Extreme weather blasts US with severe thunderstorms and record-breaking heat: Latest forecast

Extreme weather -- from destructive storms to blistering heat -- continues to wreak havoc across the United States.

Since Monday, there have been more than 850 damaging storm reports nationwide with nearly two dozen tornadoes reported from Colorado to Massachusetts. Ten of those twisters have been confirmed by the National Weather Service, including an EF3 in northern New York state and an EF1 in eastern Massachusetts.

The National Weather Service currently uses the Enhanced Fujita scale to rate tornado intensity based on wind speeds and the severity of the damage caused. The scale has six intensity categories from zero to five (EF0, EF1, EF2, EF3, EF4 and EF5), representing increasing wind speeds and degrees of damage. There is also an unknown category (EFU) for tornadoes that cannot be rated due to a lack of evidence.

Thunderstorms also dumped 6 to 7 inches of rain in Massachusetts within a short period of time, producing major flash flooding that inundated roadways and neighborhoods.

At least two people were killed by severe weather on Monday -- a 28-year-old man in Florence, Alabama, and a 15-year-old boy in Anderson, South Carolina, according to local authorities.

The weather forecast for Wednesday shows a new threat of severe thunderstorms is on the move, stretching from Kansas to Georgia. Major cities like Little Rock, Arkansas; St. Louis, Missouri; Memphis and Nashville, Tennessee; and Birmingham, Alabama, could see damaging winds, hail and a few tornadoes.

On Thursday, the severe weather threat shifts east to the Carolinas, including the cities of Raleigh and Charlotte, North Carolina, and Columbia, South Carolina. The biggest threat there will be damaging winds but an isolated tornado can't be ruled out.

That same storm system is forecast to produce more thunderstorms and heavy rainfall to the north, from New York, New York to Boston, Massachusetts.

Meanwhile, down south, a record-smashing heat wave is expected to persist in Texas, Louisiana and Florida. Heat index values -- a measure of how hot it really feels when relative humidity is combined with the air temperature -- are forecast to be around 110 degrees Fahrenheit there this week.

On Tuesday, the heat index value hit 113 degrees in Miami, Florida -- the hottest ever recorded in the city. Parts of Florida's Tampa Bay metropolitan area reached a heat index of 123 degrees. Temperatures in Naples, Florida, hit 98 degrees -- a tie for the hottest recorded in the city during the month of August.

Lafayette, Louisiana, has gone a record 10 straight days of temperatures at or above 100 degrees.

The hotter-than-normal temperatures are expected to continue into the weekend and next week.

The National Weather Service has issued heat alerts in effect Wednesday morning for 50 million Americans across nine states, from New Mexico to Florida.