The jet stream over North America has been unusually far south for much of 2009. It marks the approximate boundary between cooler air to the north and warmer air to the south, and New York has usually been on the north side of the jet stream, resulting in cooler-than-normal temperatures.

Why has the jet stream been so far south this year? No one knows for certain. The position of the jet stream is affected by factors on time scales ranging from days to centuries, including multiple natural atmospheric oscillation cycles. The combined effects of the different natural cycles can be predicted in general terms, but the details remain unclear, and any effects of climate change (whether natural or human-caused) on the jet stream are even less well understood.
Source(s):
http://www.accuweather.com/regional-news-story.asp?partner=accuweather&...
http://www.cpc.noaa.gov/data/teledoc/teleintro.shtml
http://ams.confex.com/ams/87ANNUAL/techprogram/paper_116577.htm


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