More than 40 million people are at risk for severe thunderstorms capable of very large hail, damaging winds and isolated tornadoes on Tuesday, with the risk area stretching from the Great Lakes to the Gulf Coast.

The thunderstorms are expected to begin Tuesday afternoon across the Central Plains and Midwest and move south and east. For the northern half of the risk area (Kansas, Missouri, Iowa, Illinois, Wisconsin) isolated tornadoes and hail are the greatest risks.

For the southern half of the risk area (Oklahoma, Arkansas, Texas, Louisiana) very large hail and damaging wind gusts of 70+ mph or higher are the greatest risks, with isolated tornadoes possible. For this southern risk area, the storms are expected to morph into an intense line of storms that will charge south through the overnight hours. Large hail combined with hurricane-force wind gusts could do immense damage.

By midnight, a line of thunderstorms could be stretching more than 1,000 miles from southern Michigan to southeast Texas.

Cities at risk Tuesday and through Tuesday night include Tulsa, Dallas, Oklahoma City, Houston, Austin, Kansas City and St. Louis. Chicago and Memphis are on the fringe of the severe threat and could experience strong storms as well -- the Chicago area could experience thunderstorms between 5 p.m. and midnight.

Dallas can expect storms from 9 to 11 p.m., and Houston early Wednesday between 5 and 7 a.m.

On Wednesday, 10 million people are at risk for severe thunderstorms across parts of Alabama and Georgia, including Atlanta. Storms can be expected in the afternoon and will be capable hail, damaging winds and isolated tornadoes.

By Thursday, this storm system will move into the Mid-Atlantic, Northeast and New England. While some isolated thunderstorms capable of gusty winds will be possible, heavy rain will be the greatest risk. Torrential rain could bring flash flooding along the I-95 corridor. Rainfall totals could be 1-3 inches region-wide, with higher amounts possible locally.

The severe weather this week will only add to what has been the most active April for tornadoes in the United States in nearly a decade. More than 70 tornado fatalities have been reported so far this year, the most since 2011.

Elsewhere across the country, the southwest is sizzling under blistering and record-setting hot temperatures.

About 9 million people are under excessive heat warnings across the Southwest including Phoenix, Las Vegas and Tucson. Highs 10-20 degrees above average from the southwest to southern Plains could lead to nearly 50 new record highs that could be tied or set by Friday. A few all-time hottest April temperatures could be set, and Las Vegas could see its first 100-degree day on record for the month of April.

Meanwhile, in what has been nicknamed "Arctic April" by meteorologists, the northeast is experiencing a cold and dreary month that feels more like a rewind to winter rather than spring. Numerous cities in the northeast have seen colder high temperatures this month than they saw during January. The last time Washington D.C. saw 1 day or few of 80+ degrees in April was 2000, and Boston could see its coolest April high temperature on record, only hitting a maximum of 62 degrees so far this month.