It could be Air Conditioner condensation drain tube water, under the car in line with front bulkhead is black tubing passenger side just off from center.
I haven't cleaned mine so location may differ by a few inches
When it rains the A/C may be pulling more moisture out of the air.
After running A/C look under car and look for dripping, follow that up into underside.
Also check and make sure that water falling on the windshield is drained out of rubber tubes on each front underside rocker panel