It happened only once last year, but what happens when the tornado sirens go off, and it’s not a test?

A tornado warning is issued by the National Weather Service — the La Crosse office for Grant County, the Milwaukee/Sullivan office for Lafayette and Iowa counties, and the Quad Cities office for the Dubuque area — based on one of three criteria: a tornado spotted on radar, a severe thunderstorm capable of producing a tornado (rotation detected) on radar, or a tornado spotted by trained storm spotters.

The latter group includes the Platteville Fire Department, whose trucks are sent out when tornadoes are possible in the area. Fire Chief Dave Izzard estimated the Fire Department has seen “five or six” tornadoes in the past 20 years.

The Fire Department hosted one of the people who broadcast tornado warnings, Jeff Kennedy of KWWL-TV (channel 7) in Waterloo, for a storm spotter training session in late April.

The Fire Department, law enforcement and volunteer spotters, including area ham radio operators, are what Kennedy called “storm spotters,” instead of “storm chasers,” people who drive sometimes hundreds of miles in one day to chase storms in the hope of seeing tornadoes.

Kennedy said that while weather technology has improved greatly, technology has “kind of dropped the ball” in local storms, which is why local spotters remain necessary.

“There’s a lot of weather that does not get measured, and that’s a problem,” he said.

Kennedy pointed out that weather radar operates on line of sight, which detects less the farther the radar signal goes.

“That’s why eyes on the ground are needed,” he said. “That’s something we haven’t been able to replace, and I’m very happy about that.”

Wisconsin had just four recorded tornadoes last year. One of them was in Grant County — the tornado that hit a farm near Bloomington Sept. 4.

That is considerably lower than the average, since 1980, of 23 tornadoes per year. On the other hand, 2012 followed two above-average years, 2010 (46) and 2011 (38).

On average, Kennedy said, Wisconsin gets 29 severe thunderstorm watches and 11 tornado watches per year. Out of the more than 40 days with thunderstorms each year, a county on average receives five to 10 severe thunderstorm warnings per year and one or two tornado warnings per year. Grant County had the one tornado warning for the Sept. 4 tornado in 2012.

Tornado warnings formerly were issued for tornado sightings in person or by radar. Tornado warnings now are issued for rotation detected in severe thunderstorms.

“When you have weather that’s spinning, that’s usually bad,” said Kennedy.

The average Wisconsin tornado is 121 yards wide, travels 5.5 miles, lasts 9.8 minutes, and averages a 1.1 on the enhanced Fujita scale of tornado wind-caused damage, according to the NWS Milwaukee office.

“Most tornadoes in Wisconsin are what I’ll call ‘survivable’ — EF 1, EF 2, EF 0,” said Kennedy, referring to tornadoes with three-second wind gusts of 65 to 135 mph.

Kennedy identified U.S. 151 as “our tornado alley,” with Dane (61 tornadoes since 1950) Dodge (58), Grant (51), Fond du Lac (45) and Iowa (21) counties leading the state in tornadoes. Marathon County is tied with Grant County for third most tornadoes since 1950.

The group of counties along the U.S. 151 corridor has combined for 19 deaths and 452 injuries since 1950. Nine of those deaths and 200 of those injuries came from the June 9, 1984 Barneveld tornado, an EF 5 tornado. The Oakfield tornado in 1996, also an EF5, resulted in no deaths, but nearly $40 million in damage.

The most common month for tornadoes in Wisconsin is June, followed by July, May, August, September and April.
Tornadoes are not the only effect of severe weather, though they get the most attention. Severe thunderstorm warnings are issued for reports of hail of 0ne-half inch or more in diameter and thunderstorm wind gusts of 58 mph or faster. About 10 percent of thunderstorms reach severe levels, about 10 percent of severe thunderstorms produce tornadoes, and about 10 percent of tornadoes result in deaths, Kennedy said.

Between 1982 and 2012, Grant County had 438 severe weather events, second only to Dane County, Kennedy said.

Severe thunderstorm warnings are no longer issued for heavy rainfall, but flash flood warnings are issued for flooding caused by heavy rainfall in a thunderstorm. (One inch of rain falling in Grant County totals 8,223,534.6 tons, or 2,055,883,648 gallons of water, Kennedy said.)

Most flooding deaths occur in automobiles, and most flood deaths occur in urban areas, Kennedy said, adding that one foot of moving water can move a car.

The best view of a storm is from its southeast or southwest looking north. That perspective gives a better view of the rain-free base on the southwest edge of a storm from which a wall cloud and tornado may form.

Storm spotters are asked to report tornadoes, funnel clouds, rotating wall clouds, hail, wind of more than 40 mph, flash  flooding or mudslides, and rain of more than 2 inches per hour.