July 1-7, 2012 Wisconsin Heat Wave

Wisconsin experienced a major heat wave during the first 7 days of July 2012, peaking July 4-6^th.  There were several heat-related fatalities during this event and mostly likely several hundred people needed medical treatment.   As of Monday, July 23rd, there were 10 confirmed heat-related fatalities: two in Barron County, two in La Crosse County, and one in each of the following counties: Dane, Juneau, Kewaunee, Richland, Rock County, andVernon.  Additionally, there are other fatalities are under investigation to see if they are heat-related. 

The July 2012 heat wave was roughly as hot as the killer July 1995 heat wave, but less humid and longer in duration. Maximum air temperatures in the July 2012 heat wave ranged from the upper 90s to 106 degrees. However, it was cooler near Lake Michigan and Lake Superior and across parts of northern one-third of Wisconsin. Maximum heat indices (apparent temperature) peaked in the 100 to 115 range thanks to dewpoints reaching the mid-60s to mid-70s.

Dry soils across southern Wisconsin allowed for deeper mixing of the lower atmosphere due to daytime heating which brought drier down to the ground.  Consequently, relative humidities and heat indices during the July 1-7 heat wave were lower than those experienced in the July 1995 heat wave.  Information on the July, 1995 killer heat wave can be found here.

Numerous daily record maximum temperatures were tied or broken during the heat wave, as well as new record high minimum temperatures.  Details on these records are found toward the bottom of this summary.

• Fatalities (10 confirmed heat-related and possibly others under investigation) & possibly several hundred injuries (sought medical treatment)
• Buckled and cracked road surfaces
• Isolated bird & fish deaths, and possibly some pet and livestock/animal losses (assumption based on history)
• Maximum air temperatures – 100 to 106.
• Maximum heat indices of 100 to 115 (they were 118 to 128 in July 1995).
• Dew-points in the mid 60s to mid 70s (they were in the upper 70s to lower 80s in July 1995).
• Stuffy overnight minimum temperatures in the 70s to lower 80s.
• Little or no rain.
• Unusual long duration (7 days), not to mention a couple hot days centered on June 28th.
• Drought conditions intensified across southern Wisconsin to moderate levels.
• A few minor wildfire-grass-tree burns in southern counties.
• Many cities and Civil Townships banned outdoor burning and fireworks.
• Crops, trees, shrubs, lawns showing water & heat stress.
• Some ponds, marshes, and small streams have dried up (water table probably lowering?)

The heat wave was generated by a large, warm, high pressure in the mid to upper levels of the atmosphere stretching from the Kansas area east to the Ohio River Valley. Sinking air in the high pressure warmed and formed a “cap” that resulted in more sunshine than usual and hot temperatures.

The Midwest Picture

In the first row below are 4 images for the Midwest for July 1-8th, left to right, showing the average temperature, the temperature departure, precipitation, and the percentage of normal precipitation.  These images were generated by the Midwest Regional Climate Center (MRCC).  Note the greatest temperature and precipitation anomalies are located from northeastern Iowa and southeastern Minnesota across southern Wisconsin!  Click on thumbnail for larger version.

Below are images showing maximum air temperature contours across the Midwest for July 1-8th.  Click on thumbnail for larger version.

Wisconsin - Maximum Air Temperatures

Below are daily maximum air temperatures at selected cities across the state, followed by the locations that had the highest daily temperature through the period.  Note:  "AP" is an abbreviation for "Airport." 

Below are images showing daily maximum temperatures across Wisconsin during the heat wave.  Click on thumbnail for larger version.

Wisconsin - Maximum Heat Indices (Apparent Temperature)

Below are images showing daily maximum heat indices (apparent temperatures) across Wisconsin during the heat wave.  Click on thumbnail for larger version.

Milwaukee and Madison Hourly Heat Indices (Apparent Temperature)

Below are two line graphs depicting the hourly heat indices at the airports in Milwaukee and Madison.  Note the rapid rise in the mid-morning hours, and in some cases, a slight increase in the 7 to 8 pm CDT timeframe once the boundary layer of the atmosphere "decouples" from the rest of the atmosphere.  When this happens a low-level inversion develops and the surface winds decrease as atmospheric mixing ceases.  Mixing brings drier air down to the ground - when this stops, relative humidity and heat indices increase accordingly.

Southcentral and Southeast Wisconsin - Temperature Records

The tables below show observed and record high temperatures and record high mininum temperatures for July 1-7 in Milwaukee and Madison. 

Previous to this year, the last time high temperatures reached 100 degrees or higher in Milwaukee was July 14, 1995 and in Madison, July 13, 1995.

The tables below show the top ten hottest days on record in Milwaukee (records go back to 1871) and Madison (records go back to 1869). Milwaukee and Madison both had several days make the list during this heat wave.

Madison tied for 3rd for the most consecutive days of high temperatures greater than or equal to 90 degrees (table on left) and tied for 2nd for most consecutive days of high temperatures greater than or equal to 95 degrees (table on right). Not to mention, Milwaukee and Madison both tied for the 2nd most consecutive days with high temperatures of 100 degrees or higher, with Milwaukee having two consecutive days and Madison, three.

Central and Northeast Wisconsin - Temperature Records

Westcentral Wisconsin - Temperature Records

MEB/MBK/PC/ET/JJW/JCW/REM/MG/AFK
NWS Milwaukee/Sullivan