NWS LINK: Forecast for Essex, Montana

For those of us in the weather/meteorology world, the summer of 2010 is over. Just as March 1 is the start of "meteorological spring" and June 1 ushers in "meteorological summer", September 1 marks the start of the annual transitional season prior to winter here in the USA. These "meteorological seasons" are defined by weather patterns and trends, rather than by the degree of earth's axial tilt through which the astronomical seasons are defined. From September onward, the upper jet (now primarily in Canada and the extreme northern US) will begin its increasing return southward, bringing along with it the cooler Canadian air that will eventually trigger the first snows, frosts and freezes of the season. Severe storms and tornadoes will also be possible as the upper support comes back to spring-like latitudes. Signs of winter will start first in the Rocky mountains (snow is already in the forecast this week for Montana), then the northern US states, followed by the Appalachians by mid to late October.

This trend means that September is also the start of icy road season. Road ice isn't only a winter problem, it shows up in the fall and continues into spring. In past years, the season's first icy road accidents have been recorded each September, primarily in the higher elevations of the Rockies, where interstate highways carry travelers from warm weather into a sudden danger zone among the mountain passes.

In a related topic, NOAA has finished compiling preliminary data for weather hazards during the 2009 season (linked here). Here is the 2009 graphic:

Courtesy NOAA

Historically and currently, icy road fatalities are not counted in these datasets. So, here I added a bar to the NOAA graphic that includes the 2009-2010 winter season total for road ice deaths (based on data collected for icyroadsafety.com):

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