Overwarm is not a widely used word these days, but there are still plenty of synonyms to choose from. Words like overheated, overblown, or overexcited are all similar in meaning to overwarm. If you're looking for a more formal or literary word, perhaps consider using the term sweltering or feverish instead. Alternatively, you could use a phrase like too warm or overly hot in the context of describing a temperature that is uncomfortably high. There are many different options to choose from, so whether you're writing a formal essay or a casual blog post, it's easy to find a word or phrase that fits your needs.