At least six historically Black colleges and universities received threats or “potential” threats Thursday, prompting lockdowns and class cancellations.

The plunging polar vortex is bringing subfreezing temperatures to some of the southernmost points of the U.S. A forming storm system is threatening to dump snow on parts of Texas and Oklahoma, while communities in Louisiana and Florida may see frigid cold.

Much of the U.S. is being blasted by ice, snow and wind, as the polar vortex that dipped south over the weekend is keeping an area stretching from the Rocky Mountains to the coast of Maine in its frigid grip.