Thanks for the all the responses! I don't think the heat will be a big problem; the porch is enclosed with a roof and a gated brick wall around it, just not screened in, and the cage is back right next to the house, so it's shaded all day. I went out there to check on the chicks several times today, and the porch was much more pleasant than out in the direct sun. The cage is an old ferret cage with a high-sided plastic bottom that is taller than the chicks are, which I think works well to keep drafts down, and at night I cover the cage. The chicks weren't acting like they were too warm or too cold, but I do keep a check on them throughout the day to make sure.
On the other hand, the pop-up t-storms are something I hadn't considered. In the location they're at right now, I think they're fairly well protected as it's rare for rain to blow so far into the porch that it hits the back wall, but now that I'm thinking about it, it would probably be better if I moved them closer to the middle back portion of the porch instead of to one side where they're at now. Rain would be even less likely to get to them then. I could also switch to covering the cage with plastic/tarp at night in case of rain. Hmm, I need to think about this more. The storm issue is one that I hadn't taken into account when I planned to set the brooder out there. Ah, well, if all else fails, they may just end up back in the basement like my first batch of chicks.
Thanks again for all the advice!