Forecasters warn storm Alberto, a still-menacing depression after its Monday landfall on the Gulf Coast, is scattering heavy rains around the southern US amid heightened risks of flash flooding.

The first named storm of the 2018 Atlantic hurricane season, Subtropical Storm Alberto lumbered ashore Monday afternoon in the Florida Panhandle and then weakened to a depression overnight as it trekked inland.

The storm that sprang from warm waters of the Gulf of Mexico was now a vast, soggy system as it headed inland, dumping heavy rains in bursts all around the region.

Though the storm had weakened, forecasters, warned, it was capable of potentially life-threatening flash floods in the coming hours or days as the vast system spreads over Alabama and large areas of Georgia, Tennessee and the Carolinas.

Authorities did not directly attribute any deaths or injuries immediately to Alberto.

But in North Carolina, the state's governor noted a television news anchor and a photojournalist were killed Monday while covering the fringes of the large system.

A tree that became uprooted from rain-soaked ground toppled on their SUV, killing them instantly, authorities said.

Between four and eight inches (10-25 centimetres) of rain could soak the Florida Panhandle, Alabama, and western Georgia before the storm moves on.

Isolated deluges of 12 inches (30 centimetres) also are possible in spots as the system heads toward the Tennessee Valley on Tuesday and later this week into the Ohio Valley and Great Lakes region.

Earlier Monday, Alberto rolled up big waves and tides along beaches of the northern Gulf Coast.

Lifeguards posted red flags along the white sands of Pensacola Beach, where swimming and wading were banned as Alberto disrupted long holiday weekend plans for millions.