How do you say the weather is 87 degrees in spanish?

Usually, when you are speaking of weather or temperature, you use the verb "hacer", so you could also say "va a hacer noventa grados". You could say "Que va a ser de noventa grados", but this would not be idiomatically correct. Also note that most Latin countries use the Celsius temperature scale, so 90 degrees would be states as "31 grados".