Heat advisories likely next few days

Published 7:00 am Saturday, July 11, 2020

By Skip Rigney

The heat is on. The next few days are forecast to be the hottest so far this summer. Afternoon temperatures are expected to reach the middle to upper 90s. Add the effect of high humidity, and we have the recipe for uncomfortable and even dangerous, sweltering conditions.
We have approached this level of heat only twice before this summer in Pearl River County, first on June 15th and again during the last few days of June. The next few days look to be noticeably hotter than those previous events.
Soil moisture across the region is relatively high. As the ground bakes, some of that moisture will evaporate into the air, adding to our already muggy atmosphere. The more water vapor in the air, the less efficiently our perspiration can evaporate from our skin and cool us off.
That difficulty of cooling off in muggy air is the basis for the so-called “heat index,” “apparent temperature,” or “feels-like temperature.” Those are different names for an attempt to model the way our bodies perceive muggy, warm conditions as hotter than dry air with the same actual temperature. From mid-morning through early evening today and Sunday the mugginess will make it feel 10 to 15 degrees warmer than the actual thermometer reading.
One way to think about the heat index is to contrast our situation with places where there’s so little humidity that there is no difference between the actual temperature and the heat index.
The amount of water vapor in the air is the same today, as measured by a dew point temperature of about 40 degrees, in Logan, Utah, and in White Sands, New Mexico. Folks outside in those places will be cooled off about as efficiently as is possible by the evaporation of perspiration from their skin. However, it will be hotter in White Sands where the actual temperature is forecast to hit 107 compared to 97 in Logan.
Meanwhile in Picayune, the muggy air will be marked by dew point temperatures in the middle to upper 70s, which is enough to make it feel about 10 to 15 degrees warmer than if the dew point was in the 40s. So, when the actual temperature reaches 97 degrees in Picayune today, it will feel more like the 107 degrees of White Sands than it will the 97 in Logan.
At least we’re not in central Arizona where new daily records are expected to be set in Phoenix with highs climbing above 115 degrees. Low temperatures in that city may not cool below the lower 90s the next few nights. That’s hot, even if it is a dry heat.
The excessive heat is associated with a strong dome of high pressure located several miles above ground covering the southwestern United States. The high is predicted to drift toward the eastern half of the country over the next week.
In contrast with the relatively active pattern we saw the first eight days of July, the big upper high pressure system will squash most showers that try to form the next few days. However, sometimes strong clusters of thunderstorms can move down the eastern side of these big summertime, continental high pressure systems, which accounts for a continued 20 to 30 percent chance of afternoon showers and thunderstorms in the forecast through Tuesday.

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