Weather Station

Follow the light posts and you'll come to a fence.  Follow it to the right and you'll come to a shed.  Search the body there for a weapon and some ammo, and then warm up in the shed.  Exit the shed and head for the lighted area.

Head up the stairs and enter the building.  You'll find a couple of things in each of the first two rooms.  Proceed until you find a staircase.  Under the staircase you'll find some ammo.  Follow the stairs up all the way to the roof.  Jump across the gap to the roof of the next building.  Shoot out the grate on the roof and jump down - watch out for a few things.  Shoot out the next grate you find, but don't drop down right away.  Shoot the barrels you see to injure the two big things down below.  Then drop down and finish them off.

After the battle, search around for a key and then go outside.  Return to the first building to retrieve your squad.  Then head out and go to the weather dome.  In the alley between the weather dome and the adjacent building is a body with a flamethrower and fuel - watch for a few small things in the area.  Go to the weather dome's door and enter.

Inside you will be attacked by a few small things, kill them and then search the lockers for goodies.  Go to the next room and kill the things that attack as you approach the save point.  Continue on until you reach a kitchen.  You'll find some ammo in the room, but while inside a fire will break out.  Run out of the room and the sprinklers will take care of the fire.  Return to the kitchen to make sure that you got everything, including a key.

Exit the kitchen and then head upstairs.  You'll run into a large thing which generates smaller things from its stomach.  Killing the big thing will put an end to the attacks by the small ones as well.  Continue on until you find a bathroom.  Inside you'll find a big thing and some smaller ones that shoot at you.  Kill them all and then search the lockers and stalls.  Head back out to the main room.

Go up the stairs and exit through the door, initiating a cutscene.  In the room you'll find a sniper  rifle and ammo.  Also use the PC and the telescope.  Exit the room and head back down the stairs.  Shoot the barrels near the door that led to the roof and you'll bring down an antenna.  Use the antenna to cross to the next building.  Head left and go down until you reach a gate, watching for things the whole way.  

In the far corner of the room, you'll find a PC that has a tidbit to continue the story along. Once you get it, head up the stairs and outside to the dome. Once you exit the door a cutscene begins. Once the scene ends (notice I left out juicy details), search the room and pick up the sniper rifle and sniper ammo clips. Use the PC to find out what the next box on the table does. Use the viewer for the telescope. < Insert movie fan rejoicing here. >

Exit the dome, down the stairs and look behind the door that led to the roof. Notice those fuel drums; shoot them so you can use the downed antennae to cross over to the warehouse. On the roof just under the antennae is a health pack. If you need it, drop down and grab it. A few scuttlers might protest but a little buckshot goes a long way. If you decide to get it, work your way back through the building to the downed antennae.

Once across, head left and keep winding your way around, shooting scuttlers, and working your way up until you reach a grate. Shoot it out and head inside to the next level.