Don't worry about slicks in the rain: patterned tread on bike tires doesn't do squat. A road bike won't hydroplane until 80mph or so. Actually, slicks will provide the best traction, since they'll put more area in contact with the ground.
Riding in the humidity is a necessary evil, here, too. If you don't do it, then you'll suffer when you eventually go out. We end the spring/start the summer with high (105-115) heat and low humidity. Then when the Monsoon starts at the beginning of July or so, we move into high heat (105 or so) and high humidity. And that's what we have until sometime in October. In June we were blessed with 37 days in a row of 100+ temps, and now it looks like we're gonna have at least a 10-14 day run of 100+ temps. The upside is that we ain't Phoenix, where it's usually about 5 degrees warmer.
Acclimating to the heat and humidity is essential. Doing so fines tunes your cooling mechanism--i.e. sweat--and reduces the amount of electrolytes you lose in your sweat.
The bike computer has a thermometer that I've seen hit 123 on the road this year. That was a tad warm.