The City of Portland , Oregon , uses a more complicated formula to determine when to open centers ; wet or dry conditions , the night 's predicted low , the three-day trend , wind , and whether snowfall . The City of Portland , Oregon , uses a more complex formula to determine when to open shelters ; factors include wet or dry conditions , the night 's predicted low , the three-day trend of lows , sustained wind speed , and whether snow is on the ground or predicted to fall .