These places are normally cold, but this chill was unlike anything seen in the contiguous United States in decades, in intensity and duration.
The February temperature departures from normal were stunning. Several major climate locations averaged 27 to 28 degrees below normal, the most extreme departures in the Lower 48 for a full month since January 1969, according to Alaska-based climatologist Brian Brettschneider.
Great Falls, Mont., was at the heart of it. There were 11 days when the mercury didn’t rise above zero, and there were 24 nights when it dropped to zero or below. Only the first day of the month topped freezing. The average February temperature there finished at 27.5 degrees below normal.
The punishing and unrelenting cold continued into March. On March 3, the low temperature tanked to a bone-chilling minus-32 in Great Falls. Combined with a high of minus-8, the day finished a whopping 50 degrees below normal. The city concluded its longest stretch on record below freezing on Thursday.
“For February 3rd through March 4th, Great Falls, MT averaged 32.3F below normal,” Sam Lillo, a PhD student in meteorology at the University of Oklahoma, recently tweeted.
While the cold in February was remarkable for its persistence, the brutal Arctic blast to begin March delivered the most intensity. Almost two dozen official stations in Montana broke monthly records during the early-month Arctic invasion, with widespread readings in the minus-30s to minus-40s.
A state-record low for March of minus-46 also was probably established. Icing on a cake long frozen.
The graphic above, provided by Lillo, illustrates how this historic cold snap compares with unusual 30-day cold streaks in other parts of the country. In most locations, there is no historical record of cold so extreme lasting so long. Only portions of the northern and central Plains into parts of the Midwest have seen anything like it in modern history.
The chart below shows the frequency of temperatures of varying degrees above (orange) or below (blue) normal averaged over 30 days at every reporting location in the United States (as far back as the late 1800s). In the case of Great Falls’s difference from normal of 32.3 degrees over the 30-day period ending March 4, everything to the left has been colder. Pay close attention to the scale.
“There are 0.00041% of obs[ervations] in the contiguous United States that have been colder,” Lillo said. That’s 0.00041 percent of more than 30 million days of observations. In other words, it was an exceptionally rare and exceptionally cold 30 days.
So what was the driver behind this abnormal cold? Turns out, we can largely blame a meandering high pressure that delivered record warmth in Alaska during February.
“It was a warm January and February with high pressure off the West Coast," Erik Gustafson, a meteorologist at the National Weather Service in Great Falls, told The Washington Post. Then that high pressure shifted toward Alaska, which “allowed extremely cold air to come down the Rockies, in round after round,” he said.
Gustafson noted that the repeated Arctic outbreaks brought Montana “northeast winds behind fronts, and this reinforcing wind holds the cold in.” Thanks to these regular intrusions, cold air was “stuck in valleys and against the mountains with no place to go.”
Although the heart of the extraordinary cold was centered in parts of Montana and adjacent Canada, impressive cold was also observed to the east in the Dakotas.
While temperatures are still cold in the northern Plains, the pattern has shifted a bit. A less extreme version of colder-than-normal weather may persist into the next week or so. Beyond that, there are signs of closer to normal or even a few days above normal later this month.