In
making its seasonal prediction, forecasters cautioned that there are large portions
of the country for which there are no clear indications whether it will be a warmer, colder, wetter, or drier than average winter, largely due to a fickle El Niño event that may have petered out too early to have
much of an
impact on
North American winter weather.
Snowfall varies across the region, comprising less than 10 %
of total precipitation in the south, to more than half in the
north, with as
much as two inches
of water available in the snowpack at the beginning
of spring melt in the northern reaches
of the river basins.81 When this amount
of snowmelt is combined with heavy rainfall, the resulting flooding can be widespread and catastrophic (see «Cedar Rapids: A Tale
of Vulnerability and Response»).82 Historical observations indicate declines in the frequency
of high magnitude snowfall years over
much of the Midwest, 83 but an increase in lake effect snowfall.61 These divergent trends and their inverse relationships with air temperatures
make overall projections
of regional
impacts of the associated snowmelt extremely difficult.