Published: September 7, 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)
For the Midwest and parts of the East, temperatures will drop to near-average or even below-average levels as the week progresses. In fact, some cities in the Midwest may see high temperatures fall 20 degrees or more from early week into mid or late week. Even more impressive is the temperature drop from highs early this week to lows later in the week. For example, Chicago had a high of 92 degrees on Sunday but will see lows in the 50s late in the week, a drop of more than 30 degrees.
Before the cooler air arrives in the Northeast, daily record high temperatures will be threatened in multiple locations, including New York City and Boston on Tuesday.
Meanwhile, the West Coast will see the opposite impact from this pattern change with temperatures soaring 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 and the meteorology behind this pattern change 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. Parts of the Upper 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 late week.
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 was 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 Saturday. Lows in the 50s will arrive Thursday morning and continue through the weekend.
- 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. 60s are possible in Detroit on Saturday. Lows in the 50s are expected late in the week.
- Oklahoma City: Highs in the middle 90s continue through Tuesday, then dropping into the 80s starting Wednesday. Lows may dive into the lower 60s or upper 50s late in the week and into the weekend.
- St. Louis: Highs in the 90s through Tuesday will give way to 80s on Wednesday, and possibly 70s by Friday or Saturday. Lows may drop into the lower 60s or upper 50s late in the week.
- Northeast Region: As mentioned before, the cooler air will take the longest to reach the East Coast. Above-average warmth will persist through Thursday in parts of the region, with several daily record highs threatened early in the week. Some heat relief will potentially arrive Friday into next weekend with highs closer to average.
- Northeast Record Highs Possible Tuesday (current record in parentheses): New York City (93 degrees); Newark (94 degrees); Boston (95 degrees)
- Northeast Forecast: Buffalo | New York City | Philadelphia | Pittsburgh | Washington, D.C.
(MORE: Forecast Weather Maps)
West Coast Heats Back Up
Forecast Highs
As the East cools down, highs will rise 5-20 degrees above average in parts of California, Nevada, Utah, 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.
Meteorology 101: Science Behind the Pattern Change
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.Since early last week, the jet stream east of the Rockies 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. Meanwhile, the jet stream dipped south in the West, leading to cooler temperatures and even some snow in the highest elevations of the Cascades and northern Rockies late last week.
During this coming week, the jet stream pattern will flip.
A southward dip in the jet stream will bring much cooler air to the central and eastern states. Meanwhile, the West will heat up as the jet stream lifts north over that region.
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