Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2. helmikuuta 2006 klo 15:33 (GMT)
Punxsutawney Phil, the fearless rodent prognosticator of Punxsutawney, Pennsylvania, saw his shadow this morning. According to tradition, we can expect six more weeks of winter are on tap for the United States.
Your fearful human forecaster predicts about five more weeks of winter--including some very harsh winter weather in mid-February. This forecast is based on long range model forecasts from the GFS model, plus observations of a "resonance" in Earth's climate system called the North Atlantic Oscillation (NAO) that I discussed two weeks ago. The NAO has just switched from its positive phase to its negative phase. The negative phase is typically associated with a jet stream pattern that brings cold Arctic air to the Eastern U.S., and that is the pattern we saw in the five week period from November 16 - December 23, when cold air gripped much of the Eastern U.S. The positive phase of the NAO is associated with a northward retreat of the jet stream into Canada, and usually brings much above normal temperatures to the eastern half of the U.S. The positive phase of the NAO dominted during the five-week period from December 23 - February 2, and record warm winter conditions were experienced across most of the U.S. during these past five weeks.
Figure 1. The North Atlantic Oscillation for Nov. 1 2005 - Feb. 2, 2006. The postive phase of the NAO has been associated with warmer than average temperatures at Detroit and most of the eastern half of the U.S., while the negative phase has been associated with cooler than average temperatures. Image credit: NOAA Climate Prediction Center.
Well, the NAO has just flipped back into it's negative phase (Figure 1), and I anticipate that given the five-week periodicity in the oscillation we've been seeing the past few months, we're in for a five-week period of colder than normal weather across most of the U.S. These below-average temperatures should ease up in early March when the NAO flips back into its positive mode. Long range temperature outlooks from the GFS model show the first Arctic air of the year invading the U.S. after Super Sunday, peaking in mid-February. I expect below-zero temperatures will affect most of the Midwest and Northeast by mid-February, with temperatures colder than -20 F in Minnesota. The Siberian Express is on its way!
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