(CNN) - Dangerous storms are forecast Thursday for central and southern Plains states, with the National Weather Service saying Kansas and Oklahoma should be on alert for powerful tornadoes.
The service's Storm Prediction Center has issued a rare "high risk" advisory for severe weather that could continue into the overnight hours.
The weather service's tornado Twitter account said one observed twister was moving through western Oklahoma. Several areas were under a tornado warning, which means one has been seen in person or on radar and there is imminent danger.
"Strong, long-track tornadoes are likely from parts of central and southern Kansas southward into western Oklahoma," the weather service said in its forecast.
Large hail fell in Granite, Oklahoma, according to one Twitter user who posted a picture of her cousin holding three pieces of ice that were bigger than golf balls.
Tornado and flash flood watches extended from Texas to Nebraska.