Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 9:48 PM GMT on December 12, 2013
The temperature at International Falls, Minnesota, plummeted to minus 33 degrees Fahrenheit early Tuesday morning (Climate Summary for December 10). The low occurred between 4 and 5 A.M. CST on December 10, but, if you were a rookie forecaster working during the evening of December 9, you might not have predicted such an extreme minimum.
A portion of the 00Z surface analysis on December 10 (6 P.M. CST on December 9). Full analysis. Courtesy of WPC.
Indeed, a low-pressure system was slated to move southeastward and pass to the south of International Falls...see 00Z surface analysis on December 10 above (6 P.M. CST on December 9; full analysis.). Even so, frost points to the west and north of International Falls were not even in shouting distance of minus 33 degrees. By way of background, weather forecasters use the dew point (frost point) as an estimate for the nighttime low temperature on clear nights with light winds...the air temperature can't really fall below the dew point (frost point) (unless a cloud forms, but, even then, there's an imperceptibly tiny difference between the two).
The METARS at International Falls, MN, from 00Z to 15Z on December 10 (6 P.M. on December 9 to 9 A.M. on December 10). I underlined, in blue, the frost points at 00Z and 10Z. I also indicated the low temperature (minus 36.1 Celsius = minus 33 degrees Fahrenheit). Courtesy of NOAA.
One look at the METARS (hourly weather observations) at the International Falls Airport (above) indicate that the frost point fell from minus 27.8 degrees Celsius (minus 18 degrees Fahrenheit) at 00Z to minus 37.2 degrees Celsius (minus 35 degrees Fahrenheit) at 10Z (4 A.M. CST) early on December 10.
The METARS at International Falls, MN, from 00Z to 15Z on December 10 (6 P.M. on December 9 to 9 A.M. on December 10). I outlined, in blue, the wind speeds (in knots) from the evening to early the next morning. Courtesy of NOAA.
Moreover, if you look closely at wind speeds above (in knots), you can see that winds were either calm or blew from the southwest at six knots or less. This observation should convince you that any quick answer involving the advection (wind transport) of really dry air is simply not reasonable.
The meteogram below graphically summarizes the points I'm making here. Note how the frost point (green curve on the uppermost plot) decreases dramatically during the early morning hours of the 10th, even though winds were calm (zero dry advection). The low temperature (minus 33 degrees Fahrenheit) obviously occurred between hourly observations.
The meteogram at International Falls from 1255Z on December 9 to 1255Z on December 10, 2013. Note how the frost point (green curve in the uppermost plot) decreases dramatically during the early morning hours of the 10th, even though winds were calm (zero dry advection). The low temperature (minus 33 degrees Fahrenheit) obviously occurred between hourly observations. Courtesy of the University of Wyoming.
So how did the frost point get so low during the nighttime hours? Whatever the answer, the air temperature followed the lead of the frost point and plummeted to minus 33 degrees. Okay, I'll divulge the answer.
During winter, when International Falls is deep in an Arctic air mass, there is a lot of frost deposition on exposed surfaces such as cars, light poles, etc. This process takes water vapor in the air near the ground out of the air, thus lowering the frost point. Water vapor also likely deposited on the cold snow cover, which was reported to be 12 inches deep on December 10. The bottom line here that deposition likely took water vapor out of low-level air, decreasing frost points, and, with a mainly clear sky and light winds, the temperature "chased" the falling frost points, plummeting to an absolutely frigid minus 33 degrees Fahrenheit before sunrise.
There are also many winter nights in International Falls when ice deposition occurs in the air (in addition to exposed surfaces, snow cover, etc.). When I lived in Montreal during graduate school, ice crystals sometimes fell out of the air even though the sky was clear. Thus, I hesitate to call these hydrometeors "snow," but, technically, they are. I've seen this kind of precipitation reported as light snow (-SN) on METARS in frigid places like International Falls.
An example of an ice crystal in the shape of a hollow, hexagonal column. Courtesy of Kenneth G. Libbrecht, Caltech.
For the record, these ice crystals are often thin plates or columns (above) that form in the temperature regime from minus 20 degrees Celsius to minus 40 degrees Celsius. Even with a temperature and frost point of, say, minus 29 degrees Celsius and minus 34 degrees Celsius (respectively), the air is likely saturated with respect to ice. As a result, the deposition process readily occurs because all sorts of ice nuclei become "activated" in the lower troposphere. These activated ice nuclei grow as water vapor deposits onto them, removing water vapor from the air and lowering the frost point.
Having discussed this process, I admit there weren't ice crystals reported on the METARS at International Falls. There were, however, reports of ice crystals within 50 miles of International Falls. If they occurred, more water vapor would have been removed from the layer of air above the ground, and the frost point would have lowered.
The bottom line here is that deposition likely occurred on exposed surfaces and the cold snow cover, paving the way for frost points to dramatically decrease, even though there wasn't any dry advection. In turn, temperatures plummeting under a mainly clear sky and light winds. In effect, frigid begat super-frigid.
P.S. This is my last blog for Wunderground (it's difficult for me to comprehend that I've already been here for an entire year). I cannot adequately express my gratitude to Jeff Masters for this wonderful opportunity. Jeff...I always anxiously looked forward to writing my blogs. And many thanks to my colleagues at Wunderground for supporting me and helping me whenever I had technical difficulties. Finally, to my faithful Wunderground readers, I couldn't have done it without you. Thanks a million to everybody.
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