Cheyenne, Wyoming, broke a record on Wednesday, seeing the most drastic temperature drop in less than an hour as the country braces for a massive winter storm, days before Christmas.

The National Weather Service (NWS)  is warning of the “widespread and dangerous arctic blast,” which has already begun to sweep the U.S., making its way to the East Coast as the holidays approach. The NWS warns that the “record-breaking cold and life-threatening wind chills over the Great Plains” will spread to the eastern portion of the country by Friday.
Some cities have already experienced historic drops in temperature. Cheyenne, Wyoming, for instance, experienced a 40 degree drop in the span of 30 minutes, according to AccuWeather, which described it as “its greatest one-hour temperature drop on record Wednesday, going from 43 degrees to 3 degrees…”
Denver, Colorado, experienced a similar plunge, as temperatures swung from 42 degrees to just 5 degrees in less than an hour. Overall, the temperature was expected to drop to minus 16, and with the temperatures in the 50s the day prior, that would result in a 68 degree drop in less than a day.
Some areas will experience even colder temperatures. According to AccuWeather:

AccuWeather RealFeel® Temperatures in Denver are expected to settle between 30 below zero and 40 below zero, and was at 39 below zero at 3 a.m. local time early on Thursday morning. Elsewhere, the RealFeel will be even harsher in cities such as Rapid City, South Dakota, and Scottsbluff, Nebraska, as both locations could record RealFeel temperatures as low as 60 below zero.

The latest update from NWS warns of subzero temperatures in areas of the U.S. That, combined with a wind, could result in wind chills as low as  minus 40 degrees “across a large swath of the Intermountain West and northern/central Plains, with more localized areas of minus 50 to minus 70 possible through the end of the week.”

Even areas as south as Florida are bracing for freezing temperatures as well. Dozens of Florida counties are under either a freeze warning, freeze watch, hard freeze warning, or hard freeze watch this weekend:

According to data from FlightAware, the winter weather is causing massive delays and cancellations across the country, with over 2,000 flights delayed and over 1,500 more canceled altogether as of Thursday 11:00 a.m. ET.