ROMEOVILLE – Up until recently, six short-lived EF1 tornadoes spawned by two storms that tore through northern Illinois June 30 might have been all but invisible to the National Weather Service.
The closest, which touched down briefly between Plainfield and Romeoville, was "embedded" in the storm front, said Matt Friedlein, a meteorologist with the National Weather Service's Romeoville office.
"All of the tornadoes happened in less time than the old radar scanning strategy takes," Friedlein said.
Atmosphere scans by the weather service's Doppler radar – the massive white bubble that towers over Lewis University Airport – once took about five minutes to complete. The Doppler – basically a rotating radar dish enclosed by the bubble – used to progressively scan the horizon from 0.5 to 19.5 degrees.
The Plainfield-Romeoville tornado was on the ground for less than three minutes, a short enough time to form and dissipate in-between scanning intervals.
But a new system installed in June at the Romeoville office now scans lower elevations every 1.8 to 2.5 minutes. The system, called Supplemental Adaptive Intra-Volume Low-Level Scan or SAILS (see graphic), emphasizes scanning at elevations 0.5 to 3.1 degrees, the area where tornadoes touch down.
"We really focus on rotation closest to the ground," Friedlein said. "A lot of storms rotate aloft, but to see that intensifying trend close to the ground is a key for meteorologists to issue tornado warnings."
While air movement itself is invisible to Doppler, the radar can see which way raindrops or hail are moving within a storm, Friedlein said. Dust clouds or insects sometimes can be used, he said.
"The lowest angle down to the ground is where the key features of storms can be found," Friedlein said. "Doppler allows us to see phase changes in the storm, such as changes in wind speed, direction and movement, including rotation."
Just in time
The system was almost made-to-order for the two June 30 storms. The powerful bow-shaped fronts, known as derechos, caused widespread damage in Will and Grundy counties, with downed trees and power lines knocking out power to nearly 400,000 utility customers at one point.
The fronts were moving so quickly, up to 60 mph according to Friedlein, that the storms only lasted a few minutes before moving on to the east. And while most of the storm damage was credited to straight-line winds, the EF1 tornadoes were a major concern.
"The tornadoes were short-lived but they covered a few miles in a few minutes, so they were really hauling," Friedlein said.
An EF1 tornado is the second lowest rating on the six-point scale used by meteorologists to rate tornadoes and is considered "weak." Still, an EF1 twister can have wind gusts from 86 to 110 mph, according to the National Weather Service.
The tornado spawned in LaSalle County by the storm, for instance, made a beeline for the village of Earlville, destroying one home and damaging several others, but caused no injuries.
"SAILS helps us to better detect rapidly intensifying storms and short-lived tornadoes," Friedlein said.
Also new for Doppler is a radial noise filter that removes solar interference known as "sun spikes" from radar images, according to the National Weather Service. Another new feature will enable the radar to automatically determine the best settings for viewing velocity data for the strongest storms in the radar’s coverage area.
Boots on the ground
Yet technology only can go so far, and forecasters still rely on weather spotters for data.
"The technology is great, but we're not a a point where we can see everything 100 percent," said Harold Damron, director of Will County Emergency Management Agency. "It's definitely a combination of eyes on the street and technology working in concert with each other.
"We get eyes-on reports from law enforcement, firefighters or other types of spotters stationed out there and feed them into the weather service. It allows them to correlate what we see with what they see on the radar."
Jeremy Hylka, director of the Joliet Weather Center, agreed.
"As much technology as we have, nothing beats the eye from a spotter in the field," Hylka said. "It goes without saying."
One great advancement has been the ability to narrow down the geographic focus of weather warnings, Damron noted. In years past, severe weather and tornado warnings would be issued on a county-wide basis.
Yet issuing warnings too frequently in too wide of an area can lead to other problems.
"You don't want to get into that 'cry wolf' area," Damron said.
Meteorologists now can define a specific warning area on-screen and issue a warning with the click of a mouse, Friedlein said.
The weather service now can issue Wireless Emergency Alerts to cellphone towers in an affected area, which in turn will broadcast an audible noise and warning text to all newer smartphones within its range.
"It broadcasts a sound you're not used to hearing," Friedlein noted.
It's a far cry from the old days when warnings were issued primarily by sirens, radio and television.
Damron said he got a clear view of the limitations of the older technology during last year's anniversary of the Aug. 28, 1990, Plainfield tornado, when he viewed a black-and-white radar image of the storm.
"It was just a shock to look at it on the traditional round radar screen," Damron said. "It's amazing how far we have come."