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      The Greatest Recent Late-August Hot Spell: 1991

      A late August hot spell cranked up to full throttle beginning on August 26, 1991. The high temperature at the National Weather Service (NWS) near Negaunee hit a record-breaking 92 degrees.

      The traditional weather pattern for a U.P. heat wave developed and matured over several days. The jet stream or storm track lifted north into Canada, while a big upper-level trough developed in the Gulf of Alaska. This configuration helped to pump up a hot, upper-level ridge over the Great Lakes (Image above).

      The high temperature reached 90 or above on four consecutive days at the NWS beginning August 26. During that time, the mean temperature came in 20.5 degrees above average. In Iron Mountain, it reached the low to mid 90s from August 27 through 30. Munising residents sweated under 90+ heat from the 26th through 30th. The hottest temperature in that streak was a sizzling 98 degrees on the 28th.

      This heat wave pushed the August 1991 mean temperature to 65.2 degrees, 2.6 degrees above the long-term average. The warm spell ended as the month closed and September came in nearly two degrees below average. This began a chilly fall. October and November 1991 were both over three degrees below normal.

      So far, August 2014 is still, despite the warm spell that peaked yesterday, running below average. The mean temperature through yesterday at the NWS was 1.9 degrees below average. We are now in another cool spell with at least the next few days expected below average.