The following blessing is traditionally recited when encountering a powerful display of nature, such as hearing a loud thunderclap or seeing a storm roll in over the sea:
Barukh attah Adonai Eloheinu melekh ha-olam, shekocho u'gevurato malei olam
Note: This blessing is associate with God's Name of El which represents divine power (ko'ach). According to Jewish Halakhah, if you see the lightning and hear the thunder at the same time, you should only recite one blessing, oseh ma'seh vereshit.