Daylight snow can have a tough time accumulating on roads at this time of year. When it does snow in mid-November, temperatures are often pretty close to the freezing mark of 32. (We are still about two months out from our climatologically coldest time of year, when the average high is only 19).
I like to call our current time of year one of our “shoulder seasons,” the period when we are heading into winter, but not yet there. And in the shoulder season, daylight snow accumulation on paved roads is particularly dependent on three things, elevation, snow intensity and sun angle. When the sun is higher in the sky, as it is now, as compared to late December, more solar energy makes it to the surface, even on a cloudy, snowy day. If temps are hovering around freezing, it is harder for the snow to stick. However, this can be overcome by heavy snowfall rates. A prime example of this is the 8 inches of snow which fell in Presque Isle on May 16, 2016, during daylight. In fact, the heavy, accumulating snow lasted deep (pun alert) into the morning, only 35 days before the summer solstice.
One of my favorite climate fun facts is that on that date, May 16, Caribou’s official total was 4 1/2 inches of snow, while just six days later, albeit in a different year (1977) it was 96 degrees on May 22. 96 is Caribou’s hottest temperature on record.
Regarding elevation, let’s use Portage and Wallagrass as examples. Portage is out on Rt. 11 and is 725 feet above sea level, but keep cruising north to Wallagrass, and you get up to about 1,000 feet. When temps are in the neighborhood of freezing, that relatively small difference in elevation can make all the difference between wet blacktop at the lower elevation, and snow-covered or slushy roads at the higher elevation, since it may be a degree or two cooler just a few hundred feet higher.
Another good example of elevation gain is heading north on 161 out of Caribou. You gain about 400 feet as you head up toward Jacobson Hill. So if you leave Caribou and it’s 34 and raining, he aware that you could well hit snow, simply due to the elevation gain. In certain weather setups that elevation difference isn’t as much of a factor, but it is certainly something to be aware of.
Switching gears, this winter the National Weather Service is adding a warning type to its arsenal of advisories. It will be called a “snow squall warning.” I’ve noticed that many people call in snow showers as “snow squalls.” Perhaps folks do it because snow showers, like summer rain showers, can come on suddenly. However, a true snow squall is way more intense than a snow shower, and it can be deadly if you are out on the road, due to the whiteouts they produce. It’s like trying to drive with a blindfold on. In addition, snow squalls can produce rapid, heavy accumulations over a localized area.
These snow squall warnings will be issued for localized areas, similar to how severe thunderstorm warnings look on a map. In terms of the duration of the warning, snow squall warnings will be similar to severe thunderstorm warnings, lasting in a given place usually for no more than an hour. So listen carefully for snow squall warnings this winter. If a snow squall warning is issued, read it or listen to it carefully to see if you are in its path, especially if you are getting ready to head out, or if you know someone on the road who is, unknowingly, driving toward one. Last winter there were some remarkable squalls that dropped up to 10 inches in a very localized area, and in a very short period of time. A good analogy is summer thunderstorms, which can produce torrential rain in one town, with nary a drop just 10 miles away.
Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been chief meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.
