/execute @e[type=Snowball] ~ ~ ~ /summon Fireball ~ ~ ~ {direction:[],ExplosionPower:10} you can upgrade the command with this:
/execute @e[type=Snowball] ~ ~ ~ /summon Fireball ~ ~ ~ {direction:[],ExplosionPower:100} or :
/execute @e[type=Snowball] ~ ~ ~ /summon Fireball ~ ~ ~ {direction:[],ExplosionPower:1000} or:
/execute @e[type=Snowball] ~ ~ ~ /summon Fireball ~ ~ ~ {direction:[],ExplosionPower:100000}