WASHINGTON

A record-breaking heatwave is tearing through the central and northeastern US on Wednesday as the southern state of Texas prepares for the first named storm of the season.

Excessive heat warnings have been issued across a broad swathe of the country stretching from New York City in the northeast to northern Indiana in the Midwest as officials prepare for heat indexes to again surpass 100 degrees Fahrenheit (38 Celsius).

Such prolonged extreme heat this early in the summer is exceptionally abnormal as scientists continue to sound the alarm about the mounting effects of climate change.

In Maine, the US' most northeastern state that is usually home to cool Atlantic breezes, heat indices have already soared past 100 degrees Fahrenheit amid stiflingly high humidity.

"A dome of very warm high pressure over the area will bring an exceptionally hot day today with inland highs in the mid 90s. Very humid air with dew points in the low to mid 70s will result in heat indices over 100 across most inland locations," the National Weather Service warned.

The heat wave is expected to persist through the week's end, but forecasters are warning that above-normal temperatures could continue to wrack the region well into July.

Tropical Storm Alberto, meanwhile, is already lashing southern Texas with flash flooding and a storm surge as it forms in the Gulf of Mexico ahead of making landfall in northeastern Mexico, likely early Thursday.

The National Oceanic and Atmospheric Administration has warned of a busy upcoming hurricane season with between 17 to 25 tropical storms expected in what would mark the worst hurricane season in two decades. Between eight to 13 storms are likely to become hurricanes, including four to seven major hurricanes, NOAA said in May in its hurricane outlook.