You've probably noticed that lizards sometimes have a thunderstorm symbol. A few minutes before the rain starts, the lizards run around with this symbol.
After the rain ends, this symbol disappears for all lizards.
I assume that they get it about 15 minutes before the storm starts. And since there are lizards in all locations, the appearance of the symbol will be a sign to gather to kill Gigantix. The storm may be random, but we have the means to predict it. Or... not ^^