High for a single location. Highs would refer to multiple locations, and usually would be more general. "Today's high was 89 degrees." "Today we had highs in the upper 80s"

‘High’ is usually used in the singular when talking about weather. “Today’s high is 30 degrees.”

Hi, I’ve actually heard both used before, but I’d lean more towards “highs” being the more accurate and commonly used, simply because a weather forecast is usually predicting various temperatures for more than one area.