By KATHLEEN MADIGAN, Wall Street Journal

NEW YORK -- Monday marks the first day of winter in the Northern Hemisphere, and the snowstorm that blanketed the East Coast this weekend was just the latest reminder that weather holds a tremendous sway on economic activity.

Winter's impact presents a particularly big risk to the outlook. The second half of 2009 has seen fairly robust activity as the U.S. has come out of recession. But if the weather drags down first-quarter growth, expect to hear more talk of a double-dip downturn. Economists will have to separate the problems caused by Mother Nature versus the longer-term weakness from deleveraging and high unemployment.

Weather is so important to the economy that the Bureau of Labor Statistics tracks the number of people who cannot get to work because of weather. Two extreme examples are 2005's Hurricane Katrina that caused more than 200,000 of Gulf Coast residents to be off the job, and the blizzard of January 1996, when 1.8 million workers were snowed in. (Since the BLS is finished with its surveys for the month, this weekend's storm probably won't show up in the December data.)

The problem for 2010 is that consumers will tend to compare the weather mostly with last year's conditions, rather than what they have experienced in years further back, says Evan Gold of Planalytics, a company that advises companies on how to prepare for the impact of weather on their business.

But the 2009 winter saw temperatures slightly above normal, with minimal snowfalls for most of the country. "Last year on the East Coast, winter shut off in late January," says Mr. Gold.

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