Mayor Bill de Blasio accidentally called a tornado warning for New York City, but it's just under a tornado watch from Tropical Storm Isaias

• At a Tuesday morning press conference in preparation for Tropical Storm Isaias, Mayor Bill de Blasio said a tornado warning was issued for New York City.

• But the National Weather Service has issued a watch, which means tornadoes may occur based on the kind of storm in the area.

• A tornado warning is far more rare and means that a tornado has been spotted or indicated by radar and residents need to immediately take action.

• Follow our live updates of Isaias here.

• Visit Business Insider's homepage for more stories.

On Tuesday, New York City Mayor Bill de Blasio said during a live press conference that a tornado warning — the more serious of the two alerts — had been issued for the city. That's not the case.

"When we talk about a tornado warning, that is such a rarity here in New York City. It may sound to some people like that's not something to worry about," the mayor said. "Look, it is something to worry about. That tornado warning is in effect from now until 4 pm. today."

What the National Weather Service has put in place because of Tropical Storm Isaias is a tornado watch, which means that there is a potential for tornadoes to develop based on the kind of storm in the area, NWS Meteorologist Nelson Vaz told Insider.

A tornado watch means that one is imminent.

The mayor updated his comment by the end of the press conference.

"I misspoke earlier using the phrase tornado warning," he said."It's a tornado watch, which is less severe than a tornado warning, but doesn't change the basic reality – be aware because, again, even though we don't think of New York City as a place where you have tornadoes, it can happen here."

"It's still a tornado watch. You know, the watch and warning terminology gets people confused," Vaz said. "A tornado watch means that there is a potential for tornadoes to develop, basically for thunderstorms to develop that can produce a tornado. That threat is possible thru 4 or 5 p.m. for the New York City area."

Earlier on Tuesday, the mayor had retweeted the Notify NYC account telling residents about the tornado watch:

Vaz said meteorologists would elevate the watch to a warning if they saw something more indicative of a tornado over the city.

"We're watching radar, and if we see a storm that has a rotation that's pointing toward the strong potential of a tornado, then we'll issue a warning," he said. "The watch is for the whole city. The warning would be for a small portion of the city where the tornadic storm could track."

There hasn't been a tornado warning issued for New York City in several years, he said.

What the city can expect from Tropical Storm Isaias is heavy rain totaling between 1 and 3 inches through 4 p.m., with the threat of a storm that could produce a tornado.

It is still serious, and residents should take shelter and avoid driving, if possible.

"Strong winds are going to be the key thing," with sustained speeds between 40 to 50 mph until 7 p.m, Vaz said.

During that time, he said winds could gust up to 70 mph.

Tropical Storm Isaias has been bringing tornadoes, strong winds, and heavy rainfall to Maryland and Pennsylvania on Tuesday, after making landfall as a Category 1 hurricane near Ocean Isle Beach, North Carolina, on Monday night.

As of 12:30 p.m. ET on Tuesday, the storm was 70 miles southwest of Philadelphia. It's predicted to start impacting the New York City area by afternoon, then progress to New England on Tuesday evening. There were some reports of tornadoes touching the ground in southern New Jersey earlier on Tuesday, NJ.com reported.

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