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What causes ice storms like the one that recently wreaked havoc in the Twin Cities?

The Dec. 28 storm resulted in around 500 crashes, dozens of severe injuries, and two deaths.

The ice storm on Dec. 28 resulted in around 500 crashes, dozens of severe injuries, and two deaths.
The ice storm on Dec. 28 resulted in around 500 crashes, dozens of severe injuries, and two deaths.

Early morning on Dec. 28 a surprising and deadly glaze of ice descended on the streets of the Twin Cities metro area. As rain fell through the warming air and landed on the cold ground, it promptly froze into a glaze of ice just a few hundredths of an inch thick that covered every surface in the city.

The resulting landscape wreaked havoc on the automobile infrastructure of the city. Cars, semis, and even fire trucks careened into each other or slid down any hill or ramp they could. Some brave drivers inched along, while elsewhere, hockey kids strapped on skates and glided down sidewalks. Others, myself included, simply looked out their door, said “no thanks,” and canceled plans.

But the storm resulted in around 500 crashes, dozens of severe injuries, and two deaths. It was a terrible example of how ice storms can challenge our cities in new ways that might — or might not — be becoming more common in an era of out-of-control climate change.

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City glazing 101

Ice storms like the one on Dec. 28 — and an earlier equally tragic event in November 2010 — happen because of inversion between the air temperature and the pavement temperature. Rain hits the colder ground and instantly freezes, creating the kind of glaze found only in doughnut shops — only deadly.

“With a freezing rain event, the precipitation forming in the clouds aloft starts out as snow, because temperature is below freezing up in the middle levels of the atmosphere,” explained Caleb Grunzke, a meteorologist at the National Weather Service (NWS). “But as snow is falling to the surface, the temperature gets warmer and goes above freezing. Those snowflakes melt into raindrops, and then just before the rain drops fall to the surface, the temperature dips back below, freezing at the surface.”

Compared to a snowstorm, this kind of event is difficult to predict for meteorologists like Grunzke. There’s a higher degree of error because, around the freezing point, small differences can have big impacts on the form of precipitation.

“It can be very tricky just to determine if the [air] layers are warm enough to be able to melt those snowflakes and turn them into liquid water,” explained Grunzke. “You only have to be 1 or 2 degrees off in temperature, and not even that, to completely change your precipitation type.”

That means that ice storms can often come a surprise for maintenance crews. And, because rain easily washes salt away, even if there is some advance notice, there’s not much that maintenance crews can do to prepare streets for the freeze.

How a city gets a grip on an ice storm

Twin Cities’ public works have long been geared toward dealing with the challenges and rigors of snowfall, where teams of plow drivers work in shifts for days, heading out to the roads, sanding and salting, and clearing away the accumulation. Even if it’s difficult, it’s a challenge that the crews are used to.

Ice is another story, and poses different dangers. The rain that fell on the 28th is a good example, where Minneapolis maintenance trucks could not do much for most of the city’s streets.

“What happened was that we got some rain very early in the morning,” explained Mike Kennedy, the director of Transportation Maintenance and Repair for the City of Minneapolis. “At 5 to 6 a.m., some rain came down and then the pavement temps dropped below freezing and the rain froze to the pavement. It was a black ice situation. It happened all over the entire metro, and it posed a real problem for drivers and for everyone.”

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 The Minneapolis crews eventually began sanding the roads, but with so many city streets to cover, they could only sit and wait for the weather to warm up.

“One of the biggest problems is that it happens everywhere, it isn’t just a couple of streets,” said Kennedy. “With 1,000 miles of streets flash frozen like that, sometimes it seems like there’s a public expectation that we can be instantaneously on every street. But it takes a whole day to get through every street, even with the most aggressive response. [Meanwhile, the ice] sort of basically took care of itself by noon.”

But in the intervening hours, before the melt, the entire city’s transportation network was thrown into chaos. For hours, Metro Transit canceled all bus service, and only the light rail lines, which perform well in almost every kind of weather, remained running.

Sign of things to come?

The kind of street-glazing weather event is not typically what people expect in Minnesota, where blizzards, below-zero temperatures, and months of snowpack have long been the norm.

“It doesn’t happen too often up here that we get very significant ice storms like that,” explained Grunske from the NWS. “It’s commonly more of an issue down south in the southern plains, [which are] naturally warmer. Sometimes a cold front makes its way down to Oklahoma or Kansas, and has a layer of subfreezing air at the surface where the rest of the low level air above it is warmer.”

Here in Minnesota, ice storms have typically been more prevalent in parts of the state with some topography. But with climate change disrupting longstanding patterns, it is changing Minnesota winter most of all. As many maps point out, the future Twin Cities’ climate is more likely to resemble that of places like Kansas City, Tulsa, or Evansville, Indiana. The warmer winters will bring more precipitation in general, and more freeze-thaw cycles.

However, the climatologists I asked were unanimous in hedging their bets about whether climate change translates into more freezing rain events.

The future Twin Cities’ climate is more likely to resemble that of places like Kansas City, Tulsa, or Evansville, Indiana.
The future Twin Cities’ climate is more likely to resemble that of places like Kansas City, Tulsa, or Evansville, Indiana.
“There are places in Minnesota much more susceptible to freezing rain events than the Twin Cities, such as the Buffalo Ridge in southwest Minnesota and the north shore of Lake Superior,” said Peter Boulay, a climatologist at the Minnesota Department of Natural Resources. “While the elevation change isn’t very great [in Minnesota], it is enough that the air is slightly cooler and has a greater chance for ice accumulation. Stronger winds at higher elevations may play a role too.”

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For his part, Kennedy, who has worked on plowing Minneapolis streets for three decades, has been noticing the warmer winters. But he wasn’t willing to say whether or not ice storms are becoming more common.

“The trends? I’m just here in the cheap seats, but we seem to be seeing a greater variation in the weather,” said Kennedy.

Kennedy is far more confident in predicting his annual budget, which he says has become much more volatile during his career.

“In the last 10 years or so those swings have been much greater,” he said. “We’re either way over the budget, or way under the budget. We’ve seen that trend.”