Unseasonable means inappropriate or unusual for the time of year. When a snowstorm hits in April or temperatures reach 80 degrees in December, that's unseasonable weather: it doesn't match what normally happens during that season.

While we most often use it to describe weather, it can apply to anything that seems out of place for a particular time. Wearing shorts and sandals in January might be considered unseasonable clothing. A store selling Halloween costumes in March would be displaying unseasonable merchandise.

Unseasonable is different from unseasonal, which is less common. It's also different from unseasonably, the adverb form, as in “It was unseasonably warm yesterday.”

When meteorologists report unseasonable temperatures, they mean the weather doesn't follow the normal patterns for that date. An unseasonable cold snap in late spring can damage crops that farmers planted expecting warmer temperatures. An unseasonable heat wave in fall might confuse animals preparing for winter.

The word carries a sense that something is off-kilter or unexpected. Nature follows rhythms and patterns, and when those patterns break down, we notice. That's when we reach for the word unseasonable.