The Mid July 2011 Central US Heat Wave
Left: surface temperatures analyzed at 2pm CDT on July 17, 2011. Right: surface dew points analyzed at 2pm on July 17, 2011. The combination of high moisture levels and hot temperatures produced a large heat wave across the central United States in mid July 2011.
This web page will continue to be updated with additional information over the next several days as we compile some more statistics about the heat wave that impacted the area in mid July.
On Sunday, July 17th, heat index values rose to between 100 and 115 degrees across much of the NWS Duluth area of responsibility. Temperatures were in the upper 80s to low 90s, while dew points were unusually high - mostly in the upper 70s and low 80s. The combination of hot air temperatures and large amounts of low level moisture led to the dangerously high heat index values. The highest heat index recorded in our area on the 17th was 118 degrees, just northeast of Hayward, Wisconsin in the town of Seeley. The maximum heat index that day at the Duluth airport was 98 degrees, and the highest dew point observed was 77 degrees.
Hourly observations were available to search for Duluth back to 1979. The highest dew point observed at Duluth since 1979 was 78 degrees, which occurred on August 24, 2003. In fact, a dew point of 77 or 78 degrees has only been reported 6 times in the last 32 years at Duluth prior to July 17, 2011 (on four different days). Note that dew point values are not officially tracked, so this is an unofficial record.
|Highest Dew Points at Duluth since 1979|
|8/24/2003||2pm, 3pm||78 F|
|7/17/2011||4pm, 5pm, 6pm, 7pm||77 F|
|6/22/1983||5pm, 6pm||77 F|
Above is a table of the highest dew points on hourly observations at the Duluth airport since 1979. The 77 degree dew point was the 2nd highest recorded at Duluth since 1979.
On July 18th, a weak area of high pressure in southern Canada pushed a slightly drier air mass into the area from the north. Dew points across extreme northern Minnesota fell into the 50s during the afternoon, while temperatures soared into the lower 90s. Therefore, it was still a hot day in parts of the Arrowhead, and north of the Iron Range, but heat index values did not reach dangerous levels. Further to the south, low-level moisture persisted longer, and thus heat index values rose above 100 degrees once again, particularly along and south of U.S. Highway 2. The highest heat index values were around 115 near the St. Croix River in Burnett County, and near some of the inland lakes of Crow Wing County, Minnesota.
On July 19th, persistent thunderstorm activity and cloud cover suppressed a warm front mainly south of the area. It was a cooler day across northeast Minnesota and northwest Wisconsin. Some severe weather occurred across the area as well, mostly in the form of damaging winds.
On July 20th, hot and humid conditions returned to the area once again as the warm front pushed north into Ontario. High temperatures were generally in the upper 80s to low 90s, with dew points rising into the mid 70s again at many locations. The result was that most locations away from Lake Superior experienced heat index values peaking between 100 and 110 degrees. The maximum heat index at the Duluth Airport was 100 degrees. As the chart below shows, that is not a common occurrence at Duluth. Only five other days in at least the past 32 years have experienced a heat index of 100 degrees or more. Note that heat index values are not officially tracked, so this is an unofficial record.
|Triple Digit Heat Indices at Duluth since 1979|
|Date||Hour(s) with HI at least 100
||Max Heat Index
|7/7/1988||5 hours||103 F|
|8/5/2001||5 hours||102 F|
|7/3/1990||1 hour||102 F|
|8/16/1988||1 hour||101 F|
|7/20/2011||2 hours||100 F|
|7/6/1988||1 hour||100 F|
13 of the 18 counties in the NWS Duluth area of responsibility had at least one observation with a heat index of 110 degrees or higher during the heat wave, which lasted from July 16-20, 2011. Additionally, over half of the observation sites we examined (61/111, 55%) recorded at least one instance of a dew point of at least 80 degrees.
We have created maps using GIS software of the highest observed heat indices and dew points across our area. The creation of the map may have smoothed out some isolated extremes, but the overall pattern is captured.
Not all the stations used in creating the map are maintained by the NWS, and therefore their absolute data quality cannot be guaranteed. However, all of the observations were quality controlled to eliminate values that were erroneously high or low before the analysis was done. A listing of the data, along with the source of the observations, is available below in a PDF format.
|Download a PDF table of the highest heat index values and dew points from across our area during the duration of the heat wave (click here) - 298KB|
Below is an animation of approximate high temperatures, as forecast by the NAM model, during the heat wave from the NOAA Environmental Visualization Laboratory:
What Is The "Heat Index"?
National Weather Service heat-related products are based on the heat index. The heat index is basically a measure of how hot it really feels, when factoring relative humidity into the air temperature. For the same given temperature, more humid conditions will result in a higher heat index value. The heat index is sometimes referred to by meteorologists as the "apparent temperature".
It should be noted that heat index relationships were devised for shady conditions with light winds. Exposure to sunlight can make it feel another 15 degrees hotter than the reported heat index value. The risk of heat related illnesses increases quickly for heat index values above 100 degrees, although long stretches with slightly lower values can have similar effects.
Other NWS Office Web Pages On The Heat Wave