Pattern Change to Bring Heat Relief to Midwest, Plains, East

Chris Dolce
Published: September 6, 2015

Hot temperatures have dominated parts of the Midwest, Plains and East during the first week of September, with highs topping out well into the 80s and 90s at times. While some might be enjoying this late-summer heat and humidity, others are probably ready for the air to have more of a fall feel east of the Rockies. For those in the latter camp, we do have some good news on the horizon thanks to a rearrangement of the jet stream pattern.
(MORE: Where Snow Falls in September)
The jet stream is a ribbon of fast winds aloft in the upper atmosphere that helps dictate the weather conditions we see near the surface of the earth. Given that the fall equinox is just over two weeks away, we know that weather conditions are becoming much more changeable as the jet stream strengthens and amplifies, resulting in more movement north and south.

Late Week Weather Pattern

Since early last week, the jet stream has been well to the north in central and eastern Canada, resulting in the widespread above-average warmth we've been seeing lately in the Midwest, Plains and East. During this coming week, the jet stream pattern will flip.
For the Midwest and parts of the East, the jet stream will dip south, allowing temperatures to drop to near-average or even below-average levels as the week progresses. In fact, some cities in the Midwest may see high temperatures drop 20 degrees or more from Sunday into late this week.
Meanwhile, the jet stream will build north along the West Coast, sending tempeatures well above average all the way into the Pacific Northwest.
(MORE: Northwest See Summer's Weather Extremes)
What does all of this mean for your forecast? We have those details below.

Cooling Down in the Midwest, Plains and East

Monday's Highs

Friday's Highs

Forecast Highs Next Five Days

The cooler air will come in two waves this week.
Through Wednesday, an initial cold front will drop temperatures to near or even below average across much of the Plains and Midwest. That cold front will also bring rain and thunderstorms to those regions.
(MORE: Severe Storms, Flash Flooding Possible) 
During the second half of the week, another surge of cool air will move into the aforementioned regions as an area of high pressure dives south from Canada. Much of the Midwest, Plains and Great Lakes may see highs 5-15 degrees below average by Friday. The cooler air will take the longest to reach the East Coast, perhaps not until next weekend.
Here are a few examples of the temperature changes we will see this week. In general, most of these cities have been above average to start September, but can expect near or even below-average temperatures in the week ahead.

• Chicago: Monday will be the seventh straight day with highs in the upper 80s or low 90s. Cooler air arrives Tuesday-Thursday with highs in the 70s, and then possibly not getting out of the 60s Friday into next Saturday.
• Detroit and Cincinnati: Highs in the upper 80s to low 90s are forecast to continue through Tuesday. Expect highs in the 70s beginning Wednesday or Thursday into next weekend.
• Oklahoma City: Highs in the middle 90s continue through Tuesday, then dropping into the 80s starting Wednesday. Temperatures may then hold in the 70s Saturday.
• St. Louis: Highs in the 90s through Tuesday will give way to 80s on Wednesday, and possibly 70s by Friday.
• Northeast Region: As mentioned before, the cooler air will take the longest to reach the East Coast. Above-average warmth will persist much of this week, with several daily record highs threatened in the region Tuesday. Some heat relief will potentially arrive Friday into next weekend. The most significant heat relief will be found in the interior Northeast, where highs may plummet below average starting Thursday.
• Northeast Forecast: Buffalo | New York City | Philadelphia | Pittsburgh | Washington, D.C.

(MORE: Forecast Weather Maps)

West Coast Heats Back Up

High pressure aloft will build along the West Coast as the jet stream surges northward into Canada in response to its southward dip east of the Rockies. This will allow highs to rise 5-15 degrees above average in parts of California, Nevada, Oregon, Idaho and Washington state as the week progresses.
Triple-digit heat is expected this week in California's central valley, including Sacramento, Fresno and Redding. In fact, some daily record high temperatures may be threatened late this week.
The building area of high pressure in combination with a weak offshore wind flow will also bring hot temperatures to coastal portions of California. Los Angeles (downtown) may top out in the upper 80s or low 90s this week, while inland valley locations like Ontario and Riverside soar into the 100s.
In the Pacific Northwest, Portland, Oregon, will make a run at 90 degrees late in the week. The city has already seen a record number of 90-degree days this summer.

MORE: Most Extreme Temperatures in Each State