With snow and an Arctic blast expected to hit the Philadelphia region Thursday, Montgomery County declared a "Code Blue" emergency.
The alert went into effect Wednesday at 9 p.m.
Montco also closed all courts and government offices Thursday morning. Bucks, Delaware and Philadelphia counties followed suit.
"A Code Blue Cold Weather Declaration is made in Montgomery County when winter conditions pose a threat of serious harm or death to individuals without shelter," a Montco statement said.
"A Code Blue is called when the combination of air temperature and wind chill is anticipated to be 20 degrees or less."
Montco's declaration is set to expire at noon Saturday as temps are expected to warm above freezing.
NBC10 First Alert Weather chief meteorologist Glenn "Hurricane" Schwartz issued a warning of his own -- a First Alert.
"The wind chills here will be in the single digits from Thursday morning through Friday around noon. That’s more than 24 straight hours! The coldest it should feel is near zero during Thursday night," Hurricane wrote in his blog post.