Mount Washington Fall Almanac and a 2024 Seasonal Outlook

By Alex Branton

As we move deeper into the month of September, New England’s most notorious season, fall, is nearly upon us. Reflecting on a warmer than normal and slightly wetter than normal summer in the White Mountains, folks across the region are ready for cooler temperatures and autumn colors. On Mount Washington, the atmosphere has already given us a glimpse of the season ahead with two icing events (Image 1) and our first measurable snowfall. Meteorologically, fall has already begun, with the season being defined by the months of September, October, and November. Astronomically, we are still in the season of summer until the Autumnal Equinox on September 22nd, 2024. The Autumnal Equinox is one of two days in a calendar year – the other being the Spring Equinox – where day and night are of equal length. According to astronomy, the fall season persists until the Winter Solstice on December 21st, 2024. This is the day in which the northern hemisphere receives the least amount of sunlight during a 24-hour period; A.K.A. the shortest day of the year.

Regardless of which seasonal calendar one chooses to follow, New England can experience some major shifts in conditions in the month of September. In this first month of autumn, New Englanders usually experience shortening days, cooling temperatures, and the first signs of fall foliage. In New England, temperatures start dropping in the month of August and leaves begin changing colors in late September, but this can vary from year to year. The Mount Washington Observatory has been collecting weather data on the summit of Mount Washington for over 90 years, and in fact, the first weather observation taken by MWOBS was done in the fall. This is known as our founding day, which took place on October 15, 1932. So, although there are many factors that influence a season’s weather, our 90+ year dataset can give us insight into what a typical autumn looks like on Mount Washington.

Frosty sunrise after first snowfall with tower and summit sign.

Similar to the phenomena experienced at lower elevations, the summit of Mount Washington experiences decreasing temperatures and shortening days in the fall. In addition to these variables, the highest elevations of the White Mountains also get increasing wind speeds and a transition of the dominant precipitation type to snow. Although there are no trees in the alpine zone, grasses lose their green coloring and we can see vibrant fall foliage at the lower elevations. Using MWOBS’ climate data set, see Figure 1 below for the Mount Washington Summit Fall Almanac.

So far this season, we have seen both expected and unexpected changes in the weather on Mount Washington. The monthly average temperature in September so far (as of this writing; 43.9°F) is lower than the monthly average temperature in August (49.4°F). The summit weather station has recorded four days with icing so far this month (September 2nd, September 3rd, September 8th, and September 9th) and received the first measurable snowfall of season (0.01”; September 8th). Temperatures have since warmed back to above seasonal averages during an extended period of anomalous high pressure, beginning on September 12th. Wind speeds are slightly below average for the month of September, with an average speed of 26.6 mph as of this writing. Alpine grasses have lost their green coloring and we can already see some fall color at the lower elevations north of the summit.

With a seemingly early onset of fall foliage, many are anxiously awaiting more consistently cool temperatures and the first big snow event of the season. The Climate Prediction Center (CPC) has released a seasonal outlook for meteorological fall (update coming September 19th), giving their best projection for the season ahead. According to the CPC, above normal temperatures are expected across the majority of the United States this fall. The highest probability of above normal temperatures is in New England. Precipitation is also expected to be above normal all along the East Coast, including in New England (Climate Prediction Center Three-Month Outlook, Forecaster Brad Pugh).

While above normal temperatures is forecast this fall, an anticipated uptick in tropical activity in the Atlantic Basin due to anomalous upper-level divergence and a favorable Madden-Julian Oscillation could interrupt the projected mid-latitude circulation pattern. This could result in fluctuating temperatures to both above and below normal values in New England (Climate Prediction Center Three-Month Outlook, Forecaster Brad Pugh). Regardless of whether or not we meet seasonal norms, September marks the beginning of decreasing temperatures, shorter days, and fall colors. The summit of Mount Washington will begin to experience its notoriously extreme conditions as we move through the rest of the fall while the lower elevations enjoy New England’s most famous season. Whether you are looking forward to vibrant foliage or watching the summit of Mount Washington become capped with snow, happy fall from the Mount Washington Observatory!

Mount Washington Observatory is a private, nonprofit, member-supported institution with a mission to advance understanding of the natural systems that create Earth’s weather and climate. To learn more about our weather and climate research and how to become a supporting member, visit mountwashington.org/.