For the thunderstorm with body percussion, it works best with a very large group (whole school, etc.).
1st group, rubs palms together in circles (the damp, misty type of rain)
2nd group, snaps fingers, randomly, not in rhythm (rain drops tapping on windows)
3rd group, pats on legs, quickly (steady downpour)
4th group, stomps feet quickly (rain with thunder)
When all 4 are going, have the thunderstorm die down by taking the groups back out, from 4th to 1st.
It doesn't work as well with just a class or two, best used with a whole school :-)
Have you tried a rainstorm with newspaper?
Each child has an open newspaper sheet on floor in front of him/her. They are told to dowhat the teacher does.
Teacher starts out by lightly tapping on the paper, then wrinkling the paper, wadding and crinkling the paper louder and louder, gradually get quieter and quieter with a few isolated "taps" as you end the storm.
Use one child on the timpani and another at the light switch for occasional bursts of thunder and lightning.