We will see our first excessively hot and humid stretch of the season this weekend, as temperatures are likely to reach well into the 90s with heat indices pushing above 100° at times. Just how high the air temperature climbs will have a number of factors that play into it.
One of those factors is called atmospheric mixing. The atmosphere is a three-dimensional construct, with differing air masses East/West and North/South, but also near and above the surface. Mixing occurs when dry air aloft drops down to the surface, often dropping surface moisture content a little as the drier air displaces the moist air. This can be limited by an atmospheric cap, the same feature which also acts to prevent thunderstorm activity in other scenarios.
If we see more of this mixing occur, that could allow for dew points to fall a bit. That drier air heats up faster in the daytime heating, however. This could allow temperatures to end up in the mid to upper 90s. While the air temperature will be higher, the drop in dew point will mean a slightly lower heat index.
Conversely, if we see less mixing, surface dew points may remain in the mid-70s. This would keep temperature a little lower due to the higher moisture content, but heat index values would be higher within the higher humidity. As of now, the better chance for more mixing and higher temperatures appears to be Sunday and Monday, while dew points Saturday remain higher as temperatures stay a touch lower.
Either way, these changes in mixing vs no mixing only nudge temperatures up or down a couple degrees. The main takeaway is that it will be very hot and humid for an extended stretch this weekend and early next week. We will not see much relief overnight, as overnight lows only reach the low-mid 70s for the next several nights. Be sure to take heat safety precautions if you plan to be outdoors this weekend.
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