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Naomi Rappaport asks: I am so curious to know how snowfall totals are determined, who measures them and how reliable it is.
New Bedford residents dug themselves out of over 3 feet of snow last week — at least, that’s what they were told.
Snowfall totals came into the National Weather Service’s tracker slow and scattered Monday, leaving many Massachusetts residents wondering exactly how much snow was out there. Wind can cause snow drifts that make it seem like more snow fell than actually did, and even stray factors like grass can muddy the metrics.
As the inches pile up, measuring snowfall might seem more like an art than a science. How do the meteorologists actually do it?
This is the latest installment of a series that answers questions about what’s going on in New Bedford. Ask the Light your question here and our reporters will look into it for you.
The National Weather Service may station observers in major metropolitan areas to take measurements throughout the storm. But in many parts of the country, including in New Bedford, the NWS relies on trained volunteers, or “snow spotters,” for their measurements.
Frank Nocera is a NWS warning coordination meteorologist, helping organize the vast network of forecasters, NWS staff, and trained volunteers during major weather events. The NWS stations employees or contractors in the country’s major airports to measure rainfall, windspeed, and yes, snowfall. But in places like New Bedford, the organization relies much more on its volunteer network, Nocera said.
“It’s a pretty dense network,” Nocera said. “And we do quality-control it. Sometimes the observations are not good, and you have to toss them out.”
Some measurements that might get tossed stand out too prominently from the recordings in nearby areas, Nocera said. In Rhode Island and the South Coast, where Nocera is stationed, most measurements easily broke 30 inches during last week’s blizzard, but the wind can create snow drifts that make snowfall look higher than it actually is.
“This particular storm that just took place, one of the challenges was that the wind was so, so strong that we had so much drifting snow,” Nocera said. “It was challenging, even for a meteorologist, to do it accurately, just given that we had hurricane force winds at times.”
To get accurate measurements, volunteers are encouraged to find an area shielded from wind and use a snowboard or a 16” by 16” piece of wood painted white and measure against a flat surface. (Even grass can artificially inflate snow height.) Volunteers lay out two snowboards before snowfall starts and make multiple measurements throughout the storm.
While the flurries are still flying, observers and volunteers make two different measurements: new snowfall and snow depth.
New snowfall: New snow is measured against the snowboard, which may need to be cleared off between measurements. The snow should be measured to the nearest tenth of an inch, and it may take multiple attempts to get an accurate measurement depending on wind. Snow spotters take a new measurement once every six hours, Nocera said, and should not take more than four snowfall measurements in 24 hours.
Snow depth: The second snowboard is left out over the entire weather event and measures the total amount of snow on the ground. If multiple measurements are taken, observers average all of the measurements and round to the nearest inch.
Once submitted, these recordings show up in the National Weather Service’s snow report with a label indicating where the measurement was taken.
During last week’s blizzard, measurements in New Bedford were taken by an unnamed trained spotter near the city’s center at 5 p.m. and at a location in New Bedford two miles north-northeast of Dartmouth’s Bliss Corner (this is roughly in downtown New Bedford) at 8 p.m. The highest measurement taken was 37 inches: 11 inches above the city’s previous record during the Blizzard of 1978.
Want to become a snow spotter or help measure other natural phenomena? The Community Collaborative Rain, Hail, and Snow Network trains and mobilizes volunteers, reporting findings directly to the NWS and to other scientists and meteorologists. These recordings can include rainfall, hail, soil moisture, and other natural conditions.
Nocera said the volunteer networks help fill the reporting gaps left after the National Weather Service cut its force in 1993, switching to more automated forms of forecasting that require less on-the-ground manpower.
Much like clearing a street or sidewalk, measuring snowfall is a group effort.
Email Brooke Kushwaha at bkushwaha@newbedfordlight.org.
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