summer-like weather in late spring
Describes unseasonably warm, summer-like conditions during the spring season. Often used in weather reports or seasonal commentary.
今日は春暑しで、半袖でも過ごせるくらいだ。
Today is summer-like for spring; it's warm enough to wear short sleeves.
Compound of 春 (spring) and 暑し (classical adjective meaning 'hot'). The classical form 暑し is used for a poetic or fixed-expression feel.