Weather Whys, February 24, 2013

A review of this winter
Today we will look at how this winter compares to normal conditions and if a concept of normality can even be applied to this season. Some people are grumbling about this winter and are ready to move on to another season. Perhaps that’s normal enough.

The first problem with defining winter is deciding when it begins and ends. For climatologists this season takes place from December 1 to the end of February, the three coldest months of the year. For astronomers and on our calendars the dates are approximately December 21 and March 21 for the finish. Spring this year comes on March 20, early in the morning.

Some advice to groundhogs: err on the side of caution. Winter always extends four or six weeks beyond Groundhog Day.

Of course, winter weather in our region typically extends earlier and later than these dates. Last fall was somewhat unusual in Thunder Bay because no measurable snowfall took place in October. This was followed by November with normal snowfall, although most of that came in one day. Some alert skiers may have taken advantage of this but by month’s end the landscape was snow free.

Snow was in short supply in the city of throughout December and marginal snow conditions persisted until January 20. More recently, snowfalls on February 10 and 16 have resulted in about 25 cm on the ground in urban areas. It is a different story as one moves inland; near Thunder Bay and generally in the region in the region a snow base of 40 to 55 cm is typical.

This snow season reinforces a trend. In the middle 20th century, about 140 cm would have been counted by this date at Thunder Bay Airport. So far, an estimate of the total is 94 cm.

Several factors contribute to this snow decline. Winters are warmer than they used to be and both temperatures and rain (and freezing rain) are more common. Several winters in the 1970s had no measurable rain.

Contrast this with rain this winter (DJF) with a total of 20 mm; only a couple of traces so far in February. This winter seems to conform with a new normal of the 21st Century; more rain and less snow. Winter precipitation was almost exclusively snow In Northern Ontario 30 to 40 years ago.

The threshold of 0°C, snow with colder temperatures and rain with warmer explains some of snow decline but another factor is the measurement of snow. Record keeping by Environment Canada used to be meticulous. This made sense in Canada because considerable infrastructure, snow loads on roofs for example, road maintenance, tourism and so on, are directly affected by snow amounts.

I know, it can be tiresome to talk about the “good old days” but indulge me for a moment. Staff at airport weather stations informed the public and pilots as to current and forecasted weather, recorded weather on the hour (24-7) and measured snow both by instruments and manually. Measuring snow depth on the ground was a procedure – a survey conducted at precise times. The routine to determine snow depth required taking 10 representative samples and divide by 10. This combination of methods insured that windy and drifting snow conditions did not result in suspicious numbers – increases or dramatic decreases in snow depth when no snow had actually occurred.

Staff cuts and changes to procedures result in compromised snow records. Snow amounts that occur prior to mid-October or after mid-April no longer included in a snow season total. Sometimes it does not matter. No measurable snow fell in Thunder Bay last fall prior to November 11. Other years it does matter. In the past any snow that did fall in May or September or, if it had dared to happen in summer, would be included in an annual snow total.

I have received inquiries about the arrival of the spring. Temperatures are likely to average above seasonal for the next week or two. Is this the end of winter weather? Highly unlikely.

March and April are very likely to be warmer than average. After saying that, here comes one of those disclaimers – one of the features of our new weather is more variability. Expect an early spring but with occasional winter reminders.