'Heat dome' weather system to bake America with temperatures beyond 90F for weeks

A blistering "heat dome" will blanket large swathes of the US this weekend with temperatures above 90F degrees, and up to 121F, that will last for weeks as experts warn of increased danger in heat-related illness and deaths.

It comes as Tropical Storm Fay is set to batter the east coast from the mid-Atlantic to New England, with heavy rain to deluge New York and a flash flooding watch issued for parts of New Jersey.

The National Weather Service on Friday begun issuing excessive heat watch alerts for "dangerously hot conditions" and are forecasting more than 75 record high temperatures to be approached or broken from Friday to Tuesday, and growing the following week.

The prolonged heatwave would increase the odds of heat-related illness and deaths, Weather Underground founder Jeff Masters told CBS News.

"The heatwave will be very long-lived, lasting multiple weeks in some areas with only a few days of near-normal temperatures during that span," Mr Masters told the outlet.

While temperatures up to 121F are possible in the Desert Southwest, areas in Texas like Amarillo will record 10 days of temperatures above 100F and up to 110F.

Las Vegas and Phoenix, meanwhile, are forecast to rise above 110F on Saturday. The 100F+ degree temperatures are predicted to spread across the mid-Atlantic, Ohio Valley and the Southern Plains.

A heat dome occurs when the atmosphere traps hot ocean air like a lid or cap, according to the National Ocean Service.

A team of National Ocean Service scientists established the Modelling, Analysis Predictions and Projections program to investigate what triggers heat domes, and found the main cause was a strong change in ocean temperatures from west to east in the tropical Pacific Ocean during the preceding winter.

"This happens when strong, high-pressure atmospheric conditions combine with influences from La Niña, creating vast areas of sweltering heat that get trapped under the high-pressure "dome", the service says.

The National Weather Service issued a La Nina watch on Thursday, predicting a 50-55 per cent chance the weather phenomenon could develop in the months ahead.

As well as increasing chances for a heat dome the following summer, the La Nina weather system can accelerate the Atlantic hurricane season and increase the number of hurricanes and tropical storms.

Tropical Storm Fay is the sixth-named tropical storm of the season and will be the earliest sixth-named storm on record when it makes landfall on Friday.

The US National Hurricane Centre in Miami said that Fay was located northeast of Cape Hatteras and was expected to move north over the next couple of days with sustained winds of 45 mph, according to the Associated Press.

Forecasters issued a tropical storm warning from Cape May, New Jersey, to Watch Hill, Rhode Island, including Long Island and the Long Island Sound in New York.