A rare tornado warning was issued Tuesday for the New York City area, triggered by severe thunderstorms that rolled through the region. While it’s uncommon for twisters to hit major cities like New York, scientists say that may not always be the case.

There’s no special reason why tornadoes seem to miss big cities. It just comes down to chance. Densely populated urban areas such as Los Angeles, Chicago or New York are relatively small specks amid vast stretches of forests, mountains or otherwise rural regions.

“Cities are small targets,” said Victor Gensini, a climatologist at Northern Illinois University in DeKalb. “If you think of a dartboard, a dense urban area is really the double bull's-eye. It’s very rare for a downtown metro business district to get hit.”

But, as cities grow and developments spill beyond city limits, these areas become part of an “expanding bull's-eye” effect, as described by a 2015 study published in the magazine Weatherwise.

“There’s an enormous increase in vulnerability and exposure just due to population density,” Gensini said. “I could give numerous examples in Illinois, where I live, where tornadoes would have normally blown through corn fields, but now they’re going through urban areas.”