It must be "heinen" , へいねん in hiragana, 平年 in kanji.
平年 means average year / normal year.
平年(へいねん)より気温(きおん)が高(たか)くなるでしょう。
The temperature will be higher than average year.
The averages in weather in Japan are averaged based on each element in the past 30 years which is updated every 10 years.
The currently-used averages in Japan are based on between 1971 and 2000.