The summit of Mount Washington has experienced freezing temperatures and rime/glaze ice three times (so far) this month. And with the recent cold snap, the summit also received some snow/sleet. When we posted images and videos about the cold, snow, and ice, a common comment each time was something along the lines of, “It seems early.” For valley locations around here, you would likely be correct. However, for mountainous terrain in NH, winter weather in September is common. In fact, for the summit of Mount Washington, September is the month when we typically see our first freeze of the season, our first rime/glaze ice event of the season, our first snow of the season, and our first inch of snowfall for the season. But, lets dig a bit deeper.

Let’s start with temperatures. The average daily temperature goes from 47F on the 1st down to 38F by the end of the month. Contrast that to August where the average daily temperature on August 1st is 50F and the average on the 31st is 47F.

September’s ~10 degree change is typical for nearly all shoulder season months. October goes from 37 to 26F, November goes from 25 to 16F, and December goes from 16 to 8F. So, September marks the start of colder weather returning to the higher terrain. This has to do with less daylight, shifting weather patterns, and colder northern air returning.

While colder temperatures are more common starting in September, freezing temperatures can occur at any point of the year. While they are less common in the summer, they do occur. In fact, examining our daily record lows over the course of the year (Jan 1 to Dec 31), they range from 47F below to 34F above. If we only count the days where the daily record low is >32F (so record lows above freezing), there are only four days where that has occurred. If we expand it just a bit and count the days where the record low was 32F or greater, then there are only 9 days where that has occurred. So while it might not be common to see freezing temperatures in the summer, it clearly is not impossible.

With cold temperatures comes the possibility of rime and glaze ice. The summit is in the fog over 60% of the year. As a result, as temperatures drop to below freezing, the super-cooled water molecules that make up fog allow for the formation of rime or glaze ice as they hit various surfaces (want to learn more about rime ice, check out this blog). Additionally, if the surface is cold but the air aloft is warm, we might see ice accumulations from freezing rain or freezing drizzle. As colder air returns, it is not a question if we will see rime but when. And since we typically see our first freeze of the season, the first icing typically occurs at some point in September.

For snow, it all comes down to timing. Some years, we might see cold, rime, and snow all happening at once. However, most years we typically see cold and rime return first as they occur after a cold front sweeps through and after the precipitation has left the region. But as ambient air temperatures continue to decrease over the month, the cold air returns sooner and sticks around longer until eventually it arrives prior to the precipitation tapering.

Statistically speaking, the month of September averages 1.2 inches of snowfall. Most of this occurs on the backend of a cold front or during upslope events behind an exiting cold front. While snow in September can occur, it is not known for being an exceptionally snowy month as the record maximum monthly snowfall for September is only 7.8 inches. While snowfall totals might not be significant most years, it is enough to whiten up the summit, slicken trails, and result in our first snowfall and first inch of snowfall for the season.