(Portland, OR)  --  Authorities are confirming that a tornado touched down in Portland over the weekend.  The National Weather Service says the twister touched down briefly Sunday night on a farm near Cornelius Pass Road in northwest Portland.  The agency says the tornado was rated EF-0, which is the weakest category of tornadoes.  It's the second tornado to touch down in Portland this summer after an EF-0 rated twister uprooted trees and tore shingles from roofs in northeast Portland on July 1st.