An Entire Winter in One Weekend
By Heidi Sonen & Roscoe Shaw
Crozet averages 18 inches of snow per winter. After the warmest December in history, it seemed as if winter had been cancelled. But, on Friday, January 22, an entire winter’s worth of snow and cold came in just one weekend.
My official measurement was 20.5”, which made it the fifth biggest snowstorm ever here. The king of them all was the 24 inches in January of 1922 (see insert). Measuring snowfall is difficult and inexact, so ranking storms historically is not that meaningful. But no matter how you slice it, this month’s storm was historic.
So how do you measure snowfall? It seems easy. You just stick a ruler (or in this case, a yardstick) in the ground. But the January storm had a lot of wind so amounts were uneven. When that happens, you need to measure over a grid and take an average. Also, wind compacts the snow, leading to errors. The weight of the snow packs down the snow beneath, so a foot of snow on top of another foot of snow might end up being only 23”. When the ground or air is warm, snow accumulates differently, depending on the surface. Also, some snow might get packed down by sleet and then have snow start accumulating again. In other words, there is no easy way to measure it perfectly.
One of the easiest storms I ever measured was on December 19, 2009. Twenty-two inches of snow fell in just 20 hours with hardly a breath of wind. Everywhere I measured, it was exactly 22” deep.
One of the most fascinating aspects of this month’s storm was how well it was forecast. On Sunday, a full five days before the snow started, Heidi looked up from her computer and said, “Have you seen this? Every weather model on the planet has us getting nailed on Friday.” She was giddy.
There are at least a handful of well-known and different mathematical global forecast models. Their five-day forecasts are often wildly different. But this time, they were all the same. That causes a meteorologist to stand up and take notice because it’s a very good sign that the solution is correct.
At about 11 p.m. Sunday night, Heidi woke me up. “They all still have it,” she said. Most models run twice a day. The new data was in. The solution was the same. And it never wavered right up to the first flakes Friday morning. The entire event was a proud moment for modern weather forecasting.
The month averaged 36.4 degrees, which is 2.2 degrees below normal. That came in sharp contrast to the warmest December ever, which was a full 12 degrees above normal. The coldest was 11 degrees on January 6. Ice formed on our pond that morning, which was quite a shock after the warm holidays. The month went out like a lamb with 60 degrees on the 31st, creating a massive melting mess.