The heat index and wildfire forecast

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Next Friday morning, June 21, the official start of summer begins in the Northern Hemisphere. However, we’ve already seen very hot temperatures across parts of the country, especially in the Southeast where highs recently topped the 100-degree mark and humidity levels made heat index readings over 120 degrees in places.

The feels-like temperatures are also known as a “heat index” which 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 percent relative humidity level would push the heat index up to a very dangerous 122 degrees Fahrenheit.

The region of the country with the highest levels of heat and humidity are east of the Rockies, especially in the Deep South and the Southeast. Warm and humid air will often stream in from the Gulf of Mexico making conditions very uncomfortable, most often in July and August.

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 close to 30,000 Americans have died during big heatwaves, mainly in the desert Southwest and regions east of the Rockies, particularly the deep South and the Southeast.

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 percent, hence only a slight rise in the overall heat index.

However, the Inland Northwest will have days with temperatures in the 90s. Although, the heat and humidity levels are much lower in our part of the country when compared to areas east of the Rockies, one could get heat stroke in very hot weather.

Cliff and I see warmer and drier weather conditions across the Inland Northwest this upcoming summer season. By late in the week, we may be flirting with our first 90-degree day of the season. With the drier conditions expected, many are wondering if we’re going have another tough fire season.

On June 1, the National Interagency Fire Center updated its three-month outlook for the chances of wildfires across the country. It continues to show above average chances for significant wildfires in parts of the far West during this summer.

The updated forecast shows the southern two-thirds of Idaho expected to have a relatively “normal” fire season through September. However, during the summer, the chances for wildfires are high across southern Arizona, southern Nevada, extreme southwestern Utah, much of the western portions of California, northwestern Oregon, western and the northeastern portion of Washington and North Idaho. Coeur d’Alene is on the southern edge of the region that has an above-normal chance for significant wildfires.

Officials are concerned that the “abundant winter and spring moisture should translate to a heavy crop of fine fuels that will become increasingly receptive to fire activity. California has already had two back-to-back massive fire seasons and officials are concerned for another one in 2019. The above-normal precipitation this year will undoubtedly lead to more brush and fuel for fires.

Last month, many stations in California had record rainfall, which led to more growth of brush and other plants. Now, temperatures in the Central Valley of the Golden State are much warmer as readings have already hit the 100 degree mark. If conditions continue to be warmer than usual, then it’s possible that California may have another rough season.

In the meantime, thanks to the above-normal moisture totals in the far West and the rest of the U.S., the 2019 fire season has been off to a slow start. Since Jan. 1, there have been a total of just under 16,000 fires. Last year and in 2017, there were nearly 25,000 fires through early June. In terms of acres, there have been slightly less than 350,000 acres burned so far this year. But in 2018, there were nearly 1.8 million acres that were charred and close to 2.3 million acres that were hit by wildfires in 2017.

In terms of our local weather, May’s precipitation finished at an above normal 2.83 inches. The normal for last month is 2.37 inches. Despite moisture totals above average, we only had 8 days last month with measurable rainfall.

For June, our normal rainfall is 1.93 inches. We’re off to a good start as Cliff has measured .82 inches over the first 9 days. We’re expecting conditions to turn drier than normal across the Inland Empire toward the middle to the end of the month.

Our summer season is expected to be dry, but we don’t think it will be quite as dry as the previous two summers that helped create the disastrous wildfires. However, with the possibility of some isolated thunderstorms, there’s always the chance of dry lightning. Hopefully, we won’t see that kind of pattern later this summer.

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Contact Randy Mann at randy@longrangeweather.com

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