Ghost of Polar Vortex? Were strange Mississippi ice sounds weather driven?

It could be Minnesota's version of the Loch Ness Weather Monster.

Some in the Twin Cites awoke to eerie, even ghostly sounds near the Mississippi River this morning.

Curious, even alarmed calls poured into Ramsey County 911. MPR News reporter Tim Nelson investigated.

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Was it an animal in distress? Heavy equipment tearing up concrete before dawn? A train accident?

Not that anyone in St. Paul could tell. But a distant, howling scraping sound woke people up about 4 a.m. this morning in St. Paul’s Highland Park and Mac-Groveland neighborhoods.

The sound echoed for miles through the city.

Ramsey County dispatchers initially thought it might have been demolition at the site of the former U.S. Bank offices near Highway 5, across the river from Fort Snelling. But they sent officers down to check it out and authorities now think it’s ice shifting on the Mississippi.

So what caused the unusual sounds on Old Man River this morning?

I'm no expert on ice sounds, though I've spent enough time on frozen Minnesota lakes to hear my share or cracking, groaning and popping.

The leading explanation is probably the weather changes overnight.

Tonka ccarsons skaters
Skaters on Lake Minnetonka. Image Paul Huttner/MPR News

Ice: A living weather barometer

We tend to think of ice as static, solid and unchanging. Ice is actually a moving, breathing reflection of the weather and the water surrounding it.

Changes in temperature and wind affect ice. Ice expands and contracts as temperatures shift, and wind exerts more force on ice than you might think.

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After 17 consecutive sub-zero  nights, temperatures in the Twin Cities actually rose overnight into Thursday morning. Shifting winds varied in speed and direction. Here's a look at the changing conditions at Minneapolis-St. Paul International Airport early Thursday.

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Image: Twin Cities NWS

River currents also exert force on the ice from below. The combined and changing forces may have been the trigger for the slow seismic shifts in the Mississippi River ice sheet overnight that woke nearby residents from a long winter's sleep.

Or not. This weather mystery may remain unsolved for now.