Unlike past years, fall weather has wasted little time asserting itself this September in the eastern United States, and, as October begins, the most intense blast of chilly air yet will crash southward.

The jet stream, the high-altitude wind current that divides cold and warm air, is set to plunge out of Canada this weekend, carving out a trough or dip over the eastern United States for much of next week and perhaps longer.

The core of the cold is initially forecast to focus between the Plains and the Appalachians, where temperatures may hover 10 to 15 degrees below normal for much of next week in the Midwest, the Great Lakes and the Ohio Valley. The East Coast is predicted to lie along the periphery of the expansive cold pool, with temperatures a little above normal early in the week but trending colder during the week’s second half.

In Chicago, after climbing into the 80s over the weekend, highs next week are forecast to plummet to only near 55 Thursday and Friday. Nighttime lows are likely to sink to near 40, with frosty 30s in its colder suburbs.

“Potentially summerlike temperatures Saturday are expected to give way to conditions more suited to mid to late October by Wednesday, as three successive cold fronts bring colder air and unsettled weather to the region,” the National Weather Service office serving Chicago wrote.

Here are forecast highs and lows for several other cities at this time next week (Oct. 1), subject to change some because predictions this far into the future are lower in confidence:

  • Minneapolis: high 49, low 39. (Normals: 65/45)
  • Milwaukee: high 55, low 42. (Normals: 65/48)
  • St. Louis: high 58, low 45. (Normals: 74/54)
  • Cleveland: high 60, low 45. (Normals: 68/50)
  • Nashville: high 67, low 49. (Normals: 76/54)
  • Pittsburgh: high 62, low 46. (Normals: 68/52)
  • Washington: high 72, low 53. (Normals: 74/56)

The cold fronts ushering in the chill will bring periodic chances of showers and perhaps even some wet snowflakes in the Upper Peninsula of Michigan and northern Minnesota.

Whereas the coldest air will remain just west of the East Coast through the middle of next week, longer-range forecasts for the period of eight to 14 days into the future indicate increasing chances for the chill to penetrate eastward.

The National Weather Service has declared a moderate to high risk of much-below-normal temperatures from the Midwest to the Mid-Atlantic between Oct. 1 and 7.

During this period, numerous areas could see their first frost or freeze, especially away from urban areas.

The surge of cold set to enter the Lower 48 states comes just one week after scores of locations in interior New England and western New York set record lows in the 20s and 30s. The chill even reached the Mid-Atlantic, where Washington observed lows in the 40s on four straight days in September for the first time since 1950.

The weather pattern becoming established next week is the opposite of October’s first week one year ago. The jet stream took on the reverse configuration, rising over the eastern United States, resulting in record warmth. Dozens of locations in the East observed their hottest October weather on record in 2019, including Washington, which soared to 98 degrees on Oct. 2. The forecast high for Oct. 2 in Washington this year is closer to 70.

While colder-than-normal air pours into the East over the next one to two weeks, the jet stream will rise over the West. A heat dome will form underneath that bulge in the jet stream, resulting in abnormally high temperatures while elevating the fire danger.

Beyond the first week or two of October, however, longer-range forecasts generally do not suggest that this extreme, bifurcated weather pattern will persist. Computer models suggest milder weather will return to the East with flatter west-to-east flow across the country. However, models differ on exactly when this cold pattern will break down.

Matt Rogers, a long-range-forecasting specialist with the Commodity Weather Group, said it’s rare for a pattern like this to sustain itself in October during La Niña conditions, currently in place.

“It is unusual for La Niña to deliver a cold October in the East,” he said. “This time of year, we shouldn’t expect things to lock in, and it’s reasonable to expect some moderation.”

Fall color emerging in the mountains and the north

The rapid onset of autumn-like conditions is bringing out the fall foliage in northern parts of the Upper Midwest and in northern New England.

Peak color is emerging in northern Minnesota and the high altitudes of New Hampshire, Vermont and Maine.

In the next week or two, aided by the chilly weather, peak foliage will lower in elevation in New England and bleed into the higher elevations of the Mid-Atlantic. It will also expand over much of the Great Lakes region.

Here are several photos of some of the emerging foliage from the past week:

Vermont

Wisconsin

Minnesota