The Upper Midwest Heat Wave of July 2012

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 on July 2nd for portions of the region, with Excessive Heat Warnings covering much of the Upper Mississippi River Valley from the 3rd through the 6th.

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

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