The Weather Guys

Though the first really nasty cold snap of the year is already underway, by the middle of this week we should be in the depths of a deep freeze, flirting with a daily high temperature no higher than 20 degrees Fahrenheit.

Such cold maximum temperatures, though not unprecedented for early December, are also not common in Madison. In fact, only 18 of the last 40 Decembers in Madison have had at least one daily maximum temperature of 10 degrees Fahrenheit or colder.

Upon dividing the month into early December (Dec. 1-10), mid-December (Dec. 11-20) , and late December (Dec. 21-31), we find that seven of these years had at least one such cold day in early December (1976, 1977, 1978, 1985, 1991, 1995, and 2009). Six of the years had the first such day in mid-December (1983, 1989, 1999, 2000, 2004, and 2008) while the remaining 5 didn’t get that cold until late December (1973, 1980, 1990, 1993, and 1996).

So about every third December we can expect to get this cold before Dec. 21, and we might just do that again this week. Our chances will be enhanced thanks to the several inches of snow that blanketed the area on Sunday.

With a fresh snowcover on the ground, a number of effects that can deepen a cold spell come immediately into play. First, the snowcover is an excellent insulator and, if as deep as 3 inches, it effectively eliminates the vast amount of heat stored in the ground from helping to warm the air.

Second, snow is very good at cooling overnight, and that helps to lower the surface air temperature. Third, the snow is very good at reflecting sunlight during the day, which helps to minimize the daytime heating of the air near the ground.

October ended up to be about normal in terms of temperature despite the chilly last couple of weeks. In fact, 14 of the last 17 days of the month were at or below normal. A natural question arises in the face of this cool spell — namely, does this portend a cold start to the winter?

The official November-January forecast from the Climate Prediction Center of the National Centers for Environmental Prediction (NCEP) gives us a slight probability of above-average temperatures for the next three months and even chances for normal amounts of precipitation over that interval. This forecast may arise from the fact that sea-surface temperatures in the central tropical Pacific Ocean are nearly normal heading into the cold season.

Our own harbinger of the early winter focuses on the presence or absence of snow cover in northwestern Canada. The idea is that if there is snow cover there by mid-late October, the chances of producing cold air masses there increase as the daylight hours decrease in late fall. Cold air produced in northwest Canada is often involved in our early season cold air outbreaks.

We are testing this idea in current research. So far this year, there has been unusual warmth (and consequently little snow) in northwest Canada. Thus, our prediction is in line with that of NCEP — the early part of the winter will not be memorably cold.