Isaias was a hurricane. Why do tropical weather systems cause tornadoes?

At least 20 tornadoes were reported along the East Coast late Monday and into Tuesday as Hurricane (now Tropical Storm) Isaias roared through, adding another layer of worrisome weather to nervous residents.

At least two people were killed and several others were unaccounted for after an Isaias-spawned tornado destroyed 10 mobile homes at a trailer park in Windsor, North Carolina, Bertie County Sheriff John Holley said.

Tornadoes also were reported in Virginia, Maryland, Delaware and New Jersey, the National Weather Service's Storm Prediction Center said. And as of midafternoon Tuesday, Weather Service offices had issued nearly 100 tornado warnings across 10 states, said Weather Channel meteorologist Greg Diamond.

Tornadoes like the ones that hit the East Coast during Isaias aren't uncommon during tropical cyclones, (which is the umbrella term for all hurricanes, typhoons, and tropical storms): In fact, almost all hurricanes and tropical storms that make landfall in the United States spawn at least one tornado, provided enough of the storm's circulation moves over land, according to the National Oceanic and Atmospheric Administration (NOAA).

Infographic: An inside look at the birth and power of hurricanes

Hurricanes and tropical storms offer all the necessary weather elements to spawn tornadoes: They carry small, spinning supercells inside their outer bands, which are thunderstorms that can form twisters, Live Science has reported.

Tornadoes spawned by tropical cyclones are also known to develop very quickly and be very transient in nature, the Capital Weather Gang said in a report last year. They’re also usually tough to see because of low cloud bases in hurricanes.

Fueled by warm, moist ocean air, hurricanes, with their strong wind shear near the ground, can create an unstable atmosphere when they make landfall, and their swirling vortices can flip vertically into funnels.

Live updates: Isaias leaves 1 million without power; New York City under tornado watch

Gulf Coast states tend to have the most frequent and significant hurricane tornado events, NOAA said, partly because of their tendency to have at least one state fully exposed to the right-front quadrant of the hurricane when landfall occurs there. The right-front quadrant of hurricanes contain the ingredients for tornadoes to form.

Although hurricanes can spawn tornadoes up to about three days after landfall, statistics show that most of the tornadoes occur on the day of landfall or the next day, NOAA said.

The worst tornado outbreak occurred during 2004’s Hurricane Ivan, which caused a multiday outbreak of 127 tornadoes. The deadliest hurricane-spawned tornado was in October 1964, when 22 people died in Larose, Louisiana, during a twister from Hurricane Hilda.

This article originally appeared on USA TODAY: Tropical Storm Isaias was a hurricane. Why did it cause tornadoes?