Meteorologists warn of heat-related illnesses as record-breaking temperatures are expected in many large US cities.

As temperatures are set to soar to record-breaking levels across the southwestern part of the United States this week, the health dangers cannot be underestimated.

After a relatively mild May and early June across California and the southwest, a heatwave is under way and is expected to persist for many days to come.

Predictions are for as many as 10 very hot days, with numerous high-temperature records likely to be broken as the heat sets in.

Warnings are in place for this period of what is expected to be heat of dangerous intensity and duration in a significant number of highly populated regions, including the large US cities of Phoenix and Tucson in Arizona, Las Vegas in Nevada and Sacramento in California.

READ MORE: Heatwaves hit much of Asia earlier than expected

Cities closer to the Pacific coast will also see very hot conditions, but not to the same degree as those locations further inland.

“This period of intense heat is due to a strong and broad area of high pressure that is developing over the west coast and southwest interior and which will move slowly west over the coming days,” Daniel Swain from the California Weather Blog said.

READ MORE: Heatwave grips northern China

As it does, the hottest air and highest temperatures will move from the southern Arizona deserts to the California interior through the early part of the week.

Added to this heat is the extra moisture that will be in the air, making things feel even worse, as overnight temperatures will not fall as they usually do, giving places and people a chance to cool off.

Lack of overnight cooling is one of the biggest problems during a prolonged heatwave and is often the cause of heat-related illnesses.

One other factor in all of this is the added element of isolated afternoon thunderstorms.

So just how hot will it get?

Jeff Masters of Weather Underground says that “most of the Central Valley will see temperatures approaching 43C and possibly up to 46C”.

He added that the “southeast deserts will see temperatures up to 49C and incredibly, Phoenix in Arizona may see temperatures rising to 51C. These values could well break daily temperature records in many places.”

City Predicted High Temperature All-time Highest Temperature

Needles, CA 52C 51C July 17, 2005

Phoenix, AZ 49C 50C June 26, 1990

Las Vegas, NV 47C 47C June 30, 2013

Palm Springs, CA 48C 50C July 29, 1995

Fresno, CA 43C 46C July 8, 1905

Heatwaves in California very rarely last longer than a few days. At most, they generally last one week.

But this heatwave has the potential to last much longer and various computer models are predicting the extreme heat could last for up to 10 consecutive days.

However long the heat lasts, with temperatures so high, it is likely to be an intense spell of very uncomfortable weather for many people.