In my experience, the showers are really heavy, but it remains humid and fairly hot. Most people shelter in a shop or undercover, because sometimes, its just a passing shower and the sun comes out in 10mins or so. Sometimes its more set in, and thats when indoor pursuits are a better option. I'd pack a foldup poncho thing just in case. You will find umbrellas on sale there, so I wouldn't bother taking one with you. You'll like find ponchos too, but they are so small, they don't take up any space.
I generally take a light pashmina. Its handy on the plane if it gets cold and can be wrapped up as a temporary pillow. If you go into any temples, especially in Thailand/Bangkok (The Grande Palace), shorts on both men and women aren't acceptable. A pashmina can be wrapped around your waist and used a temporary skirt so you can enter. Shoulders should be covered too, so no vests or spaghetti straps in temples. Some are more lenient than others. A light cardigan is good for evenings, when you might be sitting outside at a restaurant or on the beach. Ideally cover your legs if you are on the beach in an evening and use mosquito repellant.
I find loose linen/cotton breaths and feels cooler in the heat. Anything that gets wet, generally dries on the back of a chair on a balcony in the sun.
Take tampons if you use them. I found them tricky to find in Thailand because most women use pads. I had an awkward hand gesturing moment with a male phamacist!
Have a fabulous time. I'm very jealous 😊
The Weather Station Emergency Poncho - ASDA Groceries