My preference is to use a combinatorial tool that uses temperature and dew point. It looks like the runners connect calculator also takes this into account, but its adjustment is much larger than the one I use:
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Add Temperature and Dew Point together (T+D). Adjust pace based on the % listed underneath the T+D range. This adjustment is based on the average person's experience. Being heat acclimated, where you train normally vs where you're racing (in terms of T+D) can influence these numbers. When I'm home in WI, I generally don't need an adjustment when in the middle of summer at less than a T+D of 135. At the beginning of the summer (when not heat acclimated), I need adjustments starting at 100.
For example, if your ideal temperature current fitness HM pace was an 8:50 min/mile.
It was a Temperature of 75 with a Humidity of 70% (60 Dew point, use this calc (
LINK)), then the T+D is 135.
The adjusted 8:50 pace would be a 9:03 min/mile.
I always advise trying to start slower than this pace and work your way up to it. See how the effort feels in relation to similarly paced runs under ideal conditions. The cautious approach almost always wins out in cases of high heat.
Word of advice to most, with the race being two weeks out, and most starting their taper, this is the perfect time to start heat acclimating if you aren't training in a T+D of 135+. Pay less attention to the pace of your runs (read, go slower) and overdress for your current conditions. Here in WI, it's less than 30 degrees today. If I were preparing to run DS, I would start wearing double leggings, double tops, jackets, heavy gloves, hats, etc. All in an effort to force my body to adapt to hotter/humid conditions. Create a micro-climate around your body to force it to adapt to thinning the blood and increasing sweat production. Just be mindful that if you do decide to heat acclimate like this - cut your cool down way down (because you'll be sweating and could get the chills) and make sure you drink plenty of water/electrolytes since you'll be sweating more than a normal cold day. Research shows it takes about 14 days or 10 workouts to be acclimated to a hotter condition. So best to start now, then waiting to start with a few days prior to the event. I've used this strategy for several WI winter (see -50 degree wind chill) to Dopey Challenge races (as high as T+D 143), and have done quite well being heat acclimated.