The cold weather that started last Friday in Arizona is leading the weatherman to dig into the record books.

The winter system that began with scattered rain and snow showers was followed by a deep pocket of cold air that had its origins in the Arctic. Since then, daytime highs haven't cleared 50 and overnight lows have plummeted to the upper teens, about 20 degrees below average.

Those kind of temperatures aren't entirely unusual in the Sonoran Desert. But the duration of the cold is. The last four-day cold snap was in 1988. And before that?

"You've got to look back to the mid 1950s, early 1960s to see an event like this last for this long," says J.J. Brost, science officer with the National Weather Service's Tucson office.

Brost looked back at weather history to see when the region last endured such a prolonged cold spell. According to the data, Brost says, this event is already tied for the third coldest four-day stretch ever recorded, going back to the late 1800s.

"So this is really a once in maybe a 30-year event or even once in a 50-year event. Very rare," Brost says.

Wednesday morning will be well below freezing again, Brost says. But high pressure returns to Arizona later in the day, and highs are expected to be in the mid 60s by the end of the week. Lows will be in the low 40s.

The overnight low Tuesday morning at Tucson International Airport was 17 degrees, which is the coldest temperature recorded there since December 24, 1974. The average overnight low for this date should be 40 degrees.