Story highlights A tornado causes scattered damage in Norman, Oklahoma

Severe storms are expected in Plains states on Saturday

A tornado outbreak is likely in high-risk areas, forecasters say

Kansas, Missouri forecasters testing graphic warnings

A tornado touched down late Friday afternoon in Norman, Oklahoma, bringing scattered structural damage and a taste of more severe weather -- including a tornado outbreak -- expected Saturday in the Plains.

No serious injuries were immediately reported in Norman, home to the University of Oklahoma and the National Weather Service's Storm Prediction Center.

Forecasters have said there is a "high risk" of severe thunderstorms on Saturday in portions of Nebraska, Iowa, Kansas and Oklahoma.

A severe thunderstorm is defined by the National Weather Service as a thunderstorm that produces at least one or more of the following: winds of at least 58 mph, hail 1 inch in diameter and tornadoes.

According to CNN meteorologist Sean Morris, "high risk" areas could possibly endure EF3 to EF5 tornadoes, packing winds of 136 mph or stronger.

A tornado outbreak is likely across the central and southern plains from late Saturday afternoon through the evening and overnight, according to the National Weather Service.

CNN meteorologist Dave Hennen said the Interstate 35 corridor -- from Oklahoma City, Oklahoma, to Wichita, Kansas -- is a high-risk area.

A second high-risk area includes portions of Iowa and Nebraska, where dangerous long-track tornadoes are also possible. Lincoln and Omaha might be affected.

An elevated "moderate" threat extends from Iowa and Nebraska, southward into the tip of northern Texas. This includes Tulsa, Oklahoma, and Kansas City, Missouri. A "slight" risk area extends all the way from Texas to southern Wisconsin.

The predicted severe storms come as five National Weather Service offices in Missouri and Kansas are conducting an experiment on how to better convey risks from tornadoes and severe storms.

The "impact based" warning test, which began earlier this month, comes on the heels of the May 22-27 Midwest/Southeast tornado outbreak, including a tornado that killed 158 people in Joplin, Missouri.

The National Weather Service is ratcheting up its efforts to combat complacency, with the help of graphic terms to ensure people find safe shelter. A tornado is confirmed, on average, only once for every four formal warnings.

Forecasters in the test area will continue issuing traditional tornado warnings, but for "significant" and "catastrophic" scenarios, they can add information at the bottom of the warnings issued to media outlets.

When a storm has the potential to cause "significant" damage, meteorologists may include terms such as "major house and building damage likely," "complete destruction possible" or "major power outages in path of tornado highly likely."

In a "catastrophic" outlook, descriptions may include "This is a life-threatening situation," "You could be killed if not underground or in a tornado shelter," or "complete destruction of entire neighborhoods likely."