Lately, some of our readers have been asking if there’s a link between a wet summer and snowfall the following winter. It’s always tempting to connect the dots in weather, especially when extreme events happen (think Snowmaggedon winter of 2009-10, the June 2012 derecho, or the all-time warmest summer of 2010). Washingtonians just experienced the fourth-wettest June on record and, with 10.97” recorded for the season already, a wetter-than-normal summer (12” is normal) is all but assured. We have an excellent chance of logging our wettest summer since 2006, when a little over 18.5” fell.
Does all this rain mean anything for next winter? In short, no. A snowy winter does not a wet summer make. But my investigation did uncover another possible indicator for snowy Washington winters. It turns out that you, dear reader, have been looking for snow in all the wrong places.
For starters, there is virtually no correlation between the cumulative June-July-August (JJA) rainfall amount and total snowfall the next winter. The wettest D.C. summers – 30 seasons with at least 15 inches total – have been followed by above-normal snowfall 10 times and below-normal snowfall another 15 times (five of the wettest summers occurred prior to 1888, before which authentic snowfall records are not available).
1905 and 1906, the top-two wettest summers of all time (27.05” and 24.22” of rainfall, respectively), were followed by 28.3” and 25.7” of winter snow, respectively. However, five of the six all-time wettest summers starting with the eighth wettest JJA period in 1955 (19.39”) and ending with the 13th wettest JJA period in 2003 (18.28”) resulted in below-normal winter snowfall.
Though in recent times our snowiest winters have followed dry summers, the historical record shows this hasn’t always been true. The summers of 2009, 2002, 1995, 1986 and 1982 all recorded drier-than-normal rainfall amounts, and were followed by these respective anomalously high winter snowfall totals: 56.1”, 40.4”, 46”, 31.1” and 27.6”. In the more distant past, the summers of 1963, 1945, 1933, 1917 and 1891 experienced above-normal rainfall; exceptional seasonal snow totals – 33.6”, 24.5”, 30.7”, 36.4” and 41.7” – also followed these summers.
The statistically insignificant correlation between summer rainfall and winter snowfall helps explain why snowier winters nowadays tend to follow dry summers, as the long-term trend shows JJA periods becoming less wet. Drier summers in D.C. are simply more common and, therefore, the snowier winters that do develop have a higher chance of occurring after a dry JJA.
Since the relationship between rainfall and snowfall yielded nothing meaningful, I decided to focus on the temperature-snowfall relationship. It turns out that no statistical significance exists between the average summer temperature in Washington and total snowfall the following winter, either. A hot summer is almost equally as likely to result in a snowy winter as a cold summer.
Something did capture my interest though in plotting the trend in average summer temperature and winter snowfall over time (since 1888). The mean JJA temperature has been increasing, while the total seasonal snowfall has been decreasing. These observations confirm other data that show a warming and less snowy D.C. Note, though, that the warming trend in summer temperatures has negligible impact on winter snowfall.
What could be the one true link to D.C. snow, then? Move ahead a few months to autumn. October, a month known for idyllic Washington landscapes, holds the key.
The District has had 47 winters with 20” or more of snowfall. Of these 47 winters, 36 were preceded by a cold October! The average monthly temperature for all 47 Octobers combined is 1.5 degrees colder than the 1981-2010 climatological normal temperature. For reference, both October 2002 (40.4″ of snow the following winter) and 2009 (a record 56.1″ the following winter) were about 1 degree colder than normal, and October 1987 (25″ the following winter) was about 5 degrees colder than normal.
To be sure, not every cold October will lead to a snowy winter, and not every warm October will preclude a snowy winter. The outcome really depends on the type of pattern that eventually develops for the winter; blockbuster snowfall seasons in D.C. are driven considerably by “blocking” in the atmosphere (high pressure and warmth locked in over Greenland and surrounding areas across the high latitudes).
October temperatures might just be the new bellwether for D.C. winter weather, just as October polling serves as a key indicator for Washington elections in November. Let the debate begin.