The Upper Midwest Heat Wave of July 2012

The start to July of 2012 was a hot one as a strong ridge of high pressure in the upper atmosphere allowed for a persistent feed of very warm and humid air from the south. This ridge also served to "cap" the atmosphere, preventing  relief in the form of showers and thunderstorms.

Highs climbed into the 90s for the  2nd through 6th, with some locations topping 100 on the 4th and 6th. When the high humidity was factored in, afternoon and early evening heat indices ranged 100 to 115.  Heat Advisories were in effect Monday for portions of the region, with Excessive Heat Warnings covering much of the Upper Mississippi River Valley from Tuesday through Friday.

Tragically, the extreme heat also was deadly as several people lost their lives from heat related factors. Five people perished from July 4th through July 10th across western and central Wisconsin, mainly from lack of air conditioning in their residences.

 Factors that led to the prolonged, oppressive heat wave:

  • Drier than normal conditions
  • Strong, persistent ridge of high pressure
  • Little to no rain

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La Crosse

 

Rochester

  High Low Average Heat Index
July 2nd 99 73 86 105
July 3rd 99 75 87 105
July 4th 103 81 92 110
July 5th 98 79 89 111
July 6th 103 78 91 109
 
  High Low Average Heat Index
July 2nd 92 73 83 98
July 3rd 93 73 83 102
July 4th 93 75 84 104
July 5th 94 73 84 105
July 6th 96 74 85 103

Graph of the early July 2012 heat at La Crosse

 

 
Graph of the early July 2012 heat at Rochester

 

Records that were broken or tied in La Crosse, WI:

  • July 4th: High of 103 (previous record 100 in 1911)
  • July 4th: Warmest low of 81 (previous record 80 in 1999). This also tied for the all-time warmest low (most recently July 13, 1995)
  • July 4th: Average temperature of 92 (previous record 88 in 1999). Note that this is also the 3rd warmest average temperature on record
  • July 5th: Warmest low temperature of 79 (previous record 77 set in 1900)
  • July 6th: Warmest low temperature of 78 (tied previous record set in 1886)
  • July 6th: High of 103 (previous record 100 in 1989)
  Records that were broken or tied in Rochester, MN:
  • July 4th: warmest low of 75 (previous record 74 last set in 1999)
  • July 5th: warmest low of 73 (tied record set in 1982)
Temperature - Heat Index - Dew Point Traces for La Crosse and Rochester
Temperature graph
Heat Index graph
Dew point graph

 

Numerous sites across west central Wisconsin, southeast Minnesota, and northeast Iowa saw repeated days of upper 90s to low 100s highs, with heat indices rising to the 100 to 115 range every afternoon and evening. The table below highlights some high temperatures from NWS Cooperative Observer and other automated reporting stations.

  July 2nd July 3rd July 4th July 5th July 6th
Charles City, IA 93 95 95 91 97
Decorah, IA 97 98 99 99 101
Oelwein, IA 91 93 93 93 95
Osage, IA 94 94 96 92 98 
Austin, MN 95 95 97 93 99
Preston, MN 97 96 99 97 101
Winona Airport, MN 99 97 100 95 102
Winona Dam 5A, MN 96 96 99 94 100
Boscobel, WI 102 99 103 102 105
Lancaster, WI 96 96 98 99  99
Medford, WI 90 88 91 90 91
Necedah, WI 99 98 102 101 102
Neillsville, WI 95 92 97 94 97
Owen, WI 92 87 93 90 92
Prairie du Chien 97 97 100 99 102
Richland Center, WI 100 97 102 102 103
Sparta, WI 97 97 100 97 100

 


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