Saturday, April 01, 2006
Weather columnist Kevin Myatt: 'Average' snow totals drift over the years
Kevin Myatt is The Roanoke Times' weather columnist.
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We have reached the point when I think it's safe to assume that we're entirely done with accumulating snowfall for the season in Roanoke.
While we have actually had at least a small amount of measurable snow in April about one year of every six since the late 1940s, I don't see it happening with the weather regime that has set up. It will be too warm and too dry. Snow can fall at our highest elevations way into May, but in the valley, we're done, and really have been for almost a month.
A reader requested that we run a chart plotting Roanoke's annual snowfall. We haven't really done that in several years, so I thought it would be a good idea, as it is a fascinating statistic that can vary drastically from year to year.
There are some problems to overcome in plotting this, the biggest being the 1997-2004 snowfall data gap for Roanoke's official weather data site, the Roanoke Regional Airport. A change to entirely automated precipitation data eliminated snowfall measurement during this time. As of January 2004, it has been restored. But the gap plays havoc with climate records.
I filled the gap using data supplied to me by Cave Spring weather observer Wendell Prillaman. Though, as we've documented previously, the Cave Spring site seems to commonly get more precipitation than the airport over the years, it should be pretty close.
Another significant issue is the 1986-87 problem. The Virginia State Climatology Office and Southeast Regional Climate Center data list that as our greatest snowfall year on record with 72.9 inches of snow. The National Weather Service at Blacksburg, however, in a listing of our 10 biggest snow years, lists Roanoke's snowfall for 1986-87 as 56 inches.
When I mentioned the 72.9-inch figure previously, some readers objected and provided information that would put it in question. After reviewing Prillaman's measurements for the same period, I'm going to have to concur, pending further research. He actually measured only 52.1 inches in 1986-87, so I'm having a hard time believing the airport got 20 more inches. For now, I'm going with the National Weather Service's figure of 56 inches, still one of our three biggest snow years on record.
Enough ground rules. What does the chart show?
Our 2005-06 snow total in Roanoke was 10 inches. That puts us above 16 winters on record, but below 40, and equal with another. It was also a ton less snow than my 35- to 40-inch "guesstimate" in late November, and even less than the 15- to 20-inch updated forecast in late January ... but never mind that now.
The 1960s snow bulge is always intriguing, as I highlighted in another column. In the decade of the 1960s, Roanoke averaged 33 inches of snow. Our long-term average since records began being kept at the airport in the late 1940s is a bit over 21 inches.
Compare those figures with the average snowfall of our past 10 years: 13.3 inches. Our 10-year average snowfall dropped nearly 6 inches since last year because that snowy 1995-96 season is no longer within the period. We've had only one above-average snowfall year in the past 10.
So if you perceive that it is snowing a lot less than it used to in the 1960s, 1970s and 1980s, you're entirely correct, backed by official statistics. But the 1950s had even less snow, averaging 12.4 inches, so it hasn't always snowed more.
In a location where annual snowfall has varied from 2 to 63 inches a season, I think the most misleading statistic that's ever offered is an "average" or "normal" snowfall total. That figure has bounced between 20 and 25 inches over the years, depending on what long-term period of record is used, but only a third of our snowfall seasons have topped 20 inches. The few really big years tend to pull the average up.
If you list all the snowfall totals over 58 years and find the middle, with the same number of years above and below, that point rests at 16.95 inches. This is the median annual snowfall, and probably a better, though still imperfect, indicator of what one can commonly expect in a Roanoke winter.
