Another 'heat dome' may be headed our way

Late last month, many of us heard about the huge “heat dome,” a huge ridge of high pressure, that was locked in over much of the country. On Friday, July 22, the National Weather Service reported on that particular day, it was the hottest during the same period in 2012. The big ridge of high pressure that’s been causing all the tremendous heat was actually bigger than the one four years ago that brought tremendous heat and humidity to much of the U.S.

This high pressure ridge at the end of last month was so massive that it stretched from Los Angeles all the way eastward to Columbia, S.C., and northward into Canada. The heat dome was so big that it covered a whopping 29 states. On Sunday, July 24, 26 states had heat warnings and advisories as temperatures climbed into the 90s along with high humidity levels.

There has been some relief to the oppressive high heat and humidity east of the Rockies, but Cliff is talking about another round of this type of weather developing later this month. Here in the Northwest, we had a small taste of heat and humidity pattern early last week as temperatures climbed into the 80s to near 90 degrees along with the added moisture in the air that led to scattered thunderstorm activity.

The high heat and humidity in the central portions of the country in late July had “feels like” temperatures past 110 degrees. In Minnesota, there was a feels like temperature change from January to July of this year of nearly 150 degrees. Wind chills in January were around minus 35 degrees to heat index temperatures close to 110 degrees in July. Now that’s what I call extreme.

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.

One of the most deadly heatwaves in our nation’s history occurred in Chicago between July 13-15, 1995. In the southwestern part of that city on July 14, 1995, the heat index soared to near 130 degrees Fahrenheit, killing more than 500 people. The day’s high was a record 109 degrees with a relative humidity level of 80 percent.

The region of the country with the highest levels of heat and humidity are 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 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.

The western portions of the country rarely experiences the hot temperatures and high humidity levels. 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.

In terms of our local weather, the scattered shower and thunderstorm activity pushed our rainfall total for July to an above normal 1.11 inches at Cliff’s station. The average rainfall for last month is .92 inches. As of late last week, it’s almost hard to believe that our average temperature for July of 2016 was about a degree below normal levels across the region despite the hot temperatures last week. Prior to the stretch of hot days, readings were averaging about 4-9 degrees below normal from July 4-13.

It appears that the strong high pressure ridge should persist over much of the region for much of this week as moisture will be confined to around the Canadian border. However, I wouldn’t be surprised to see some scattered showers or an isolated thunderstorm develop over the mountains in the next several weeks. And, with this crazy weather pattern, one of those bands of showers or a thunderstorm may drift over the lower elevations from the north.

Cliff and I believe that there is also the chance of some showers in our region around the early to mid portion of next week, the end of the “new moon” lunar cycle, but mainly to the north and over the mountains. However, we may see an increase of rainfall toward the middle to the end of this month, especially near the “full moon” lunar cycle of Aug. 18-23, as the big high pressure system is expected to move more toward the east, putting us on the “back” side of the ridge. Stay tuned.

Contact Randy Mann at randy@longrangeweather.com