My undergraduate astronomy professor once said, “there’s the Great Orion Nebulae, and then there’s everything else.” In Alaska, there’s the great September 1992 cold snap, and then there’s everything else.
Every location has a cold period that stands out above all others. In many cases, it is a several days long snap, to something around 10-14 days. In September 1992, the entire month was sharply below normal in Alaska.
Winter in Alaska typically “arrives” in mid-October for most of the state. Once the snow starts flying, the dwindling solar energy is easily reflected back into space. As the calendar approaches mid-October, daily temperatures slowly, but steadily drop. In September 1992, snow and cold temperatures arrived by the 10th day of the month and never left. The airmass was cold to begin with, but once the snow fell, the airmass temperature fell off a cliff.
The chart above shows the average monthly temperature for all Septembers between 1925 and 2017 across Alaska. Notice how anomalously cold September 1992 is. There’s no other year even remotely close. Again, there’s September 1992, and there’s everything else.
In the central and southern mainland portion of Alaska, the September 1992 cold was historically anomalous. Using the NCEP/NCAR (R1) reanalysis, I mapped the most extreme cool/cold September departures from normal for the 1948-2017 time period. The map shows standard deviations below the 1948-2017 mean.
Any location in purple on the map indicates three or more standard deviations below the mean. Several parts of North America have experienced a -3.0 or lower September anomaly in the last 70 years. Only Alaska has experienced a -4 or lower standard deviation event.
The difference between the coldest September (1992) and second coldest (1983) September in the southern half of the state is up to 8°F! This is the largest differential between the coldest and second coldest September anywhere on Earth between 60°S and 85°N (non-polar regions).
As noted earlier, the snow that accompanied the first waves of cold enabled the airmass to cool down substantially. As many as several feet of snow fell across the eastern half of mainland Alaska. In Fairbanks, only three times has any measurable snow fallen in the first half of September. In 1962, there was 0.5”, in 1980, there was 3.3”, and in 1992, there was a whopping 17.4″! Unlike most out-of-season fleeting snowfalls, this one did not melt, and in fact, stayed on the ground until April/May.
September 1992 was a major outlier from the global march of increasing temperatures. Areas in white on the map below show where September 1992 was the coldest September during the 1949-2017 time period. Both September 1992 and 1993 were far below the strongly trending temperatures since the early 1960s at the global scale. This may be a lingering impact of the Mt. Pinatubo eruption in the Philippines – but I’ll defer to others on that specific connection.
Many thanks to Rick Thoman of the Fairbanks National Weather Service Office for insight and first-person recollections of the event. Rick wrote a three-piece blog about the amazing cold snap a few years ago. The blogs are found here: