Colder Pattern Emerges This Week in the Midwest, East and October May Kick Off With Potential Frost and Freeze

Jonathan Erdman

Published: September 28,2020

A much chillier weather pattern will plunge into the Midwest, East and South this week, bringing with it the potential for frost and freeze conditions as the calendar turns to October.

The autumnal equinox officially kicked off fall in the Northern Hemisphere last Tuesday, and as the calendar is about to change to October this week, it should also feel like fall in much of the eastern half of the nation.

This change will be driven by an abrupt shift in the jet stream, which will take a sharp southward nosedive over the central and eastern U.S. in response to a pair of high pressure systems bulging north over western North America and the western Atlantic Ocean.

This jet stream plunge will tap chillier air from Canada and pull it through much of the central, southern and eastern U.S. At the same time, it will bring drier and hotter weather to the fire-fatigued West.

This pattern may hold into the first weekend of October, and perhaps sometime after that.

High/Lowlights

A pair of cold fronts will usher in this chillier pattern this week.

The first has already swept south through the Plains and Midwest, and it will move across the South and East early this week.

A second, more potent cold front arrives into the Northern Plains around Wednesday, then quickly plunges through the South Thursday and Friday. Afterward, daytime highs may struggle to rise out of the 40s in the northern Great Lakes, 50s in much of the rest of the Midwest and 60s as far south as the Tennessee Valley and Carolinas.

(MAPS: 10-Day U.S. Forecast Highs/Lows)

Morning lows late this week could plunge into the 30s in the northern and central Plains, parts of the Midwest, and even in a few of the coldest spots of the Appalachians. Some of these areas could see their first frost or freeze of the season late this week.

Lows in the 40s are possible into the Tennessee Valley, parts of Georgia, the Carolinas and Southern Plains.

(MORE: The Earliest You Could See a Freeze in Your Area)

Lake-Effect Flakes?

There's one other interesting facet to this chillier pattern.

Water temperatures over the Great Lakes are usually still fairly near their annual peak as fall begins because of a summer's worth of heating.

So colder air flowing over these relatively warm bodies of water will generate bands of lake-effect precipitation, primarily rain, at times this week.

The larger the difference between the cold air and warmer lake, the more vigorous the lake-effect precipitation.

In this week's case, there could be lake-effect thunderstorms, even some waterspouts, given the air-lake temperature difference and the strength of spin in the atmosphere, or what meteorologists refer to as vorticity.

(MORE: October-December Temperature Outlook)

The setup for lake-effect rainbands later this week in the Great Lakes. A few wet snowflakes could also fall in the northern Great Lakes.

Parts of the northern Great Lakes could even see a few first snowflakes of the season mix in behind the second cold front late this week.

If this happens, it would primarily be away from the warmer lakeshore over the interior of Michigan's Upper Peninsula, extreme northern Wisconsin or perhaps the Arrowhead of Minnesota.

Quite the way to usher in October, right?

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.