When a cluster of violent thunderstorms began marching across the Midwest, forecasters were able to draw a bright line across a map showing where the worst of the weather would go.
Their uncannily accurate predictions, combined with television and radio warnings, text-message alerts and storm sirens, almost certainly saved lives when rare late-season tornadoes dropped out of a dark autumn sky. Although the storms howled across 12 states and flattened entire neighborhoods within a matter of minutes, the death toll stood at just eight.
By Monday, another more prosaic reason for the relatively low death toll came to light: In the hardest-hit town, most families were in church.
"I don't think we had one church damaged," said Gary Manier, mayor of Washington, Ill., a town of 16,000 about 140 miles southwest of Chicago.
The tornado cut a path about an eighth of a mile wide from one side of Washington to the other and damaged or destroyed as many as 500 homes.
Daniel Bennett was officiating Sunday service before 600 to 700 people when he heard a warning. Then another. And another.
"I'd say probably two dozen phones started going off in the service, and everybody started looking down," he said.
What they saw was a text message that a twister was in the area.
Bennett stopped the service and ushered everyone to a safe place until the threat passed.
A day later, many in the community believed that the messages helped minimize the number of dead and injured.
"That's got to be connected," Bennett said. "The ability to get instant information."
Another factor was forecasting, which has steadily improved with the arrival of faster, more powerful computers. Scientists are now better able to replicate atmospheric processes into mathematical equations.
In the last decade alone, forecasters have doubled the number of days in advance that weather experts can anticipate major storms, said Bill Bunting of the National Weather Service.
But Bunting, the forecast operations chief of the service's Storm Prediction Center in Norman, Okla. said it was not until Saturday that the atmospheric instability that turns smaller storm system into larger, more menacing ones came into focus.
Enter another key piece of the weather predicting equations: Observation.
Information coming from weather stations, weather balloons, satellite imagery and radar told scientists that there was more than enough moisture — fuel for storms — making its way northeast from the Gulf of Mexico.
Despite Sunday's destruction and at least eight deaths, 2013 has been a relatively mild year for twisters in the U.S., with the number of twisters running at or near record lows.
So far this year, there have been 886 preliminary reports of tornadoes, compared with about 1,400 preliminary reports usually received by the weather service office by mid-November.
Similar slow years were 1987 and 1989.
An outbreak like the one that developed Sunday usually happens about once every seven to 10 years, according to tornado experts at the National Weather Service's Storm Prediction Center and National Severe Storm Lab in Norman, Okla.
There were similar November outbreaks in 1992 and 2002, with the 1992 one being even bigger than this year's, said top tornado researcher Harold Brooks at the storm lab.
The outbreak occurred because of unusually warm moist air from Louisiana to Michigan that was then hit by an upper-level cold front. That crash of hot and cold, dry and wet, is what triggers tornadoes.
Like most November storms, this one was high in wind shear and lower in moist energy. Wind shear is the difference between winds at high altitude and wind near the surface.
Because it was high in wind shear, the storm system moved fast, like a speeding car, Brooks said. That meant the storm hit more places before it petered out, affecting more people, but it might have been slightly less damaging where it hit because it was moving so fast, he said.
About 90 minutes after the tornado hit Washington, rain and high winds slammed into downtown Chicago, prompting officials at Soldier Field to evacuate the stands and order the Bears and Baltimore Ravens off the field. Fans were allowed back to their seats shortly after 2 p.m., and the game resumed after about a two-hour delay.