A battle between searing heat from Mexico and a chilly blast from the Arctic threatens to hit the US with violent thunderstorms and a barrage of bulldozing twisters.
Tropical southern-hemisphere summer air will flood into Texas, Arizona, New Mexico and surrounding States through the coming days pushing temperatures to 105F (40C).
Bubbling heat will hold out under high pressure as the tail end of Hurricane Lee pulls Arctic air down the east coast, while a separate cold plunge hits the west.
The two air masses will collide, triggering deep atmospheric instability to unleash a barrage of “severe” thunderstorms and the risk of tornadoes.
WATCH NOW: Jim Dales discusses the weather
Jim Dale, US weather correspondent and senior meteorologist for British Weather Services, said: “The heat is back, coming up out of Mexico and from the southern hemisphere which we must remember is now going into summer.
“We are looking at temperatures of 40C in some states, such as New Mexico, Texas, Nevada and Arizona.
“Where this hot, dry air clashes with cooler, damp air, we will see instability, and there will be the risk of significant thunderstorms and tornadoes.
“Tornadoes are spawned from these large thunderstorms that form in these conditions, so the potential to see some of those things will be there.”
Risk of ’severe’ thunderstorms where cold meets hot
Weather.us
Weather models show the highest risk of severe thunder through the coming days across Texas, Oklahoma, Kansas, Iowa, and Illinois.
Western regions including Utah, Nevada, Colorado and Idaho are also on alert for rain and light to moderate eruptions.
Mr Dale said: “This is interesting from a meteorological perspective, when we see this clash of airstreams and the resulting instability.
“The severity will be there, and the tornado potential will be there as the atmosphere is fed with a mix of hot air from the south and cold air from the north.”
Temperatures nudge 100F across the south
Weather.US
Cold northerly air could bring snow to some US states this week as an early blast of winter arrives.
Most at risk will be high ground over northern states and Canada, while surrounding regions turn cooler.
Domenica Davis, meteorologist for The Weather Channel, said: “A cold front moving into the west is going to shift the weather pattern.
“Above average temperatures that we're seeing through parts of the plains are going to calm down.
Scorching heat from the south
The Weather Channel
“Also, out to the northwest, we're going see more fall-like air that could even cater to some high elevation snow by late week.
“As we go through the middle half of the week…those temperatures get significantly cooler.
“We'll see the coolest pool of air moving into the northwest, and we will also have the bulk of the precipitation coming in with this front.”
Further hot weather is set to arrive in parts of the US as Louisiana endures the most savage wildfires of the season.
Swathes of the region have been ablaze for almost a month, destroying more than 30,000 acres and forcing people from their homes.
A dearth of rain has left the state unusually dry for the time of year while searing summer heat turned it into a tinder box.