The quick answer is 33 degrees temp/wind-chill whichever is lower. In Clackamas County this temp/wind-chill is based on the national weather system site.
It goes by what the Joint Office of Homeless Services predict. Generally, it is when the temperature is expected to be 32 degrees or below for at least 4 hours overnight. You can sign up for email updates at 211info.org and whenever the temperature gets low, every night they send out updates on the predictions, and which of the main shelters will be open. Hope this helps.
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