Concern is rising that the first major storm outbreak on the verge of spring could strike the nation’s heartland, putting millions of Americans from Texas to Iowa at risk of potentially strong tornadoes.

Some scattered severe thunderstorms were expected to begin late Thursday in the Texas Panhandle and across western Oklahoma and parts of Kansas, the National Weather Service said. Large hail, damaging winds and possibly a few tornadoes were also expected, according to the forecasters.

The strongest storms are forecast to develop Friday in a zone that includes much of Oklahoma, Kansas and Missouri and areas of some nearby states.

“This is probably our first real event this season where people are really starting to pay attention getting into the spring storm season,” said Melissa Mayes, deputy director of the Washington County Emergency Management Agency in Bartlesville, Oklahoma, north of Tulsa.

More than 6 million Americans are at the highest risk of severe weather Friday in an area that includes the metropolitan areas of Kansas City and Tulsa, Oklahoma, according to the national Storm Prediction Center. Another 22 million people are at a slightly lesser risk in a zone that includes Oklahoma City; St. Louis; Omaha, Nebraska; and Milwaukee, Wisconsin.

The general setup for the strong storms is a clash between warm air streaming north from the Gulf Coast and cooler Canadian air behind cold fronts, according to meteorologists with the private forecasting service AccuWeather.

The spring storms in the forecast come near the start of what many call tornado season, which generally begins at different times in different parts of the United States.

In what has historically been known as Tornado Alley — a designation that typically includes Kansas, Oklahoma and Texas — the peak of tornado season is May into early June. But the season starts earlier in what is often called Dixie Alley made up of southern states such as Arkansas, Louisiana, Mississippi, Alabama and Georgia.

Experts recommend a few simple safety steps to take before tornadoes hit.