A derecho can cause damage more like an inland hurricane than a quick, but more powerful tornado.
At least nine people were killed, including a 5-year-old girl, and millions of others were left without power.
At least one person was killed when one of the storm's twisters hit a mobile home park in North Carolina.
The governor said the state is anticipating power outages and asked residents to have a week’s supply of water, food and medicine on hand.
Forecasters downgraded Hanna to a tropical storm early Sunday.
The storm is expected to keep inundating the northern Gulf Coast well into Monday.
Roads flooded in coastal Louisiana and Mississippi, and thousands were without power even before the the storm made landfall.
Bertha's maximum sustained winds were near 50 mph as it came ashore but it was expected to weaken to a tropical depression after moving inland.
Arthur formed Saturday in waters off Florida, marking the sixth straight year that a named storm has developed before June 1.
Meet Brody, the Tampa Bay weather dog winning hearts across social media.