The winter of 1972-73 hit early and hard. Cold arctic air masses began filtering into the central U.S. in mid-November 1972. By the first week of December, mid-winter low temperatures were occurring over Upper Michigan. Three record lows were set at the National Weather Service site (NWS) near Negaunee from the 4th through the 7th, including an 18 below zero low on December 7, 1972â??the coldest so early in the season. Iron Mountain also reached 18 below for its early-season record low. Cold arctic high pressure built in and brought clear skies and light winds over a snow cover (Image 1 above).
The snow came a couple of days earlier. A widespread, moderate to heavy snow fell over the Western Great Lakes including parts of the Upper Peninsula on December 5th. Another tough winter seemed to be setting up for the region, which just experienced one of its most protracted cold seasons the year before. However by mid-December, the weather pattern completely flipped. A mild regime set up about a week before Christmas and held. The bulk of winter of 1972-73 turned out to be one of the mildest, least snowy seasons on record in Upper Michigan. December did end 5.2 degrees below average, but January wound up 2.6 above and February about a degree above normal. It really warmed in March, with the Weather Bureau in Marquette recording a remarkable mean temperature over 8 degrees above average. Less than an inch of snow fell in March 1973. The coldest sustained period of the winter relative to average occurred in early December 1972.
This winter is also starting quick. The question is, will it, like the winter of â??72-73 turn mild? It may not stay this cold compared to average, but the chances of a very mild winter look slim in that our global pattern is quite a bit different than 41 years ago. The winter of 1972-73 was a strong El Nino year. El Nino is a short-term (from about six months to two years) cyclical warming of the equatorial Pacific from the coast of South America to about the dateline. Back then, the phenomenon was well-know, but its affect on global weather wasnâ??t. Now we know that a strong El Nino correlates strongly to warmer than average winter temperatures over the north-central U.S. including Upper Michigan. During this cycle, there is a tendency for winter to set in early over Upper Michigan, then about the time El Nino is peaking (in December), the cold breaks down as mild Pacific air floods the United States. This same general pattern also set up in 1982 and 1986; two other strong El Nino winters. In 2002, anomalous cold locked in during the latter half of October through the second week of December. Then from Christmas through the first 10 days of January, temperatures average much above normal over the U.P. That year, however, El Nino weakened and cold weather charged in and held from late January into early March.
This year, the waters of the equatorial Pacific are locked in a neutral mode. Water temperatures are close to average from the Coast of South America to the dateline (Image 2). That means other global indicators will drive our weather. Right now, these indicators are producing very cold weather over North America.