I think it's common for TV signals to weaken after storms. I often see this myself.
Chances are, there are some trees, walls, buildings, or other obstacles between your antenna and the towers. When these get wet, they scatter more of the signal, leaving less to reach your antenna. Also, high winds and lightning from severe storms can damage transmitting equipment, so a few stations may take a bigger hit than average or even get knocked off the air completely for a while.