Strong-to-severe thunderstorms are expected to pop up from the Midwest down into the Plains and across the mid-Atlantic.

HIGHWAYS CLOSED, OIL REFINERY THREATENED AS GREEK WILDFIRES RAGE ON

Some of these storms will contain large hail, damaging winds and isolated tornadoes, along with heavy rain that could lead to flash flooding.

This comes as dangerous heat bakes the Southern Tier of the country, with tens of millions of people under heat alerts once again on Wednesday. 

In addition, smoke from the Canadian wildfires is causing poor air quality over sections of the Great Lakes and Northeast.

Meanwhile, Tropical Storm Calvin will move south of Hawaii. 

However, heavy rain and tropical-storm-force winds will still be felt, especially for the Big Island.