A standard breaker is much cheaper, and maybe they'll get one for you.)