Fall Preview Shifting Toward East; Record Heat Bakes California, West

Chris Dolce
Published: September 11,2015
Much cooler and drier air has spread into parts of the Great Lakes, Midwest and Plains this week thanks to a large-scale weather pattern change. The cool air mass will push into the East and South through the weekend.
(MORE: Where Snow Falls in September)
An initial cold front dropped temperatures to near or even below average across the Plains and Midwest this past week.
Into this weekend, another surge of cool air will move into the aforementioned regions as an area of high pressure dives south from Canada. Parts of the Midwest, Plains, Ohio Valley and Great Lakes will see highs 5-15 degrees below average through Saturday. This air mass will then drop temperatures below average across the South this weekend, while also spreading into parts of the Northeast.
This pattern change is having the opposite impact on the West Coast, with temperatures soaring above average from California into the Pacific Northwest. Some record highs have already been set, including Thursday at Stockton, California, where the temperature reached 108 degrees, outpacing the previous record for the date by six degrees. More record heat is anticipated in the coming days.
Below are the forecast details from coast-to-coast.

Cooldown Forecast: Midwest, Northeast and South

Forecast Highs Next Five Days

Midwest/Plains:
Following a stretch of hot weather to start September with highs well into the 80s and 90s, temperatures have now retreated back to near-average levels in the Midwest. Through this weekend, temperatures will dive well below average across the region. For some cities, this may be the coolest air mass in months given we are now at the tail end of summer. For example, Des Moines, Iowa, Minneapolis/St. Paul and Chicago may see their low temperatures drop into the 40s this weekend for the first time since late May or early June. Patchy frost is expected near the Canadian border.
A streak of 101 consecutive days with highs above 70 degrees in Indianapolis ended Friday when the high temperature only reached 65 degrees. Cooler air continues to surge south and east behind an advancing cold front.

• Chicago: Monday was the seventh straight day with highs in the upper 80s or low 90s. Cooler air has now arrived with highs in the 60s expected through Sunday. Lows in the upper 40s or low 50s through this weekend.
• Detroit and Cincinnati: Highs in the 60s or 70s are forecast through the weekend. Lows in the 50s or upper 40s are expected this weekend into early in the week ahead.
• Oklahoma City: After a stretch of 11 consecutive days with 90s through Tuesday, highs will be in the upper 70s or low 80s through Sunday. Lows may dive into the 50s this weekend.
• St. Louis: Following nine consecutive days in the 90s through Tuesday, highs slipped into the 80s Wednesday and may only be in the 70s or upper 60s by through Sunday. Lows could drop into the 50s this weekend.

Saturday's Forecast Departures From Average Highs

Sunday's Highs Compared to Average

Northeast:
The Northeast set several daily record highs in advance of the cooldown to start the week. New York City hit 97 degrees Tuesday afternoon, beating its daily record high for Sept. 8 of 93 degrees. New York's Central Park finally snapped its record streak of 62 straight 80-degree-plus days Thursday, when the high only reached 79 degrees.
Boston set a new daily record high of 96 degrees on Tuesday and then followed that up with a daily record of 93 degrees on Wednesday. Allentown, Pennsylvania reached 90 degrees for the seventh time this month on Wednesday, which ties the record of seven September days set in 1983.

• Northeast Forecast: Highs near average to finish the week will continue to cool further this weekend. The interior Northeast and Mid-Atlantic will see highs 5-15 degrees below average on Saturday and Sunday.
• City Details: Buffalo | New York City | Philadelphia | Pittsburgh | Washington, D.C.

South:

• Southeast Forecast: Temperatures will be 5-15 degrees below average in parts of the South this weekend, with highs in the 70s and low 80s from Louisiana to the Carolinas. Lows in the 50s will move into parts of Arkansas, Tennessee, northern Louisiana, Mississippi, Alabama, Georgia and the Carolinas this weekend into early next week. Some 40s are possible in the Tennessee Valley and southern Appalachians.
• City Details: Atlanta | Charlotte | Dallas | Nashville 

(MORE: Forecast Weather Maps)

West Coast Heats Back Up

Forecast Highs

As the East cools down, record heat is now searing parts of California, Nevada, Idaho, Oregon and Washington yet again.
(MORE: Record Hot Summer in Northwest)
Tuesday, San Francisco (downtown) topped out at 95 degrees, making it the hottest day there since Aug. 24, 2010.
Triple-digit heat has already reached portions of central and southern California. Downtown Los Angeles reached the century mark for the first time in 2015 after their high reached 101 degrees on Wednesday. Areas such as Sacramento, Fresno and Redding will see 100s in the coming days.
Sacramento toppled their daily record high by four degrees on Thursday when they reached a maximum temperature of 106 degrees.
Bakersfield, California tied their all-time record high minimum, or warmest low temperature, on Thursday when the temperature only fell to 75 degrees.
In the Pacific Northwest, Portland, Oregon, may see highs in the 90s through Saturday. 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 week, the jet stream pattern is flipping.
A southward dip in the jet stream is bringing much cooler air to the central and eastern states. Meanwhile, the West is heating up as the jet stream lifts north over that region.

MORE: Most Extreme Temperatures in Each State