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U.S. Blizzard Statistics (1959-2000)
Schwartz, R.M. and Schmidlin, T.W.  2002.  Climatology of blizzards in the conterminous United States, 1959-2000.  Journal of Climate 15: 1765-1772.

What was done
The authors examined past issues of Storm Data - a publication of the U.S. National Weather Service (NWS) - to compile a blizzard database for the years 1959-2000 for the conterminous United States.  Once compiled, they performed a series of analyses on the data to determine blizzard temporal trends, spatial patterns and relationships with ENSO.

What was learned
A total of 438 blizzards were identified in the 41-year record, yielding an average of 10.7 blizzards per year.  Year-to-year variability was significant, with the number of annual blizzards ranging from a low of 1 in the winter of 1980/81 to a high of 27 during the winter of 1996/97.  Linear regression analysis revealed a statistically significant increase in the annual number of blizzards during the 41-year period; but the total area affected by blizzards each winter remained relatively constant and showed no trend.  If these observations are both correct, then average blizzard size is much smaller now than it was four decades ago.  As the authors note, however, "it may also be that the NWS is recording smaller, weaker blizzards in recent years that went unrecorded earlier in the period, as occurred also in the official record of tornadoes in the United States."

With respect to ENSO effects, a weak but marginally significant relationship was noted, with a tendency for two to three more blizzards to occur during La Niņa winters than during El Niņo winters.

What it means
The frequency and intensity of extreme weather events are typically projected to increase in response to global warming; and the data of this study suggest that - with respect to U.S. blizzards - frequency may possibly have responded as predicted, but if it did, intensity likely did the opposite.  On the other hand, the study's authors suggest that the reported increase in blizzard frequency may well be due to an observational bias that developed over the years, for which there is a known analogue in the historical observation of tornados.  That this possibility is likely a probability is suggested by the study of Gulev et al. (2001), who analyzed trends in Northern Hemispheric winter cyclones over essentially the same time period (1958-1999) and found a statistically significant decline of 1.2 cyclones per year using NCEP/NCAR reanalysis pressure data.  In addition, we note that there were fewer blizzards in warmer El Niņo winters than in cooler La Niņa winters, which also runs counter to predictions of more extreme weather in warmer as opposed to cooler periods of time.

Reviewed 18 September 2002