slate tile on exterior entryway

we have a 4x4 entryway to front door. The house is pier and beam, so i have a wood subfloor . Plan on laying cement board over subfloor, but my question is what kind of thinset and grout to use for slate tile. Can i use pre-mix morter like they have at lowes? The premixed i saw at lowes, said for tile, and stone, but said interior, didnt see anything that said for exterior use. Was hoping to buy premixed, so i wouldnt have to buy extra stuff to mix up bag type(drill, mixer attatchment)

Thanks