I know it's late and hopefully you got it by now.
There was a sandstorm about 11:20 AM at the Chasm and the Silithids were all over the place.
Funny thing is the rare and most of the uncommon were balanced - could not find Speed or Power.
I was there alone and the storm was still on when I left - at least 20 minutes long.
You did not have to go into tunnels - they were on the ground.