Why is it raining so much?

The rain didn’t wait for the Three Rivers Arts Festival, which opens Friday in Downtown.

In some parts of Butler County, 4-5 inches of rain fell on the night of May 29 and portions of Route 28 and Interstate 279 were closed by flooding. Homes and businesses were inundated by rainfall and overflowing creeks last week.

According to Lee Hendricks, a meteorologist at the National Weather Service, Pittsburgh tallied 5.97 inches of rain in May and the region has received a total of 19.37 inches of precipitation in 2019, nearly 4 inches more than we typically see by the first week in June.

“We’ve been in a wetter than usual weather pattern for the last 18 months,” he said.

What’s going on? Why is it raining so much?

“This [weather] is just kind of an odd duck,” Mr. Hendricks said.

He suggested that El Nino, a natural climate cycle that warms ocean surface temperatures, is to blame. In an El Nino cycle, the circulation of cool water from ocean depths to warmer ocean surfaces slows. This causes surface water in the Gulf of Mexico to heat up. The hotter-than-normal water evaporates and that warmer, moistureladen air travels north to places like Pittsburgh. When the gulf air meets the region’s cooler, drier air, rain falls.

“If you have warm, moist air in front of a strong cold front, you’ll get a stronger reaction,” Mr. Hendricks said.

But Michael Mann, a professor of atmospheric science at Penn State University, thinks climate change is a factor, too. He sees a pattern in our erratic weather.

Last summer was Western Pennsylvania’s wettest on record, according to the weather service, and we’re heading into another equally soggy season, according to Mr. Mann. “Clearly, climate change is playing a role.”

A warmer atmosphere can hold more moisture, so Earth’s warming creates conditions conducive to more rainfall, he said. The continental U.S. just had its wettest 12 months in 124 years of records.

Mr. Mann said that’s not normal, and he uses a game of craps to illustrate. Natural weather cycles should turn up “sixes” — persistent rain — once in every six rolls, he said. But with climate change, the dice are loaded and sixes are coming up more often, he said.

“Part of what’s going on is that there’s a change in the configuration of the jet stream,” Mr. Mann said.

The jet stream is driven by contrasts in temperature. The greater the differences in air temperatures, the quicker the jet stream moves and the weather changes. Due to climate change, the Arctic is warming faster than the rest of the Western Hemisphere. When the differences in global temperatures are smaller, the jet stream slows.

That’s why Pittsburgh can’t dry out, Mr. Mann said. The warm, moist part of the jet stream is staying over the region, and the extreme weather persists. But he said climate change isn’t solely responsible for our rainy days and property damage.

“It’s probably a combination of natural variability, human-caused climate change, and land use and development.”

Tobiah Bilski is a research manager at Jones Lang LaSalle Pittsburgh, a local commercial real estate firm. He said developers in Allegheny and Butler counties have graded and built upon 54.5 million square feet of land since 2005. So when 57.83 inches of rain falls in the region, as it did last year, there is less undeveloped land to catch it.

Grass, trees and plants absorb rainfall and water from creeks and rivers. When these natural sponges are replaced with pavement and brick, rainwater swells waterways, floods roads and finds its way into basements.

Of course, archaic drainage systems clogged with debris also contribute to flooding, Mr. Mann noted.

All is not dreary, however. Mr. Hendricks sees hope of drier days ahead. “The pattern seems to be slowing down a bit.”

Just in time for the arts festival.