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Heat index and big heatwaves

by RANDY MANN / Contributing Writer
| July 10, 2024 1:00 AM

Several communities in the region had their first 90-degree temperature last week as we’re now in a period of extended excessive heat and dryness. 

The intense heat that has been experienced in California and the desert Southwest will be bringing very hot weather to the Inland Northwest this week. High temperatures in Coeur d’Alene and surrounding regions will likely be at or above the 100-degree mark over the next few days before we have some slight cooling around the end of the week. The long-range computer models are indicating some cooling next week, but more hot weather may be felt toward the end of the month.

Unless there are a few thunderstorms that will produce some heavy rainfall, the rest of July is expected to be drier than normal, so we’ll likely end up below the .92-inch normal. Early August should also start out dry with very warm to hot temperatures. Later next month, however, it looks like we’ll see some increase in moisture along with cooler temperatures as we start to see the effects of the “La Nada” sea-surface temperature event.

The western U.S. has been in the grips of one of the most intense heatwaves in recorded history. The Central Valley of California has reported high temperatures of at least 110 degrees. Since July 1, readings have been over the 100-degree mark in many valley locations, including Sacramento. In fact, Sacramento averages about 23 days per year with highs near or above 100 degrees. The record for the number of days in a row of 105-degree plus afternoons was seven back in July 2013. There have been four days in a row with highs over 105 degrees with a record-breaking 110 degrees July 5 in Downtown Sacramento.

Last Friday, about half of the U.S. was under a heat advisory or an excessive heat warning. Medford, Ore., was 109 degrees July 5. Since July 3, Death Valley has reported high temperatures over 120 degrees with a blistering 125 degrees July 4. It’s possible that this torrid desert region could come near the 130-degree mark later this week. Death Valley also holds the world record for the hottest temperature. On July 10, 1913, the thermometer reached an incredible 134.1 degrees Fahrenheit at the Furnace Creek Ranch.

With the intense heat and dryness, wildfires have been breaking out across California. As of early July, approximately 150,000 acres have been burned, compared to around 8,000 acres at this time last year.

As temperatures get hot, any additional humidity in the air can make it feel even hotter. The feels like summer temperatures are also known as a “heat index.” This was developed by R.I. Steadman of the National Weather Service back in 1979. The heat index is calculated by combining air temperature and relative humidity levels. For example, a 90-degree temperature combined with a 90% relative humidity level would push the heat index up to a very dangerous 122 degrees Fahrenheit. In the southwestern U.S., a 120-degree temperature often feels more like a dry heat as humidity levels are very low.

The region of the country with the highest levels of heat and humidity is east of the Rockies, especially in the southern portions of the country. Depending on the age and health of individuals, prolonged exposure to heat index temperatures over 100 degrees can lead to sunstroke, heat exhaustion, muscle cramps and even heart attacks. Since the 1930s, it’s estimated that at least 30,000 Americans have died during big heatwaves.

Here in North Idaho, most of our very hot days typically have lower humidity values. Thanks to the Rocky Mountains, the humid air from the Gulf of Mexico is blocked from entering the Far West. Although we may receive some triple‑digit readings, the relative humidity levels are generally less than 20%, hence only a slight rise in the overall heat index.

One of the most severe and deadliest heatwaves to strike the U.S. occurred in 1936 during the Great Depression and in the middle of the infamous “Dust Bowl Days.” Data from Wikipedia states that many of the high-temperature records that were set back in 1936 stood until the big heatwave in 2012. A reading of an incredible 121 degrees was reported in Steele, N.D., a record that still stands today. Highs of over 110 degrees were felt in Ontario and Manitoba in Canada.

On Aug. 4, 1961, Coeur d’Alene hit 109 degrees, our all-time record high. It was 108 degrees in Spokane on that date. However, on June 29, 2019, the Spokane International Airport had its hottest day ever recorded with a blistering high of 109 degrees.

During that intense heatwave in June 2021, the Coeur d’Alene Airport in Hayden also topped out at 109 degrees, which tied its all-time record for the highest temperature. It was even hotter at North Idaho College as the thermometer hit 110 degrees June 29. Kellogg, Worley and Rathdrum each reported 108 degrees. Hayden and Athol hit 107 while Post Falls went to 106 degrees June 29.


Contact Randy Mann at randy@longrangeweather.com.