If your sim goes outside during a heavy rain, she might stop what she’s doing for the action shown below.
The tooltip on that action queue icon in the upper left corner is “Get Struck By Lightning” which is exactly what’s about to happen…
Except for the flash from the lightning and the boom from the associated thunder, the effect is the same as if the sim were electrocuted, like when a sim with a very low handiness skill tries to repair something electrical (especially if there’s a puddle of water where the sim is repairing it).
Once the lightning has dissipated, the sim is singed. Bathing will clean the sim up again, but don’t be surprised if she doesn’t want to go out in the rain again right away.