Residents and visitors walk through debris at the River's Edge apartment complex in Dayton, Ohio, after a tornado struck the city. Photos / AP
Residents and visitors walk through debris at the River's Edge apartment complex in Dayton, Ohio, after a tornado struck the city. Photos / AP
Less than a week after tornadoes tore through Missouri's capital and levelled buildings in Oklahoma, a violent tornado has struck another population centre - this time on the north side of Dayton, Ohio.
At times likely nearly a kilometre wide, the "large and destructive" tornado was the product of several ingredients.
In the latest tornado swarm, one person was killed and about 90 were injured. Storms in Ohio and Indiana were among 53 twisters that forecasters said may have touched down across eight states stretching eastward from Idaho and Colorado.
The National Weather Service gave the Ohio twister a preliminary EF3 rating on its 0 to 5 scale for tornado intensity, estimating that its peak winds reached about 225km/h 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 update yesterday, the National Weather Service's Storm Prediction Centre 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.
At first, it was expected that the storms near Chicago would merge into a line that would eventually sweep eastward and decay. Squall lines don't usually produce intense tornadoes; instead, they're like an atmospheric snowplow, heralded by a swath of potentially damaging straight-line winds.
But later, meteorologists at the Storm Prediction Centre 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, emphasising that these loner storms would "pose a more substantive risk for a strong tornado or two." It was improbable, but it happened.
The warm front moved north of Dayton, focusing the approaching storm system's "warm sector" on southern and central Ohio. Temperatures shot into the upper 20s. Most of the area was in a steamy, hazy fog while a gentle southerly wind blew, brewing tornado weather.
The Weather Service forecast office in Wilmington, Ohio, warned that the atmosphere would "support organised storms with supercells likely."
The warm front paused just north of Dayton. 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 wind swirling and increase tornado potential.
Things turned ugly fast as the jet stream strengthened around nightfall local time. NWS Wilmington issued a tornado warning for Wayne County, Indiana, the first of more than 30 such warnings that would be issued.
The most dire came as a catastrophic supercell was taking aim at Dayton: "TORNADO EMERGENCY for SECTIONS OF CENTRAL MONTGOMERY COUNTY" read the alert.
Pedestrians pass along storm debris on North Dixie Drive in Dayton, Ohio.
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 6100m, a sign of an intense tornado.
The nearly 16km-high mesocyclone - the rotating part of the storm - tracked just north of downtown, an atmospheric sink drain trapping warm air from the south and a blast of upper-atmospheric chill from the north.
Another tornado warning was issued for some of the areas and a second twister touched down a few kilometres north shortly afterward.
As daylight dawned, the scope of the devastation is just beginning to come into view.
Meteorologists at the National Weather Service in Wilmington anticipate that surveys will take several days.
The Storm Prediction Centre has received more than 500 tornado reports in the Lower 48 over the past 30 days, one of the most active stretches on record.
