The images above comes from a 2001 paper by Hirsch et al. (here in PDF) titled, An East Coast Winter Storm Climatology. The top curve shows all East Coast winter storms, and the bottom shows the most intense storms. for the period 1948 to 1997.
As the figure implies, they concluded in that analysis:
the frequency of ECWS show a downward tendency over the study period but at insignificant levels. One test found a decreasing trend in strong ECWS significant for an alpha = 0.10.
So there was no trend 1948 to 1997, or a slightly downward trend. This is interesting because over the latter half of that period one analysis (Willett et al. 2010) found an increase in the water content of the lower atmosphere. So those who argue for a simple relationship between increasing water content and storm strength, the data do not support such a claim over this multi-decadal period.
