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Later this afternoon, the dew point will drop and we’ll get a little bit of break in the humidity that’s been around for the past couple of days. There might be a few showers south of Boston and it won’t be hot. But this is just a short reprieve before the big heat arrives this weekend.

Drier air filters south later Thursday. (COD Weather)

It looks as though Boston should have an official heat wave with temperatures at 90 degrees on Friday, then approaching and perhaps reaching 100 degrees on Saturday and still in the 90s or even 100 again on Sunday.

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The heat has been building across the center of the country, where heat advisories and warnings are posted. There’s over a 150 million Americans under some sort of heat alert today, and that number will remain high through the weekend. Presently, there is an excessive heat watch for southern New England.

Much of the eastern half of the country is experiencing high heat.
Much of the eastern half of the country is experiencing high heat. NOAA Data

Saturday (and likely Sunday) is going to be one of those days where it’s dangerous to be outside doing activities. That doesn’t mean when you walk outside your front door, you’re going to vaporize in 2 minutes. However, heat exhaustion and heat stroke are real things and can lead to serious medical complications. Be smart about how long you stay outside and stay hydrated. Park your car in the shade and don’t leave any living thing in the car for any amount of time. There’s not going to be much of a breeze on Saturday, and even the coastline is going to be hot.

You’ll hear a lot about the heat index the next few days. The heat index combines the humidity and the actual temperature and gives us a number of how the air will feel. If you’re wondering, it’s calculated in the shade, not in the sun — it can feel even hotter if you’re in direct sunlight. The moisture in the air is going to be about as high as we experience in any summer.

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The heat index for Saturday afternoon will exceed 100 degrees for several hours.
The heat index for Saturday afternoon will exceed 100 degrees for several hours.WeatherBell

Friday, the heat index will get into the mid and upper 90s and then almost assuredly go over 100 for Saturday. This index will remain nearly as high on Sunday, although the actual temperature should be off the peak of the previous day.

If Boston does reach 100 degrees, it’ll be the 26th time since records have been kept going back to 1872. The last time Logan airport reach 100 was on July 22, 2011. The record high for Friday is 99 and it’s the same for Saturday. It’s likely that we tie or just break the record on Saturday. If it doesn’t hit 100, it’s still hot. The thermometer reaching 100 versus 99 is just 1 degree. Sunday’s record is 102 and we won’t be that hot, with temperatures in the upper 90s to around 100 degrees again.

Boston has reached 100 degrees 25 previous times.
Boston has reached 100 degrees 25 previous times.NOAA Data

The reason for all the heat is a strong area of high pressure off the coast. This is very typical for summer as these highs like to develop in this position. As the high get strong, it pumps a flow of warm and moist air and gives us our biggest stretches of heat.

High pressure is bringing hot and humid air to much of the country.
High pressure is bringing hot and humid air to much of the country.Tropical Tidbits

High pressure is bringing hot and humid air to much of the country. (Tropical Tidbits)

The high will start to break down early next week and temperatures will return to more seasonable levels. It’s highly likely when we look back at the summer, this weekend will end up the hottest on average.

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Cooler air will bring temperatures back to average for the end of July. (NOAA)

This cooling doesn’t mean there’s not more heat in our future, but it probably won’t be hotter than this weekend so take solace and have a glass of cold lemonade and find some air conditioning. It’s summah!


Follow Dave Epstein on Twitter @growingwisdom.