Wednesday, March 14, 2018

Winter 2017-18: Average/Not Average

Note: This was sent in before the 3/12-14 storm. Revised in italics to March 18th data. Will revise again after the 3/21-22 storm. 

How normal/abnormal was the winter of 2017-2018?

This winter’s precipitation had been in the normal range - until March. Unlike regions of the country that have wet and dry seasons, eastern Massachusetts averages approximately four inches of precipitation every month of the year. As of March 4, i.e., between the first and second nor’easters, precipitation had been one inch over average for the last 30 days and two inches above the average for the last 12 months. [As of March 18, 2.3" higher than average for last 30 days and 3.7" higher for 12 months.] Of course, the Assabet River fluctuates greatly in depth between winter and summer, but that is because in the green months, plants are taking up huge quantities of water and releasing that into the air, whereas in winter most it either sinks into the earth to replenish our town’s water supply or else runs off to the river. The Assabet has been well above average since New Year’s Day.   

Creek near Assabet River, 1/1/18
Photo taken from footbridge
Same creek, 48 hours later
Click on photos to enlarge
Temperatures were all over the place. From December 26 to the morning of January 11 the temperature never got above freezing, and several nights got to -10F. Even the fast-running parts of the Assabet River were nearly frozen over. Then, two days of temperatures in the 60s combined with steady rain almost completely obliterated the snow cover. February temperatures were mostly in the normal range of below freezing at night, warming to above freezing by day, but on February 20 and 21, spiked to record setting highs above 70F. Whatever snow and ice cover that remained was again wiped out. Tough year on local ski slope businesses. The first nor’easter of March came in like a lion, the second one (March 7-8) was more of a snow leopard. The wet snow of the latter broke branches on trees that had survived the high winds of the former. And ANOTHER wet snow storm March 12-14! And 10F the morning of March 18!!.

Theoretically, there will be some benefits from those ultra-cold January nights. Adult deer ticks can survive a moderately cold winter, to plague us in early spring. But if the deep cold killed them all off, then woods walkers have little to fear until the over wintering eggs hatch and tick nymphs become active, in May. Likewise, the cold may have killed a majority of the wooly adelgids that plague hemlock trees, providing a year’s respite (but no permanent salvation from eventual tree death). For hemlock tree owners the only options are insecticide spraying – or a chain saw.    

Chart shows average monthly precipitation, in inches, using Boston data
(raindrops and snowflakes). The swooping line is river volume. Figure
created by Felice Katz for book: MAYNARD: History and Life Outdoors. 
Total snowfall had been a tad above average. Winters put about 45 inches of snow on Boston and 65 inches of snow on Worcester. It’s a fare guess that Stow and Maynard are in between. Counting the March 7-8 storm Boston’s winter total was 41 inches. [The March 12-14 storm brought the Boston winter total to 57.2 inches]. The last two big years for Boston were 2010-11 with 81 inches and 2014-15 with a record-setting 110 inches of snow. Between the two, 2011-12 was a low snow year, at 9.3 inches.

The long-term trend is that winters have been getting shorter, but snowier. Of the ten snowiest winters since record-keeping began in 1890, six have been in the last twenty-five years. The reason is that eastern Massachusetts has been getting wetter (up 10 percent) faster than warmer (up one degree F). But at some point in the future that upward snow trend will collapse, because once nor’easters are above freezing temperatures those storms will be heavy rain events rather than snow events. Portland, Maine has already experienced a crossover. Weather records dating back to 1870 show a bit warmer, much wetter, less snow.

March 8, 2018 "Nice hat"
Of course, winter is not over until it’s over. April 1, 1997 was the infamous April Fool’s nor’easter that put two feet of snow on Boston and nearly three feet on Worcester. Two days earlier had been sunny and in the 60’s, so people were unprepared for the idea of a pending storm. The storm started as rain, but as evening fell the air temperature dropped a couple of degrees more than expected and snow was suddenly coming down at 2-3 inches per hour. Across mid-Atlantic and New England states, more than one million people lost power. Twenty years earlier there was a snowstorm on May 9, 1977. Not as widespread as 1997 (Boston got less than an inch), but suburbs west and northwest of Route 128 got more than a foot of wet snow. Because trees had already leafed out, the damage was tremendous. 

Observe that we are looking at pretty much twenty year intervals. So maybe these March storms were pre-ordained.    

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