adverb
unusually (compared to a typical year)
Used to describe something that is different from the norm for that time of year, often implying a notable deviation.
See also: 例年
今年は例年になく雪が多い。
This year there is unusually a lot of snow.
例年になく早く桜が咲いた。
The cherry blossoms bloomed unusually early this year.
例年 means 'typical year' or 'average year'; 例年になく is the adverbial phrase meaning 'unusually for the time of year'.
From 例年 (typical year) + に (particle) + なく (adverbial form of ない, 'not'). Literally 'not being like a typical year'.