Mention "Mid-July heat wave" to any meteorologist, and you'll get their attention, just as "September hurricane", "January blizzard", or "spring severe outbreak" would.
Exacerbated by a massive, exceptional drought, daily, monthly, even all-time record highs have been tied or broken in parts of the South over the past couple of months.
Read articles: Heat records | NOAA: June records
Climatologically speaking (meaning, "on average"), for most areas east of the Rockies, we've either just entered, or are about to enter the hottest time of the year, the latter half of July.
Right on cue, a dangerous heat wave will persist and spread this week. Let's lay out the atmospheric formula for this.
Suppressing storms, maximizing sunshine
A summer heat wave starts with a massive dome of high pressure aloft, what meteorologists call a ridge, strengthening and building over an area.
The relatively warm-air aloft in an upper-level ridge effectively suppresses most (but not all) thunderstorm development, and also diverts the jet stream well to the north, effectively blocking any cold fronts from penetrating southward to provide relief.
Now, the massive ridge is camping out over the Central Plains and Lower Missouri Valley, as you can see in the graphic below.
 | | Upper-level ridge position early this week
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Later this week, we're expecting the center of the upper ridge to shift east, spreading increasing heat to the East Coast. This will also help lower temperatures in the Upper Midwest and Northern Plains.
 | | Upper ridge shifting east late this week
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Of course, it wouldn't be a summer heat wave without oppressive humidity. Sweating these details
As with most heat waves, it's the air's moisture content that tips the scales into unbearable territory.
Below is a model forecast map of dew points in the Upper Midwest Tuesday. The areas shaded in red may see dew points near or above 76°, while areas shaded in red may see dew points near or above 76°. This is the type of oppressive air mass you'll experience frequently along parts of the Gulf Coast in mid-summer. See map: Current dew points  | | Model forecast dew points Tuesday afternoon
Image: Wright-weather.com
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You can thank a combination of recent rainfall providing water for evaporation and a process called plant transpiration.
Met 101: Corn Belt...Unbearable Humidity
When your body temperature rises on a hot day, as much as two liters of sweat can pour out of roughly 10 million sweat glands each hour! With dew points this high, less perspiration is able to evaporate from your skin into the air. Since evaporation is a cooling process, your body is less effective at keeping itself cool. Abundant mid-July sunshine (still rather high up in the sky, given we're not far removed from the summer solstice) enhancing "evapotranspiration" from the surface. Let's now delve into the forecast particulars regarding this uncomfortable scenario.
Next: Heat wave forecast details |
