雷雨です。
Raiu desu
It is thunderstorm.
雷雨 = thunderstorm
雷雨になっています、外は危ないですよ。
Raiu ni natte imasu, soto wa abunai desu yo
It has become a thunderstorm, outside is dangerous.
危ない = dangerous, practical warning
ゴロゴロピカピカで怖いくらいですね！
Goro-goro pika-pika de kowai kurai desu ne!
Thunder rumbling and lightning flashing, it's frightening!
ゴロゴロ/ピカピカ = thunder/lightning onomatopoeia - very Japanese, very vivid
More Japanese Weather Phrases