Why did Debby spawn tornadoes? Millions of people in four states under tornado watches as storm moves north

Millions of people in four states and the District of Columbia were under tornado watches on Friday as Debby, now a post-tropical cyclone, moved through the mid-Atlantic and Northeast.

Here are where tornado watches are currently in effect, according to the latest update from the National Hurricane Center:

• Maryland

• Eastern Pennsylvania

• Delaware

• New Jersey

• District of Columbia

There have already been at least seven confirmed tornadoes associated with Debby. The Associated Press reported that one person died in a home damaged by a tornado in Lucama, N.C., on Thursday, raising the storm's overall death toll to seven.

????? Why do hurricanes and tropical storms produce tornadoes?

Hurricanes and tropical storms are known for their high winds and drenching rains. But they can spawn tornadoes, as evidenced this week, especially after making landfall.

“When a hurricane or tropical storm moves over land, the friction increases. Friction slows the winds at the surface while the winds remain stronger in the upper levels,” WCNC-Charlotte explained. “This creates the key ingredient for tornadoes, wind shear. Wind shear is the change in wind speed and direction with height. This creates a spin to the atmosphere.”

All of the confirmed tornadoes have been categorized as either EF0s or EF1s — the lowest ratings on the Enhanced Fujita Scale, which rates tornadoes based on estimated wind speeds. EF0s have top wind speeds of 65 mph to 85 mph. EF1s have top wind speeds of 86 mph to 110 mph.

“Usually, tornadoes produced by tropical cyclones are relatively weak and short-lived,” the National Weather Service said Friday. “But they still pose a significant threat.”

??? Confirmed tornadoes associated with Debby

• Isle of Palms, S.C. (Aug. 5)
  Estimated peak wind speed: 87 mph
  Rating: EF1

• Edisto Beach, S.C. (Aug. 5)
  Estimated peak wind speed: 97 mph
  Rating: EF1

• South Edisto, S.C. (Aug. 5)
  Estimated peak wind speed: 90 mph
  Rating: EF1

• Kiawah Island, S.C. (Aug. 5)
  Estimated peak wind speed: 85 mph
  Rating: EF0

• West Ashley, S.C. (Aug. 6)
  Estimated peak wind speed: 77 mph
  Rating: EF0
  

• Lady’s Island, S.C. (Aug. 6)
  Estimated peak wind speed: 85 mph
  Rating: EF0

• Moncks Corner, S.C. (Aug. 6)
  Estimated peak wind speed: 95 mph
  Rating: EF1