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An extreme cold warning is a weather warning issued by Environment and Climate Change Canada (ECCC) and by the United States' National Weather Service (NWS) to inform the public about active or imminent severe cold temperatures in their local region.

In April 2014, ECCC replaced the "wind chill warning" with an "extreme cold warning." In the older system a wind chill warning for Southern Ontario and Atlantic Canada was issued when the wind chill dropped to -35. Thus a temperature of −37 °C (−35 °F) with no winds would not require a warning be issued. Under the new system the extreme cold warning is issued based on either the temperature or the wind chill being a certain value for at least two hours. The values range from −30 °C (−22 °F) in the warm temperates to −55 °C (−67 °F) in parts of the Arctic.^[1]^[2]

On October 1, 2024, the NWS replaced the "wind chill warning" with the "extreme cold warning." The standards governing issuance are set by local NWS offices. The 'step' below an extreme cold warning is an "extreme cold watch," which itself replaced the "wind chill watch" in October 2024.^[3]^[4]

Canada

An extreme cold warning is issued when the following temperatures are expected to last for at least two hours, which different regions have different criteria for:^[2]

South-Central Ontario and Southwestern Ontario issued at −30 °C (−22 °F)
Southeastern Ontario, the Southern Interior of British Columbia, the British Columbia Coast and Atlantic Canada (excluding Labrador) issued at −35 °C (−31 °F)
Quebec (excluding northern Quebec) issued at −38 °C (−36 °F)
Northern Ontario (excluding the far north), Central Interior of British Columbia and the Prairies (Alberta, southern Manitoba, southern Saskatchewan) issued at −40 °C (−40 °F)
Far northern Ontario, northern Manitoba (excluding northeastern Manitoba), northern Saskatchewan, Northern Interior of British Columbia and Labrador issued at −45 °C (−49 °F)
Northern Quebec (excluding Nunavik) issued at −48 °C (−54 °F)
Yukon, Northwest Territories (excluding Paulatuk, Sachs Harbour, Ulukhaktok), Baffin Island, and northeastern Manitoba issued at −50 °C (−58 °F)
Nunavik issued at −52 °C (−62 °F)
Northwest Territories (Paulatuk, Sachs Harbour, Ulukhaktok only) and Nunavut (excluding Baffin Island) issued at −55 °C (−67 °F)

United States

In the United States an extreme cold warning was an experimental weather warning issued by the National Weather Service in North Dakota, South Dakota, and Minnesota.^[5] The warning was issued if the temperature fell to −35 °F (−37 °C) or colder with a wind of less than 5 mph (8 km/h; 2 m/s).^[6] It was an experimental advisory for when the air temperature was dangerously cold, but the wind was too little to warrant a Wind Chill Watch or Warning.^[7]

The Extreme Cold Warning was used from January 10, 2011 to April 15, 2011.^[8] It was discontinued beginning with the 2011-12 winter storm season. Only five Extreme Cold Warnings were issued in that period: one for Burleigh County, three for areas outside of the Bismarck–Mandan metropolitan area, and one for West Glacier, Montana^[9]

In a partner webinar on October 30, 2018, the National Weather Service announced that based on survey feedback, the Extreme Cold Warning and the Wind Chill Warning will be consolidated into the Extreme Cold Warning, meaning that this product would once again be issued on a regular basis. This change was hinted to take effect sometime in 2021, accounting for NWS software upgrades.^[10]^[11] This was confirmed at the AMS 2024 meeting in January 2024.

In October 2024, the NWS officially implemented these changes to its wind chill and hard freeze warnings, watches, and advisories (WWAs) as part of its "Hazard Simplification initiative." The goal of these WWA name changes, per the NWS, is "simplifying a suite of cold weather forecast products to improve messaging of winter hazards and provide better decision support." The changes include:^[12]^[13]

Extreme Cold Consolidation and Renaming

Wind Chill Watch was renamed to Extreme Cold Watch
Wind Chill Warning was renamed to Extreme Cold Warning
Wind Chill Advisory was renamed to Cold Weather Advisory

Freeze Consolidation

Hard Freeze Watch was renamed to Freeze Watch
Hard Freeze Warning was consolidated to Freeze Warning

^{Local NWS offices will still determine the standards governing the issuance of such WWAs.}

Example

URGENT - WEATHER MESSAGE
National Weather Service Fairbanks AK
332 AM AKST Fri Dec 13 2024

AKZ834-835-140100-
/O.CON.PAFG.EC.W.0001.241215T0000Z-241215T1800Z/
White Mountains and High Terrain South of the Yukon River-
Fortymile Country-
Including the cities of The Taylor Highway north of MP 9, The
Dalton Highway from MP 0 to MP 38, Livengood, Chicken, Circle Hot
Springs, Twelvemile Summit, The Steese Highway from MP 85 to MP
124, Eagle Summit, Eagle, and The Elliott Highway from MP 32 to
MP 134
332 AM AKST Fri Dec 13 2024

...EXTREME COLD WARNING REMAINS IN EFFECT FROM 3 PM SATURDAY TO 9 AM
AKST SUNDAY...

* WHAT...Dangerously cold with wind chill as low as 60 below
  expected.

* WHERE...Fortymile Country and White Mountains including Eagle
  Summit, Chicken, and Eagle.

* WHEN...From 3 PM Saturday to 9 AM AKST Sunday.

* IMPACTS...The dangerously cold wind chills as low as 60 below zero
  could cause frostbite on exposed skin in as little as 5 minutes.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Dress in layers including a hat, face mask, and gloves if you must
go outside. If traveling, be prepared with extra gear to remain warm
and hydrated.

&&

$$