This weeks Cold Spell Was The Coldest since 1994.

When computer models estimated that the national average daily temperature for the Lower 48 states dropped to 17.9 degrees on Monday, it was the first deep freeze of that magnitude in 17 years, according to

Greg Carbin, warning meteorologist for the National Oceanic and Atmospheric Administration.

That stretch — from Jan. 13, 1997 to Monday — is by far the longest the U.S. has gone without the national average plunging below 18 degrees, according to a database of daytime winter temperatures starting in January 1900.

In the past 115 years, there have been 58 days when the national average temperature dropped below 18. Carbin said those occurrences often happen in periods that last several days so it makes more sense to talk about cold outbreaks instead of cold days. There have been 27 distinct cold snaps.

Between 1970 and 1989, a dozen such events occurred, but there were only two in the 1990s and then none until Monday.

"These types of events have actually become more infrequent than they were in the past," said Carbin, who works at the Storm Prediction Center in Norman, Okla. "This is why there was such a big buzz because people have such short memories."

And Monday's breathtaking chill? It was merely the 55th coldest day — averaged for the continental United States — since 1900.

The coldest day for the Lower 48 since 1900 — as calculated by the computer models — was 12 degrees on Christmas Eve 1983, nearly 6 degrees chillier than Monday.

The average daytime winter temperature is about 33 degrees, according to Carbin's database.

There have been far more unusually warm winter days in the U.S. than unusually cold ones.

Since Jan. 1, 2000, only two days have ranked in the top 100 coldest: Monday and Tuesday. But there have been 13 in the top 100 warmest winter days, including the warmest since 1900: Dec. 3, 2012.