Weather Service hits bull's-eye with tornado forecast

The National Weather Service's detailed forecast of a deadly tornado outbreak across the central USA proved highly accurate.

Late Sunday, one or more deadly tornadoes hit Arkansas, killing at least 14 people as the storms roared on the ground for about 80 miles.

On Friday, the Storm Prediction Center (SPC), the severe weather forecasting office of the weather service located in Norman, Okla., predicted a Sunday "bull's-eye" for Arkansas for tornadoes and storms by issuing a "moderate" risk for storms.

The SPC issues risk levels – including "slight," "moderate" and "high" – for a region days or hours ahead of predicted severe weather. By early Sunday, the SPC placed Arkansas under a "high" risk for severe weather, the worst level. At 4:40 p.m., the SPC issued a "Particularly Dangerous Situation" warning for central Arkansas.

The SPC expected it to become the USA's largest severe weather episode so far this spring. Strong tornadoes and hail bigger than golf balls were anticipated.

Local National Weather Service forecast offices issue even more detailed tornado warnings for a specific time and place.

By early evening, in addition to a tornado warning, the weather service in Little Rock had issued a "Tornado Emergency" for areas north of Little Rock. That signals that a large, extremely violent tornado is about to pummel a densely populated area.

The tornado lasted from 7:30 to 10 p.m., according to KTHV-TV in Little Rock.

On average, the tornado warning lead time – the time between when a warning is issued and when the tornado hits ground – is now about 13 minutes, the weather service reports.

Warnings are issued for about 75% of all tornadoes, the Bulletin of the American Meteorological Society reports. As recently as 1986, average tornado warning lead time was approximately five minutes and warnings were issued for only 25% of tornadoes.

False alarms remain an issue. Only one of five tornado warnings actually are followed by a twister, according to a 2011 study in the journal Weather and Forecasting.