Arctic air that arrived in New Jersey over the weekend spurred at least 15 out of NJ's 21 counties to issue Code Blue alerts as of Monday, Jan. 21.

Through the state Code Blue law, signed in May 2017, counties issue alerts whenever the temperature drops below 25 degrees Fahrenheit without precipitation, or below 32 degrees Fahrenheit with precipitation. Code Blue procedures also apply when the National Weather Service wind chill temperature is at 0 degrees Fahrenheit or less for a period of two hours or more.

A Code Blue Alert "enables authorities to take homeless adults to shelter programs that have agreed to make additional beds available," according to the state's NJ211 site. Warming centers are also opened and advertised through local media and social media.

Many of the designated 'warming centers' around the state are at local library branches and senior centers. The coldest day of 2019 so far happened to fall on a federal holiday, Martin Luther King Jr. Day, and many county facilities were closed.

Mercer County officials said the Lawrence, Ewing, Hickory Corner and West Windsor branches of the County Library System are open from 9:30 a.m. to 5 p.m.

Despite the strides made with Code Blue efforts, Somerset County does not have overnight shelters available. Resident there are advised to call the County Office of Emergency Management for help with overnight placement at 908-725-5070.

Other active code blue alerts: Atlantic, Bergen, Burlington,Camden, Cumberland, Essex, Hudson, Hunterdon, Monmouth, Morris, Ocean, Passaic, Salem, Union

The New Jersey Department of Human Services has shared a list of hotlines by county, for help finding emergency shelter in bitter conditions:

Atlantic County: Atlantic City Rescue Mission 609-345-5517 (daytime and after hours)

Bergen County: Bergen County Board of Social Services 888-3-BERGEN (888-323-7436, daytime and after hours)

Burlington County: Burlington County Homeless Hotline 856-234-8888 (daytime and after hours)

Camden County: Catholic Charities, Diocese of Trenton 833-254-9801 (daytime and after hours)

Cape May County: Cape May CWA 609-886-1325 or 877-886-1325 (daytime and after hours)

Cumberland County: Cumberland Family Shelter 856-825-3144 (daytime and after hours)

Essex County: Salvation Army-Montclair 973-718-9488 (daytime and after hours)

Gloucester County: Gloucester Center for Family Services 800-648-0132 (daytime and after hours)

Hudson County: Hudson CWA 800-624-0287 (daytime and after hours)

Hunterdon County: Hunterdon CWA 908-788-1300 (daytime), Hunterdon Helpline 908-782-4357 or 800-272-4630 (after hours)

Mercer County: Mercer CWA 609-468-8296 (daytime and after hours)

Middlesex County: 211 (daytime and after hours)

Monmouth County: Monmouth CWA 732-431-6000 (daytime and after hours)

Morris County: Morris CWA 800-640-9783 (daytime and after hours)

Ocean County: Ocean CWA 732-240-6100 (daytime and after hours)

Passaic County: 211 (daytime and after hours)

Salem County: Salem CWA 856-299-7200 (daytime), 877-283-8486 (after hours)

Somerset County: Somerset CWA 800-287-3607 (daytime, after hours answered by Hunterdon Helpline)

Sussex County: County Division of Social Services 973-383-3600 (daytime), Sussex Homeless Response Service 973-940-8869

Union County: Gateway YMCA, Elizabeth 908-249-4815 (daytime and after hours)

Warren County: Warren CWA 908-475-6218 (daytime), NORWESCAP 977-661-4357 (after hours)

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