A blustery cold front pushed a remarkable cloud formation across the Twin Cities on Tuesday night, prompting a flurry of social media postings and appreciation for the spectacle — even dubbed “the Hand of God” by a couple observers.
MPR News meteorologist Sven Sundgaard says the cloud wasn’t threatening, even if it looked that way.
“It was ominous, the combination of the sunset with this dying thunderstorm moving across the Twin Cities. But it was a shelf cloud, not a (tornadic) wall cloud," he said. "It’s essentially the leading edge of rain-cooled dry air moving out of a thunderstorm. It’s warm, humid air moving up over that miniature cool front.”
The spectacle also unfolded over the heads of tens of thousands of Twins fans sitting at Target Field, where it eventually brought a rain delay.
Sundgaard said the storm did cause some wind damage near Buffalo, west of the Twin Cities, and that the storm brought some much-needed rain: about 0.38 of an inch of rain at the Twin Cities airport. Sundgaard said it was the most rain the Twin Cities has had since Memorial Day weekend.
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“It’s not quite a million-dollar rain,” Sundgaard said. This summer the Twin Cities has only had an inch or two of rain in various parts of the metro area, of the nearly six inches of rain the area usually gets between June and the first half of July.
How shelf clouds form
Here’s Sundgaard’s explanation of how shelf clouds form:
A shelf cloud is different from a wall cloud. A wall cloud is a large rotating cloud that is the base of a large rotating updraft in a super cell thunderstorm, called a mesocyclone.
A shelf cloud develops on the leading edge of rain-cooled, downdraft air out of a thunderstorm. That cooled air moves out from the storm and pushes warmer, more humid air up and over it.
That moisture then condenses out into an elongated low cloud in a bow shape. Oftentimes it’s ragged — like Tuesday evening’s cloud — and those pieces of ragged clouds move upward as that warm air moves into the storm.