The past three summers have suggested a new weather adage for Minnesota: “In late July, the land gets dry.”
Wet springs and early summers have given way to drought anxieties by State Fair time for the past three years. But in 2014, the wettest June on record statewide may have overpowered the budding trend.
There’s simply too much water around — in saturated soils and wetlands, in brimful lakes and streams, in the very air itself — to break the rainy cycle, said University of Minnesota Extension meteorologist Mark Seeley.
Meanwhile, the jet stream above us should leave a storm-producing boundary between warm and dry air over much of the region in the coming weeks, said Jeff Johnson, chief science officer for the Burnsville-based weather division of the global energy management firm Schneider Electric.
Johnson’s models show most of Minnesota getting 3 to 4 inches of rain in August — slightly less than normal, but more than in recent years.
The national Climate Prediction Center’s outlooks for July through September are noncommittal on rainfall for most of the state, while trending toward above-normal rainfall for the Red River Valley.
Last year, July and August were drier than normal statewide except for southeastern Minnesota. In 2012, June floods were followed by a July in which much of the state received one-third normal rainfall, and a September that was the second-driest on record in the Twin Cities area. The year before that, a snowy winter and a rainy June were the setups for the driest September-October-November season on record.
Even those wet springs didn’t contribute the kind of moisture to the atmosphere that this June did, noted Peter Snyder, assistant professor of soil, water and climate at the University of Minnesota.
Snyder added that in the Upper Mississippi Valley, about 45 percent of the moisture that falls as rain is produced locally — evaporating from plants, soils and water surfaces — rather than drifting in from other regions. It’s one of about only eight regions on Earth with such a strong hydrologic link between the land surface and the atmosphere, and a big reason why this year’s potential rain supply is particularly plentiful.
Larger atmospheric systems are also at work. Snyder and state climatologist Greg Spoden said the rains of recent months were delivered by a consistent northward flow of moist air from the Gulf of Mexico — “a monsoonal flow,” Snyder called it. That was produced in turn by the interaction between a steady area of high pressure over Bermuda and a low over the mountainous Western states. And those can be moved around by forces in the Pacific.
Springs are indeed trending wetter than in the past, and late summers are drier, Snyder noted. But the past few years don’t necessarily make a long-term climate feature.
“We’re not entirely sure why it’s happening,” said. “We’re still trying to figure out the mechanisms.”