What was done
A winter (December-March) climatology of cyclones (storms) was developed and statistically analyzed for the North Pacific Ocean (30°-50°N, 150°E-130°W) using sea level pressure from NCEP/NCAR reanalysis data for the period 1948-1998.  In situ data from various other sources were then examined to corroborate the results.

What was learned
The frequency of deep (sea level pressure of less than 975 mb) cyclones, which represented approximately one-third of all identified cyclone systems, was found to be increasing at a linear rate of 0.21 cyclones per year, although substantial year-to-year and decadal variability were noted in the record.  In addition, the intensity of deep cyclones was shown to have increased with time.

Several potential causes of the observed increases in the frequency and intensity of deep cyclones were discussed, including (a) that the increasing trend toward deeper cyclones (lower sea level pressure) was partly the product of decreasing background sea level pressure, (b) changes in the Pacific Decadal Oscillation, (c) increasing upper-tropospheric zonal winds between 25° and 40°N, (d) changes in tropical sea surface temperatures, and (e) "the observed intensification may be a manifestation of natural climate variability, and the mechanisms described may be completely unrelated to the observed changes."

What it means
It is important to consider the results of this study together with those published by Gulev et al. (2001), who examined trends in winter storms using the NCEP/NCAR reanalysis data for the entire Northern Hemisphere.  Similar to Graham and Diaz, Gulev et al. did indeed find that winter storms are intensifying, but they report that the storms are dissipating quicker.  Furthermore, Gulev et al. report that the annual number of total cyclones with minimum central pressure of 1000 mb or less has actually declined for the entire Northern Hemisphere during the last half-century.  The "big picture," if you will, thus seems to be exhibiting an improvement in this regard.