Does the urban core get more or less rainfall than the Twin Cities exurbs? Or, why is it dry in my garden?
Sitting in Mears Park at Twin City Jazz Festival after six weeks of drought, I could barely focus on the song “Summertime,” as belted by the Jazz Women All Stars. My eyes kept returning to my phone, where the forecast for the first rain in weeks appeared in bursts of chaotic color. It was a near lock, a 90% chance of rain, said the Weather Underground app. Any minute now, there’d be a downpour in downtown St. Paul.
Only there wasn’t. The clouds lingered on the horizon but, despite much murmuring, the crowds stayed dry. I watched the radar for hours as wave after wave of storms hurtled up toward the Twin Cities. Like magic, each rainstorm dissipated when it reached the suburban edge of the Twin Cities. Big blobs of clouds evaporating into nothing, thunder rumbling in the distance all evening as rain refused to fall on St. Paul.
“It’s the urban heat island keeping the rain away,” I proudly told some friends. There’s a well-documented effect around urban areas — mostly albedo (heat reflectivity) from asphalt and impervious surfaces — that keeps cities a few degrees warmer than the surrounding countryside.
Days later, watching the plants in my garden perk up after Frogtown finally got precipitation, I kept thinking about the phenomenon. Was it true? Does rain stop at the metro border? Does too much asphalt in cities keep the clouds away?
For an explanation, I turned to a long-time colleague, and my go-to weather obsessed expert, Kenny Blumenfeld, senior climatologist at the Minnesota State Climate Office. It turns out that the answer is as hazy as the weather forecast. Sometimes, urban areas push rain away; sometimes they don’t. A lot depends on the specific details of the weather event, and in this case, the answer was … cloudy.
“Urban areas are really complicated,” Blumenfeld told me. “For 50 years, we’ve known that urban areas can enhance precipitation in some areas, and remove or decrease precipitation in other areas in certain conditions.”
Uplifting news
First, the basics. Rain happens when warm air rises and cools, causing moisture to condense into raindrops and fall to the ground. A lot of factors play into that. For example, mountain ranges force air upward and famously cause rainy and dry climates. Being much smaller, cities don’t have nearly the same level of effect on air masses, though particulate pollution can help raindrops form under the right conditions.
“If you think of the change in the surface characteristics, it’s not a quick or smooth rise,” explained Blumenfeld, describing city topography. “Instead it’s got a lot of other inflections going on too, [and] air can move through and between buildings, for example.”
Lacking mountains, most rainfall in the Twin Cities relies on fronts to push warmer air upward, causing rain to fall. And as it turns out, the urban heat island doesn’t have much of an impact on that kind of precipitation formation.
That said, there’s a distinct (and somewhat controversial) urban pattern called the La Porte Weather Anomaly, named after the city of La Porte, Indiana where it was first discovered. For many metro areas, like Chicagoland where La Porte is located, precipitation tends is higher in the downwind (in this case, southeastern) parts of the metro areas. In other words, precipitation sometimes moves around cities and concentrates on the far side.
Blumenfeld explained that this only occurs when prevailing winds are weaker, and isn’t a huge deal. But it does mean that certain parts of the metro area, roughly the area around Inver Grove Heights or Cottage Grove, see a bit more precipitation than other parts of the city.
On the other hand, much of the time this La Porte effect is drowned out by other weather factors. In the end, rainfall tends to even out. The rain falls mainly evenly, everywhere.
“With that exception, Minneapolis and Saint Paul both get about as much annual precipitation as all of the rest of the Twin Cities’ metro,” Blumenfeld. said. “Minneapolis gets as much as Roseville, and Bloomington and Saint Paul get as much as Maplewood. There’s no real meaningful difference between the precipitation in Chaska and downtown Minneapolis over the course of a year.”
(There went my theory about rainstorm avoidance.)
What happened this weekend?
During the weekend, the ceaseless march of storms disappearing when they hit the Interstate 494 beltway was mostly an optical illusion. Blumenfeld explained to me that, last weekend, there weren’t enough atmospheric forces to keep the rain storms strong. Instead, they were driven by something called “outflow boundaries” (little bursts of cold air) that meant that storms formed and disappeared repeatedly throughout the day rather randomly.
“Over the weekend when those storms were behaving chaotically, I don’t think you could find a true urban influence on those storms,” Blumenfeld said. “There wasn’t any really good atmospheric processes to control everything, so the [outflows] were sort of calling the shots on where the storms were gonna be.”
My meteorological conspiracy theory dashed, the good news is that the core city (and my backyard garden) are as likely to see the end of a drought as anywhere else in the seven-county metro. After weeks of record-low rainfall, I remain hopeful that this summer won’t be as dry as the last one.
“Cognitively, people can’t really process it,” explained Blumenfeld, describing why we see patterns in the clouds. “People assume that storms are missing them and hitting somewhere else, when in fact most of us are observing storms behaving quite normally, with all their variations and bobs and weaves and head fakes. We let our perception and inability to process real time influence our judgment.”
In other words, as Blumenfeld tried to nicely put it, I was imagining things, a middle-aged man yelling at a cloud.
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