Vents Weak After Car Wash?

I'll take a guess. Maybe the wash water is getting into the HVAC box and wetting the air filter. Next time try setting the air flow to recirculate and see if this makes a difference. You might also want to check the condition of the HVAC air filter, make sure it is clean and serviceable, and check it after a car wash to see if it is wet. Strange that water is getting that far into the system. Maybe there are some missing parts meant to deflect water incursion. Or a drain is plugged up somewhere. Welcome to Prius Chat!

 

i wonder if the water getting into the intake grille below the windshield isn't draining properly

 

Have you popped the hood lately? I took a look at mine, the cabin air intake grill is about 50/50 above/below the back edge of the hood: there could maybe be a leaf build-up? They'd tend to block flow more when wet I think. The grill has a fine mesh too, might need a vacuuming and/or brushing.

 

You don't really say what HVAC mode you were in, whether you were trying to maintain a set temp or not; but one thing I've noticed after washing is you get the outside air temp sensor wet, and driving with a wet sensor will register a much colder than actual outside air temp because of evaporative cooling. So until the sensor drys off and reads the actual outside air, the HVAC will be trying to warm the cabin more than it needs to for the actual outside conditions. This can cause you to sense less airflow than maybe you believe it should be. Next time this happens, look at the outside temp display and maybe adjust the HVAC temp lower/cooler to compensate.