There's an odd regularity to brief midwinter heat wave
Wouldn't a January Thaw feel nice about now? Odds are, despite record-cold temperatures this week, the Northland still may see a period of above-normal temperatures this month. Statistics show a January Thaw comes about 60 percent of all years in...
Wouldn't a January Thaw feel nice about now?
Odds are, despite record-cold temperatures this week, the Northland still may see a period of above-normal temperatures this month.
Statistics show a January Thaw comes about 60 percent of all years in Duluth, including 11 of the last 12 winters. And it comes at about the same time so often in some areas that it's an accepted meteorological phenomenon.
"I was surprised how often it happens here. We found it going back into the late-19th century on a pretty regular basis,'' said Mark Seeley, climate expert at the University of Minnesota. "It's happening more often in recent years. And the variance we're seeing is greater. Instead of a couple degrees above freezing, we're seeing some years with 10- to 20-degree temperatures above'' 32 degrees.
The January Thaw seems to occur most often between about Jan. 20 through 27, right about the time average temperatures bottom out and begin their slow climb to July.
But the thaw usually jumps well outside the average uptick -- to temperatures more common in March -- before falling back down to finish January and February.
North American residents have probably been noticing the thaw for centuries. Science has recognized for decades. In 1954 the Glossary of Meteorology described the January Thaw as a "period of mild weather'' in late January across the Great Lakes, New England and eastern Canada which "statistical tests show a high probability that it is a real singularity.''
Singularity means a meteorological condition that tends to occur on or near a specific calendar date more frequently than simple chance would indicate.
No one is sure why it seems to happen so regularly. One theory is that when the coldest pool of winter's polar air finally moves up and out of the Great Lakes region about this time of year, it pulls a mass of warm air up from the southwest.
Another theory says that, after a month of increasing daylight, upper winds begin to change just enough to allow a blast of warmer air to shoot this far north.
Eventually that warm air moves away and another cold pattern returns us to a normal, more gradual warm-up.
We seem to notice the thaw so much because it often follows the coldest temperatures of winter, so we see a sharp contrast between below zero followed by above freezing temperatures -- sometimes a 50-degree swing.
In Duluth, climate data shows we get above freezing for at least a couple days in January about 60 percent of all years, according to a report last year that Seeley prepared for the Minnesota Department of Transportation. Seeley looked at all of January and not just the third week.
According to a News Tribune analysis, Duluth has had a snow-melting thaw in January in all but one winter (2004) the last 12 years, although it hasn't always happened during the third week of the month.
Looking at just the third week in January, Duluth has had a significant thaw in six of the last 12 winters, including the past four years in a row (2005-2008).
Some winters are so brutally cold that no January thaw ever comes, including 1977-1979 and both 1995 and 1996.
Others are so warm that we may not even notice a thaw. January 2002 was above normal all month. Yet from Jan 25-27 temperatures went even higher, topping out at 42 degrees -- about 20 degrees above normal and perfect timing to be called a January Thaw.
Not all scientists agree it's a real phenomenon. One study found that the thaw may seem regular and come during more than half of all years but also may be just a statistical, not meteorological, anomaly.
In Duluth, average daily temperatures drop until Jan. 18, about one month after the shortest day of the year. After that, average temperatures begin their march to the highest of the year, in late July (about one month after the longest days of the year.)
In some areas the January Thaw really isn't a thaw, but a regularly occurring warm-up with temperatures 10-20 degrees above normal -- but still below freezing.