Protecting microphones from rain is quite a challenge, and I haven't found the perfect solution either yet: it is not only about protecting the microphone itself from damage, but also getting rid of the sound of the rain drop impacts straight on the mic and its windshield/foam.
The best results I have achieved so far when recording rain was by putting the mic on a small stand (like for micing a kick drum) and then placing myself over it holding a thick blanket with my arms stretched out. Looks quite awkward, lasts only for a few minutes in strong rain and may cause some back pain after a while :-).
If your focus is on recording the frogs though anyway, I would rather wait for better weather instead. Even if you manage to isolate your mic from the rain drop impacts, you will get a lot of rain sounds and other ambience on top with an omni like the ECM8000; maybe you can rent a more directional microphone like a shotgun or at least a hypercardioid?