The National Weather Service gave the twister a preliminary EF3 rating on its 0 to 5 scale for tornado intensity, estimating its peak winds around 140 mph in Trotwood, which is northwest of Dayton. It is continuing to survey damage in the region and could revise these numbers.

Originally, it didn’t look like this deadly mix would come together; in its Monday morning update, the National Weather Service’s Storm Prediction Center didn’t include Dayton in even the lowest-tier tornado risk zone. The action instead was forecast to remain concentrated just south and west of Chicago.

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At first, it was expected that the storms near Chicago would merge into a line that would eventually sweep eastward and decay with time. Squall lines don’t usually produce high-end tornadoes; instead, they’re like an atmospheric snowplow, heralded by a swath of damaging straight-line winds.

But later Monday, meteorologists at the Storm Prediction Center zeroed in on something the computer models weren’t quite capturing. “There may be a period with sustained discrete storm development, including supercells,” they wrote, emphasizing that these loner storms would “pose a more substantive risk for a strong tornado or two.” It was a low-probability event, but if it occurred it would have an extremely high impact. And that’s exactly what happened.

By midafternoon, the warm front had moved north of Dayton, thrusting southern and central Ohio into the approaching storm system’s “warm sector.” Temperatures shot into the upper 70s with dew points as high as 69 degrees at Dayton/Wright-Patterson Air Force Base. Most of the area found themselves in a steamy, hazy fog while a gentle southerly wind blew. It was beginning to feel like tornado weather.

The Weather Service forecast office in Wilmington, Ohio, warned that the atmosphere would “support organized storms with supercells likely."

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The warm front stalled just north of Dayton, the boundary acting like a rail track that storm could train on later that evening. Stalled warm fronts are a recipe for disaster. Not only do they fuel flash flooding as storms ride over the same area, but they can enhance low-level turning of the winds and increase tornado potential.

Things turned ugly fast as the low-level jet stream strengthened around nightfall. NWS Wilmington issued a tornado warning for Wayne County, Ind., at 8:59 p.m., the first of more than 30 such warnings that would be issued through 2:15 a.m. Tuesday.

The most dire came at 10:57 p.m. Monday as it was clear a catastrophic supercell was taking aim at Dayton: “TORNADO EMERGENCY for SECTIONS OF CENTRAL MONTGOMERY COUNTY” read the alert in all caps.

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It was soon apparent just how bad things were. A “debris ball” showed up on radar — a blob of purple and white echoes where the radar is plotting tornadic debris like it would hail.

Debris was picked up on radar lofted to a height of 20,000 feet. When debris makes it that high, it’s usually a sign of an intense tornado.

The nearly 10-mile-high mesocyclone — the rotating part of the storm — tracked just north of downtown, an atmospheric sink drain entraining warm air from the south and a blast of upper-atmospheric chill from the north.

At 10 p.m., the Air Force Base reported light southerly winds at 6 mph; less than an hour later, they had switched around from the north with gusts as high as 46 mph as the “rear flank downdraft” brought high winds to the surface. That shot of downward-rushing air helped to tighten the tornado’s circulation, strengthening it further. The tornado cleared the north side of Dayton around 11 p.m.

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Just 13 minutes later, another tornado warning was issued for some of the areas and a second twister touched down just miles north of the first 30 minutes.

To make matters worse, this all occurred in the dead of night — making visual confirmation of the tornado virtually impossible. As daylight dawned, the scope of the devastation is just beginning to come into view.

Meteorologists at the National Weather Service in Wilmington will be venturing into the field Tuesday to survey the damage. They anticipate surveys to take several days.