How do I unlock northern range?

Did you complete Stormy Sea yet? If not, do that. If you did, try doing a couple of optional rescue missions, because Northern Range should unlock soon after that.

To get Northern Range you must have both the Southeren island and the hm surf.

To get Hm surf you must go to solar cave floor twenty and you need to have a leader that can walk across water,levitate or any abilitie that lets you go across water.The reason for this is that you have to walk across water to get this so you don't need a Key.

Well the first thing is thats only 1/2 0f what you need to do.You also need to go to SOLAR CAVE and on the 20th floor find HM SURF.
PS:You need a leader who has ALL TERAIN HIKER or the natural ability to walk on water to get SURF.

You need to get the hm ( hidden machine ) surf from solar cave floor 20,
Remember to find a key or bring one since it is locked up. It might be surrounded by water .

You have to complete stormy sea, which you have to beat koyogre while you are there, after you do that, then you have to do a few missions, and then you are able to unlock the nothern range, though you have to beat stormy sea first.

You need the 3 legendary birds

YOU DONT NEED THE 3 BIRDS U HAVE TO COMPLETE STORMY SEA THEN HAVING SOUTHERN ISLANDS AND THE HM SURF WHICH IS IN THE SOLAR CAVE THEN COMPLETE 1-2 MISSIONS