Did some Googling and looks like that part is called "drip molding..."

 

check drain holes around sun roof if they clog this happens, open sun roof with a spray bottle of water spray channel around edge where glass sets if it puddles drains clogged clear with smal wire or if really clogged you need compressed air to blow out the drain seen it a thousand times on cars at the lot.

 

Any signs of water leakage on the headliner?

 

Nope, those plastic, body color matched strips are purely decorative, they hide welds. So, unless the welds underneath have given way, doubtful.
(( BTW, to re-seat them, pull further up, clean underside of strips & valley w/toothbrush & vacuum, now the built-in clips will hold them down))

Will guess most likely water source is sunroof...1 or more of the 4 tubes that drain the pan surrounding sunroof that is. See if you can get dealer, or friendly mechanic to blow junk out of tubes from below ..that usually quickest way

 

Probably sunroof, press the button and retract the moonroof and look in the front 2 corners for dirt and twigs and dead birds in the drain holes. My friend was going through this in her Camry and there were twigs and dirt in the drain holes. You can clean it out with a zip tie or something you can poke in there. Then get a bottle of water and pour it on the drain hole and see if it comes out the bottom of the car. I went ahead and got some cleaning wipes and cleaned the tracks and put lithium grease on the rails of the moonroof track. I did the same thing for my Avalon. Smooooooth

 

Ok so those holes were definitely clogged! In fact, when I opened the sunroof, there was water sitting in there--like a pool of it basically. I got a zip tie and cleaned out just as you suggested @lucidbarrier and the water found it's way out the bottom of the car. Then I cleaned the area up.

But here's my question, still: why is that much water even getting past the sunroof? It was also a bit dirty and I could see how some could get past it, but it look liked someone poured about 8 - 10 oz of water in that sunroof compartment--the rain was admittedly strong but hadn't been raining for more than 5-10 mins. Seems like a sunroof should be better at holding back the rains.

And it seems like a lot of water to make its way into the cabin through the sunroof without me seeing it dripping from the ceiling somehow.

However, I do now recall something odd with the first leak which was that the left sidewall of the floorboard was wet that first time. So i'm now guessing it travels down the A column and permeates the floor from there?

 

Can you post a pic or two of the problem area and what you did to correct it? TIA

 

Why do you think they have a drain system that came with the sunroof? Maybe the next time it rains pull the shade back and watch what is happening under there, if possible. Torrential rain and storm debris at the same time is your recipe for plugged drains.
Never been a fan of sunroofs myself.
After two years I finally got my 51 year old sailboat's hull to stay dry. The whole topside was one leaky "sunroof". Clean your sunroof drains and the whole area where water is supposed to collect and you will eliminate the source of your problem. The rest is a cleanup due to the problem.

 

All sunroofs are designed to Leak some. And that's the reason for the drain system. A bad seal will make it leak even more.

 

Thanks for all the help everyone. Great forum!

 

Second the Motion on clearing out sunroof drains. This happened to me. I had to install a "bypass drain hose" on driver's side (don't ask me why). This cleared up the "downpour" of rainwater soaking my carpet and driver's side fuse box panel!

 

Check your ac drain too. And for those of you with a compressor and a blow gun and you hit the drains full throttle you will be reattaching the hoses up in the a pillar. Next time you see a junk lawnmower on the side of the road, grab a cable off it, perfect tool for cleaning those drains

 

I just had a similar experience with our 2018 Avalon hybrid. It is my wife’s car and when I sat in the front passenger seat one rainy evening, there was 3 inches of water on the floor!
After reading this forum and also some Highlander posts as well, it appears this is a fairly common issue. Too bad none of the maintenance documents bother to mention the issue with sunroof drains.
The dealership was very aware of this problem but has done nothing to warn customers or add it to their maintenance checklist and even after I gave them an earful, I could tell they did not care a bit and would not do anything to address this issue in the future. They are so aware of the issue, they have a “special” tool to clean out the tube but they still refuse to add it to their maintenance protocol.
what a douchey thing to do to your customers.
BTW, it is Heartland Toyota in Bremerton, WA if anyone has a similar experience with them.
I guess some maintenance issues you just have to learn the hard way. I would suggest no one buy an Avalon if they live in an area with a lot of pine needles and rain because I feel the car is poorly designed for these types of weather with a lot of areas where debris can get trapped. If you do decide to purchase one, park it in a garage or get a cover for it. Otherwise, you are going to spend an unusual amount of time cleaning out drainage areas (sunroof, front window, rear window, trunk areas, door wells, etc). Just my opinion. My old 2002 Highlander never had issues like this. I have lost my faith in Toyota vehicles overall.

 

String trimmer line works good for clearing drains.

 

I have a 2014 Toyota Avalon. I live in the NW (Seattle), lots of rain. My car was parked on a level street from Dec 21 to Dec 26. We had steady to heavy rain during that time. On the 26th I found about 1 inch of water on the driver side floor.

I read a bunch of post and watched several videos. Best test is to try to clear sunroof drain with weed wacker string and then pour water down sunroof drains to see if it drains out the bottom of your car just behind front wheel well.

I had to remove the lower interior side panel in the foot compartment to actually see the leak in action when water was poured down the sunroof drain. Had to do it anyways to suck up water under carpet.

My issue was that somehow my discharge line had disconnected at the sunroof. I had to loosen a portion of my headline to reattached the hose. FYI, my clips for the headliner did not dethatch as the video I watched on YouTube on how to remove headliner. Anyways just go slow and the video did help a little.