(CNN) The second intense storm in a week is whipping up gale-force winds and waves as high as 20 feet across an already-high Lake Michigan.

This latest storm is likely to be even stronger than last week's, which caused damaging lakeshore flooding and eroded dunes along the Great Lake's eastern edge.

Large waves on Lake Michigan, caused by high winds, crash Wednesday into the St. Joseph Lighthouse and pier in St. Joseph, Michigan.

The combination of above-average lake levels and gale-force winds will lead to another round of lakeshore flooding and beach erosion Tuesday through Thursday.

The powerful low-pressure system responsible for these waves is moving northeast across the Midwest while producing gale-force winds (39+ mph winds that are not associated with a tropical cyclone).

Because of the wind intensity, nearly 18 million Midwesterners are under a wind warning or advisory, including in Omaha, Nebraska; Des Moines, Iowa; Minneapolis; Milwaukee; and just west of Chicago.

Nearly 2 million people are also under a lakeshore flood warning, mainly along the western Michigan coast.

Large waves and strong winds will bring beach/dune erosion and flooding to the lakeshore. #wmiwx #lakemichigan pic.twitter.com/AKAiiEfJhV — NWS Grand Rapids (@NWSGrandRapids) October 22, 2019

Waves will build Tuesday to between 10 and 16 feet, while reaching 21 feet in some locations.

"Southwest-facing shorelines will encounter the worst conditions," said the National Weather Service Office in Grand Rapids, Michigan.

The storm will peak Tuesday afternoon and shift north into Canada, and strong winds and high waves will slowly ease through Thursday. Waves won't fall below 4 feet until Wednesday evening.

Near-record lake levels leading to beach erosion

Above-average rainfall this year around Lake Michigan has raised the lake to near-record lake levels. In September, the lake basin got 135% of its average rainfall, raising the lake to 30 inches above its long-term average.

The latest forecast means lakeshore flooding is likely for the foreseeable future.

"With Lake Michigan water levels expected to remain high at least through the beginning of 2020, the risk of continued erosion will remain for the foreseeable future," tweeted the Grand Rapids National Weather Service after last week's destructive storm.

This is a stark contrast to the record low levels seen earlier this decade.