Pittsburgh just had its wettest decade on record
2019 saw city’s third-highest rainfall
Pittsburgh hasn’t just felt super rainy — Year-end and decade-end totals from the National Weather Service prove it.
The year 2019 ended with 52.46 inches of rain — the third-wettest year on record since 1871. The year before had the most rain ever — 57.83 inches. And the second-wettest year also came in the last 15 years — 57.41 inches in 2004, when Hurricane Ivan hit in September.
“We do expect that this trend will continue,” said Josef Werne, a professor in the department of geology and environmental science at the University of Pittsburgh. “It will get warmer and wetter in Pittsburgh.”
The last decade in Pittsburgh was notably more wet than any decade on record dating to 1920, based on data provided by the National Weather Service. It rained 424.6 inches from 2010-19, with no previous decade getting even 400 inches. From 1920 to 1999, no decade got more than 388 inches.
One theory: Blame the jet stream, says Mr. Werne.
Climate change is warming the Arctic faster than the tropics, he said, decreasing the difference in temperature from the tropics to the poles. That change in temperature gradient is affecting the shape of the jet stream — a strong westerly wind that forms at the boundary of air masses with significantly different temperatures.
The jet stream is becoming less straight, turning more wavy and meandering, he said, making it harder for weather systems to pass from one place to another.
“Basically, the problem is that we have these weather systems — instead of moving through as they used to, they get stuck,” he said. “In Pittsburgh’s case, what that means is that we’re parked in a place producing a lot of rain for us. It’s not necessarily more rainy days, but more volume in those rainy days.”
A stuck weather system might be like the one on Sept. 1 of last year, when Pittsburgh saw 3.38 inches of rain in one day. Or Sept. 9, 2018, when 3.73 inches fell — the second-wettest day ever recorded at the Pittsburgh airport.
A few large one-day storms can have a significant effect on yearly rainfall totals.
“That’s pretty much what’s happening with some of those higher value days that we’re having in the summer,” said Shannon Hefferan, meteorologist with the National Weather Service. “Three inches in one day? Around here we get that in a month.”
The northeastern United States, including Pennsylvania, saw a 71% increase in “extreme rainfall events” from 1958 to 2012, according to Pennsylvania’s official Climate Impacts Assessment — last revised in 2015.
Those rainfalls also have real-world repercussions, from flooding to landslides to crumbling roads. A wet spring can affect farmers’ ability to get crops in the ground, said Mr. Werne, and warmer winters are thought to have some role in increasing the number of ticks carrying Lyme disease.
While the jet stream is one factor, there are also other potential causes. Warmer atmospheres can hold more water, said Ms. Hefferan, and the average temperature in Pittsburgh rose about two degrees from 1950 to 2015, according to data from the National Oceanic and Atmospheric Association.
It is also possible that increased irrigation for crops in the Midwest is evaporating back into the air and causing higher moisture in Pittsburgh, said Pat Herald, a meteorologist with the National Weather Service.
“There’s many variables involved, so you have to be careful about laying any simple answer on it,” he said.
Studies predict it will only get wetter. The Climate Impacts Assessment predicts that precipitation will increase another 8% by 2050, with a 14% increase during winters, when it will fall mostly as rain instead of snow.
One study in the journal Nature Communications predicted that Pittsburgh’s climate in 2050 will resemble the present-day climate in Jonesboro, Ark., which is 10.8 degrees warmer and 46.8% wetter than Pittsburgh.
And the forecast for the weekend calls for … more of the same. About three-quarters of an inch of rain fell on Friday by 4 p.m. and rain is expected to continue Saturday, changing to snow and accumulating about half an inch before stopping early Sunday.