Our last two winters have featured tons of snow, as I outlined in my last column. The question is, will this one follow suit? Time to dig into the details in my 2020-2021 winter forecast.
The last two winters, 2018-19 and 2019-20, were the snowiest of any two consecutive winters combined, as measured at NWS Caribou, with a total of 317.3 inches (26.4 feet). The last two winters are the only two consecutive winters on record with each having at least 150 inches of snow. Records at Caribou date back to 1939. The average snowfall at Caribou, based on 30-year climatological norms, 1981-2010, is 108.7 inches. So if each of the past two years had been exactly average, Caribou is about 100 inches above normal over the past two winters combined.
On to the forecast. With respect to energy use, I expect this winter to be similar to last winter, which had below normal heating degree-days, to the tune of about 4 percent. This means that for every 100 gallons of fuel in a normal heating degree-day season, last winter you would have used 96. Both the snow season and the energy use (degree-day) season go from July 1 through June 30th of the following year.
Now as for snow, I am again expecting an above-average snow year.
This is looking like a La Nina winter, which features cooler than average temperatures across the central and equatorial Pacific. It and its opposite, El Nino, along with other variables, influence our winters.
A common La Nina storm track features high pressure, back across the Pacific, and a storm track that dives southeastward, toward the Ohio Valley or Great Lakes, before turning back toward New England. At that point what sometimes happens is that the primary low peters out, while a new one takes shape to the east, along the coast and then intensifies. Just northwest of the intensifying low is a common location for a band of heavier snow. These “secondaries,” I have found over the years, can really jack up amounts, especially in southern Aroostook County, though all parts of northern Maine can see enhanced amounts compared to what would have fallen if the secondary had not formed.
One of the key things that influences whether one forms is the position of the jet stream above the potential significant secondary. If the jet stream is in a good position to evacuate (get rid of) rising air from the surface, a new low can develop and intensify (the needle on your home barometer would be falling) to the east of the original low, and again, intensifying lows can produce very heavy snowfall rates. Also, if there is going to be lightning in a snowstorm, it is most likely to occur during the time of most rapid intensification, meaning the barometric pressure is falling quickly. A rapid fall in barometric pressure is a drop of 0.04 inches per hour or more.
The main thing to remember about secondaries is that they can put us on the correct side of the low pressure system to receive a heavy snowfall!
A new-to-the-public term has surfaced over the past several years, and that is
“bomb” or “bomb cyclone.”
This is simply a term used to describe the rate of intensification of a low-pressure system. It is said to be a “bomb” (the term originated with meteorologists saying, “the low is bombing out”) when the pressure falls by 24 millibars in 24 hours (0.71 inches in 24 hours on your home barometer). When a low is bombing, it can produce snowfall rates of 3 or 4 inches per hour, and there are numerous cases where that rate has been exceeded. A rapid drop in pressure means the surface air is rising at a rapid rate, and this leads to significant precipitation rates.
By the way, once in a while, Buffalo, New York, makes the national news for getting some staggering total of snow. That comes from an entirely different process known as lake-effect snow.
Here is the “snow part” of my winter forecast for 2020-2021, and keep in mind again that the average seasonal snowfall for Caribou is 108.7 inches.
North of Houlton, I am forecasting 125-145 inches of snow, with any mixing with sleet or freezing rain fairly infrequent. (The thing that causes sleet or freezing rain is having a layer of air aloft that is warmer than 32 degrees.)
From Houlton south to Weston, I am expecting 105-125 inches of snow, with sleet and freezing rain mixing in during some storms. While this will hold down the seasonal total somewhat, I am also expecting this area to be more prone to heavier single snowfalls from the secondary development I previously described.
It’d be a good idea now to review a few terms. A winter storm watch means a significant amount of winter precipitation is possible, be it snow, sleet, freezing rain or a combination of all three, such that travel will become very difficult. Meanwhile, a winter storm warning means that the conditions I just specified will occur, within the specified time period of the warning.
Now, different regions of the USA have different thresholds for the issuance of a winter storm warning. Where there is less annual snow, the “trip line” for issuance of a warning will be less than it is here. In Caribou, where we average about 109 inches of snow, the threshold for issuance of a winter storm warning for a storm where the only precipitation type is going to be snow is 7 inches in 12 hours, or 10 or more inches in 24 hours.
I grew up in simpler times, when there were only “travelers advisory” and “heavy snow warning.”. Believe it or not, in northern Virginia, the Weather Bureau, as it was called pre-1970, would issue a heavy snow warning for 2 to 4 inches of snow (which is why the buried picnic table I wrote of in my last column was so significant). We only averaged 16 inches per winter, just south of DC.
I’ll close by pointing out a common mistake. Folks tend to focus on the forecast end time of the accumulating snow, while not paying attention to what the wind will do after the snow has stopped. I have seen plenty of times where the worst part of the storm, in terms of travel, follows the accumulating snow.
Don’t let your guard down. A good link for knowing whether wind is in the forecast can be had by simply googling NWS CAR AFM. Hit enter then click on the link at the top. Data there is laid out in an easy-to-understand grid, and in the grid you can see both the forecast wind speed and direction. Powdery snow will really get going and produce fairly widespread whiteouts once you get the winds gusting above 30 mph.
Whiteouts are one of the most dangerous hazards in winter’s playbook. My most “white-knuckle drives” have been coming down to the station from the Caribou side.
Back to that grid, make use of the wind direction forecast, and not just the wind speed. A west or northwest wind, for example, will make north-south oriented roads that run through open country especially susceptible to blowing snow!
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.
