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Heat and humidity stifle the Midwest...on the first day of fall

Key Points
  • Temperatures and humidity are unusually high in parts of the Midwest.
  • A high pressure ridge is allowing more heat northward.
  • The dew point is at a record high for this time of year in Minneapolis.
A file photo showing joggers on the Stone Arch Bridge in Minneapolis, Minnesota.
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It is the first day of fall, but it feels like the middle of summer in Minneapolis.

Temperatures in southern Minnesota and western Wisconsin hit more than 20 degrees above average Friday.

A temperature of 93 degrees was recorded at Milwaukee's Mitchell Airport — a record for this time of year.

The reason: A long ridge of high pressure in the atmosphere over much of the central and Eastern United States, National Weather Service meteorologist Tom Hultquist told CNBC.

"It is allowing this heat that would normally stay south come up here," he said.

This ridge is caused by a fluctuation in the fast band of high altitude winds known as the polar jet stream. Jet streams do not follow a straight pattern as they blast their way around Earth. They actually wiggle and wobble, developing bends in places as they encircle planet.

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The polar jet stream is so called because it rings the polar north latitudes, and forms a kind of boundary between the colder air in the north and the warmer air at the mid-latitudes.

If it develops a southward bulge, more cold air will travel further south — that is a trough. If it developed a northward bulge, it will allow warmer air to blow north — that is a ridge.

Just as there is a high pressure ridge over the Midwest and eastern states, there is a trough further west, which has brought some snow, Hultquist said.

Though the temperatures were in the low 90s in Minnesota and Milwaukee, the National Weather Service issued heat advisories referring to expected heat indices of 95 to 100.

This is a measure of how hot people are likely to feel, given the mix of heat and humidity.

The dew point is an exceptionally high 73 degrees today, which is unusual as well, Hultquist said. Typicall,y by this time of year, the biggest sources of moisture in the air in the area — trees and vegetation – have stopped giving off moisture and are beginning to dry out.

So the combination of the two factors is making the area exceptionally hot and humid.

"It is certainly not unprecedented, because we do set records, but it is unusual," Hultquist said.

And it is expected to continue tomorrow.

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