In Focus: Local Snowfall Trends

* A Blustery Day: Full Forecast *

In last week's Winter Outlook, we wrote about the long term downward trend in snowfall in the District. Some of our more astute readers pointed out the District's "official" observing station -- located at Reagan National Airport -- has not always been located there, but moved from a location downtown; therefore calling into question the integrity of the long-term trend. We were also questioned about the fact we analyzed the snowfall trends at just one station. These points and questions were quite valid, so I did new analysis to address them. After doing so I've concluded the story hasn't changed: snowfall totals are still declining in our region.

Keep reading for more on the analysis...

Running 30-yr snowfall averages for Reagan National (DCA), Dulles (IAD), and Baltimore-Washington International (BWI) airports.

Prior to the winter of 1943-1944, the official observing station that's now Reagan National Airport (DCA) was located at 24th and M St. So, one might be tempted to argue the drop in snowfall was mainly due to the re-location of the airport due to the warming influence of the Potomac River and lower elevation. However, the moving averages indicate snowfall declined not only before but also well after the station was moved to the airport. In fact, when we examine the trend in the moving 30-year averages since 1973 (shown above) - which includes only airport data (starting in 1944) - we find a decrease in snowfall at DCA at the rate of about 9"/century.

The story doesn't change when we analyze Dulles Airport (IAD) and Baltimore-Washington International Airport (BWI).

While IAD's record is short (starting in 1963, so we only have 15 years of 30-year moving averages), its running snowfall average is declining at a rate of about 6.3"/century.

Like DCA, BWI took over weather-record keeping from the downtown observing station in 1951. And BWI's 30-year moving snowfall averages also indicate a declining snowfall trend. Since 1980, the 30-year moving average (encompassing snowfall data starting in 1951) has been falling at the rather astounding rate of 17.6"/century.

The bottom line is that regardless how we analyze local snowfall records -- whether we use parts of records (using a single observing location) or all of records (combining different observing locations), 15-year averages or 30-year averages, fixed or moving -- we're seeing declines that are most likely real.