I'm pretty sure that GPS does not use anything in the 2.4GHz range (which is the range that is suseptible to water molecules)... Considering how GPS is desgined to work globally, it'd be pretty dumb if it wouldn't work through cloud cover, so I doubt that's the problem.
I also doubt that your unit is going bad. Try it again later and see how well it works. Did you do anything differently? What type of receiver do you have exactly?