Millions urged to prepare NOW as violent storms threaten more than a dozen US states on July 4th

A dangerous new threat is emerging as America's brutal heat wave finally begins to break.

Millions of people from Colorado and Wyoming to New York and New England are being urged to prepare now for severe thunderstorms, flash flooding and damaging winds through the Fourth of July weekend.

AccuWeather said the storm threat spans more than a dozen states, including Colorado, Wyoming, Nebraska, South Dakota, Iowa, Kansas, Missouri, Oklahoma, Texas, Arkansas, Pennsylvania, New Jersey, New York, Vermont and New Hampshire.

Some of the strongest storms could unleash wind gusts up to 85mph, comparable to those found in a Category 1 hurricane, along with torrential rain and frequent lightning.

Forecasters warn that the dangerous weather could create potentially life-threatening conditions, particularly in areas prone to flash flooding.

The storms are expected to disrupt holiday travel and outdoor celebrations, including fireworks displays across some of the nation's largest cities.

Heavy rain is expected to soak parts of the Midwest and Northeast, with some communities facing several inches of rainfall in just a matter of hours.

Officials are urging residents to stay weather-aware and prepare for rapidly changing conditions as the heat dome collapses.

The dangerous transition comes as a sprawling heat dome that has gripped the central and eastern US for more than a week finally begins to weaken.

While temperatures are expected to ease in some areas, meteorologists say the breakdown of the heat will come at a price.

AccuWeather Senior Meteorologist Brett Anderson said: 'The atmosphere is loaded with moisture, which will be wrung out as the heat dome collapses and slightly cooler air arrives from the north.'

That process has already begun, as more than nine inches of rain fell in just 24 hours northwest of Des Moines, Iowa, while some suburbs west of Chicago picked up around four inches of rain.

AccuWeather expects a broad swath of one to four inches of rain to fall from the Midwest to the Northeast, particularly along the Interstate 80 and Interstate 90 corridors.

In some areas, the heavy rain could extend south toward the Interstate 70 corridor.

Although the rain may be spread out over two or more days, forecasters warn that much of it could fall in only a few hours, dramatically increasing the risk of flash flooding. The weather company said localized rainfall totals could reach six inches.

Meteorologists are warning campers to avoid setting up near small streams and low-lying areas because water levels can rise quickly with little warning.

Drivers are also being urged never to attempt to cross flooded roads, as the water may be deeper than it appears or the roadway may already have been washed away.

The severe weather threat is also being fueled by intense heat and humidity, creating an environment capable of producing powerful thunderstorms.

Into Friday night, the risk of severe weather stretches from northern Mississippi and Alabama all the way to New York, Vermont and New Hampshire.

A particularly active corridor is expected from central Nebraska and southeastern South Dakota into central Iowa and northwestern Missouri, with another pocket of severe weather likely in southern Michigan.

The threat expands even further on Saturday, when severe thunderstorms are forecast along an approximately 1,800-mile zone stretching from Colorado and Wyoming to the Mid-Atlantic coast and southeastern New England.

Some of the strongest storms are expected to target heavily populated areas along the Interstate 95 corridor, including New York City, Philadelphia, Baltimore and Washington DC.

The storms are expected to develop Saturday afternoon and evening, potentially interfering with Fourth of July celebrations and fireworks displays. Forecasters warn that lightning could pose a serious threat to people waiting outdoors for fireworks, and lingering rain, low clouds or fog could also spoil viewing conditions.

The severe weather pattern is then expected to continue into Sunday and Monday.

On Sunday, storms are forecast from Missouri and Arkansas eastward to Pennsylvania and New Jersey, while another outbreak is possible across parts of Colorado, New Mexico, Wyoming, Nebraska, Kansas, Oklahoma and Texas.

By Monday, additional severe storms could erupt from near Pittsburgh to Virginia Beach, Virginia, and Greenville, North Carolina.

Even as the brutal heat begins to ease, millions of Americans may find themselves facing a new threat: a prolonged stretch of dangerous storms capable of bringing flooding, destructive winds and widespread disruptions during one of the busiest travel weekends of the year.