A number of unique things happened that made the evening tornadoes and stormy weather particularly unexpected.
“That’s stuff that you don’t see in the textbooks,” Meteorologist Pete Sack said.
The storms sparked in the early evening hours Tuesday, following a day of near-record warmth for the region, with temps reaching into the 70s and falling just shy of setting a record for the warmest February temps ever recorded in the city.
The storms moved from west to east, popping up consistently on a north-south line known as a “bow echo,” which according to the National Weather Service refers to how a band of storms “bow out” when strong winds “reach the surface and spread horizontally.”
Typically, with a bow echo, the strongest tornadic activity will spark on the curve of the bow and south. But Tuesday, that didn’t happen.
“We were seeing tornadoes yesterday kind of pop up in locations where we typically don’t,” Sack said, adding “we had circulations and tornado warnings all the way from the north to the south of the bow echo.”
It’s a situation Sack called “just unusual.”
The storms brought reports of touchdowns near Inverness in the northwest suburbs; Waterman in DeKalb County; Sugar Grove, Big Rock and Hinckley in Kane County; and in parts of northwest Indiana, including near Gary.
Add to it, a mesolow, or a very compact area of low pressure, formed along that same storm line, which Sack said contributed to the tornadic activity.
“That usually doesn’t happen either,” he said.
NBC 5 Storm Team Chief Meteorologist Brant Miller noted that the situation was one he’s never encountered in his years of forecasting in Chicago.
NWS officials are conducting aerial and ground-based surveys to determine where exactly tornadoes in fact touched down, and how long they were on the ground.
Damage assessments will continue through the day in the suburbs, and we will provide updates if NWS officials confirm tornado touchdowns in those areas.
So beyond the nearly unprecedented February temperatures, and the exceptionally rare February storms, it would seem the system witnessed by many in the Chicago area Tuesday evening were well outside the norm.