15:29 Fri Sep 13, 2019
When do we Average seeing our First Snow?
With winter coming fast in the White Mountains, it is always fun to look back at our records and figure out some averages. Since we are technically a sub-arctic climate on the summit, the first snow usually comes quite early relative to the surrounding locations. We have had a few good cold snaps recently with temperatures falling below freezing, though it has so far only occurred when we have dry air in the region so no snow yet.
Finding the first snow of the year ended up being a bit more of a challenge since for me, I wanted to know when the first true snow fell. Originally I had tried to find the first day after July 1st in which we recorded snowfall in the precipitation accumulation column but ran into a snag since hail counts as snow fall since it is falling ice. That made the first average snow fall into early August which is just not correct. In order to make sure that it actually snowed, I cross referenced the recorded snowfall with the hourly observations that we do to see if there was snow, snow showers, or snow grains reported since all those fall into what you would typically think of as snow. I even removed ice pellets, also known as sleet, because even though it technically is a winter precipitation, it is just frozen rain drops and doesn’t have the crystalline structure you get with snowflakes.
So for our entire record, the average first day where we see true snow is August 31st. This is thanks to seeing many early snow events in the first 30 years of our history. From 1935 to 1965, the average first snow was quite early, falling on August 29th! In recent history, we saw snow showers on August 31st in 2017.
For our most recent period from 1988 to 2018, our average is now September 5th. It is a little later than it was but this is also just seeing our first snow of the season, not first accumulating snow. That will have to be saved for a future blog!
Below is a graphic that shows the probability of seeing snow on any given day of the year. We are starting to increase the chances each and every day with almost a 20 percent chance of seeing snow by early October! I am really hoping for a good snow storm above 4000 feet right around peak fall foliage to get the contrast of fall and winter here in the White Mountains!
Adam Gill, Weather Observer/IT Specialist