Thursday, February 17, 2005
Feast or Famine: Is the 20"-25" winter dead?
Matt Ross @ 1:00 AM
Although our average snowfall in DC has dropped a bit over the past half-century (from about 18 to 16 inches), I believe much of this can be explained by the move of the official measuring station from DC proper (downtown) to poorly elevated and riverside National Airport(DCA)in 1941 as opposed to heat island, global warming, or other factors. Although I do believe that these factors have also played a role. Nevertheless, our 30 year median has dropped significantly in the past 30 years with respect to average versus the 30 years prior.
During the period from 1974-75 through 2003-04, DCA has averaged 15.5" while the median has been much lower at 12.1" or 78% of the mean. While the prior 30 years, 1944-45 through 1973-74 had an average of 17.4" and a median only slightly less at 16.3", or 94% of the mean. This is largely due to a recent pattern of below to well below average snowfall years interspersed with the occasional blockbuster. Because the floor is zero, but the ceiling is unlimited per se', this kind of seasonal pattern leads to a median well shy of the mean. Since our 25" winter of 1987-88(remember the Veteran's Day storm?), we have gone 16 consecutive winters(could be 17 after this winter) of not having a seasonal snowfall between 16" and 40". Over the 100 winters prior to 1987-88, we had not gone more than five winters in a row without a 16-40" winter. When looking more closely it is revealed that 14 of the past 16 winters have been below 16" while only two were above 40"(1995-6, 2002-3).
One explanation I have encountered is the increasing disappearance of the medium sized storm in lieu of the higher frequency big storm. This would lead to depressed snowfall figures in years that do not see one of these big storms(typically 8"+ for DC). However, I think this only scratches the surface. In my opinion, it is likely that we have gone through commensurate patterns at various times in history, but our limited data set has not shown it until now. I thought this winter was ripe to break the 16 year "drought", but as we sit at 6.6" officially, my confidence decreases with each passing day.
Photograph above of Kancagamus Highway, NH taken on 2/10 during a moderate snowstorm
Forecast: In the short range, today through Saturday will be dry and seasonably cold to a bit below normal with highs in the 30's and lows in the 20's as winter finally makes a return after a warm first half of February. In the medium range, there is starting to be some emerging recent model agreement that sends the Sunday-Monday storm to our west, thus allowing enough warm air to advect (or blow) in at the upper levels to change any frozen precipitation to rain. However, due in no small part to the existence of cold air in place and prior model discrepancy, I believe this is an event to keep an eye on for a possible southward trend and/or secondary coastal development. At the very least, I think parts of our forecast area will have precipitation type issues to deal with at the onset of the storm. So stay tuned to Capitalweather for the latest.
During the period from 1974-75 through 2003-04, DCA has averaged 15.5" while the median has been much lower at 12.1" or 78% of the mean. While the prior 30 years, 1944-45 through 1973-74 had an average of 17.4" and a median only slightly less at 16.3", or 94% of the mean. This is largely due to a recent pattern of below to well below average snowfall years interspersed with the occasional blockbuster. Because the floor is zero, but the ceiling is unlimited per se', this kind of seasonal pattern leads to a median well shy of the mean. Since our 25" winter of 1987-88(remember the Veteran's Day storm?), we have gone 16 consecutive winters(could be 17 after this winter) of not having a seasonal snowfall between 16" and 40". Over the 100 winters prior to 1987-88, we had not gone more than five winters in a row without a 16-40" winter. When looking more closely it is revealed that 14 of the past 16 winters have been below 16" while only two were above 40"(1995-6, 2002-3).
One explanation I have encountered is the increasing disappearance of the medium sized storm in lieu of the higher frequency big storm. This would lead to depressed snowfall figures in years that do not see one of these big storms(typically 8"+ for DC). However, I think this only scratches the surface. In my opinion, it is likely that we have gone through commensurate patterns at various times in history, but our limited data set has not shown it until now. I thought this winter was ripe to break the 16 year "drought", but as we sit at 6.6" officially, my confidence decreases with each passing day.
Photograph above of Kancagamus Highway, NH taken on 2/10 during a moderate snowstorm
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