Temperatures will be running 20 degrees below average in parts of the Eastern U.S. this weekend. (weatherbell.com)

The season’s first big cold snap will dive into the Eastern U.S. this weekend, bringing sub-freezing overnight lows, lake effect snow and an increased chance of waterspouts on the Great Lakes. By Sunday, temperatures will be running a chilly 20 degrees below average in parts of the Mid-Atlantic and Northeast.

A freeze warning (turquoise) is in effect for the northern Plains early Friday morning for temperatures that are expected to drop into the 20s. (National Weather Service)

Cold, Canadian air funnels into the northern Plains beginning Thursday night. Early morning lows will dip into the 20s in Minnesota, the Dakotas and parts of Nebraska.

A freeze warning is in effect for these regions, where crops and other vegetation will be killed. The National Weather Service in Bismarck is expecting a hard freeze on Thursday night, “and marks an end of the growing season,” they write.

Friday morning forecast lows
Great Falls, Minn. — 26 degrees
Minneapolis — 33 degrees
Bismark, N.D. — 28 degrees
Sioux Falls, S.D. — 30 degrees

The cold air shifts east on Friday, bringing lows in the 20s to much of Wisconsin, and near-freezing lows to Chicago. A freeze watch is in effect for northeast Illinois on Friday night.

[California floods may be just a preview of what’s to come in this El Niño winter]

Saturday morning forecast lows
Chicago — 37 degrees
Detroit — 36 degrees
Grand Rapids, Mich. — 33 degrees
Marquette, Mich. — 33 degrees
Indianapolis — 34 degrees

By Sunday morning the cold plume will have spread across the Midwest to cover much of the Northeast and Mid-Atlantic.

Sunday morning forecast lows
Rochester, N.Y. — 29 degrees
Pittsburgh — 33 degrees
Washington, D.C. — 35 degrees
Richmond, Va. — 34 degrees
Philadelphia — 31 degrees
New York City — 34 degrees

The season’s first big cold snap will sweep across the Eastern U.S. this weekend, seen here in an upper-level temperature forecast from the GFS model. (weatherbell.com)

The Southeast won’t quite get into the 30s this weekend but many will see the 40s on Saturday and Sunday mornings. Atlanta is forecast to bottom out around 45 degrees both days, Birmingham, Ala., will see lows in the mid-40s, and Columbia, S.C., could see a low of just 40 on Sunday night.

[Pic of the week: Aurora glows while thunderstorms flash]

The chilly blast will also bring the season’s first snow for many in the Great Lakes region, as cold air sweeps over warm lake water to create lake effect snow from Michigan’s Upper Peninsula to upstate New York. Most will see just a dusting — under 1 inch — but some areas east of Lake Erie and Lake Ontario could see two or three inches through Monday.

Snow accumulation through Monday from the GFS model. (weatherbell.com)

Waterspouts are also in the forecast for the Great Lakes. One waterspout forecasting technique takes into account the vertical depth of the clouds above the lakes and the temperature difference between the lake and the air above it. This model is predicting numerous water spouts in the Great Lakes starting Thursday and lasting though the weekend.

Waterspout forecast for Friday afternoon. (International Centre for Waterspout Research)