Winter storm warning: This one could break April snow record

With more than a foot of April snow potentially piling up by Friday, winter-weary Minnesotans could be forgiven for hoping reality will fall short of the predictions. Not likely.

If there's a crystal lining in the endless of barrage of snow clouds that have dumped their frozen vapor on the Twin Cities this year, it just might be the almost unerring accuracy of local meteorologists during the winter of 2013-14.

So when the National Weather Service says the storm expected to get underway Thursday night and build through the Friday morning commute could top the record of 13.6 inches set in 1983, people take notice.

"We have a chef from Los Angeles so the weather gets discussed quite regularly around here," said Jane Oyen, the owner of C. McGee's Deli in the North Loop, said Wednesday. "We've all been noticing how they've done a lot better this year, they just have. If they say we're going to have a blizzard, we generally do."

Exactly, say forecasters at the National Weather Service in Chanhassen. And they can prove it.

They keep a statistic called the Probability of Detection and that county-by-county analysis shows their winter storm warnings have been on target 92 percent of the time since November.

Experts say the enhanced accuracy comes from a man-and-machine combination of factors: Wily, longtime forecasters with well-honed "pattern recognition" skills pass down their expertise to younger meteorologists, who just happen to also have faster and higher-resolution computer models to exploit.

"It's both the computer models and the forecasters themselves," said Tony Zaleski, one of the meteorologists in Chanhassen. "We've got a forecaster who came up from South Carolina and he wasn't in the actual groove for the winter weather that we get up here the last two years, but he's improved."

Local weather guru Paul Douglas said that human factor can't be overlooked.

"If you totally depend on computers, you're going to get burned," Douglas said. "You still need people — meteorologists — to go in an interpret the models."

Douglas said this winter's 50 days of subzero temperatures actually made it easier to forecast snowfall amounts because it was so darned cold, meteorologists didn't have to mess with sleet turning to rain turning to snow and back to sleet.

"Those mixed precipitation types can mean the difference between 2 inches and 12 inches," said Lisa Schmit, another National Weather Service meteorologist.

This week's "event," she said, could be tricky as it tracks northeasterly from Iowa on Thursday evening and turns from that dreaded wintry mix of slushy rain to snow. Most of the snow will fall between 1 a.m. and 7 a.m. Friday in a swath framed by Redwood Falls, Mora, Red Wing and Rice Lake, Wis. — including the Twin Cities.

Like waiting for a baby

Predicting snow depth is like predicting what a baby will look like before the mother gets to the hospital, according to Craig Edwards, the retired chief meteorologist in Chanhassen who serves as the Twins forecaster.

"The snowflakes haven't been born yet and, a lot of times, they literally develop right over Minnesota," he said. "Laying out an amount a couple of days ahead is like saying the baby will be 20 inches and 7 pounds and 5 ounces."

The key, he said, is predicting an amount a few days ahead and fine-tuning the estimate as the storm inches closer. Better, faster computer models from Europe and Canada have made a huge difference over the last seven years, said Zaleski, who considers five different high-tech programs when preparing his forecasts.

"The models used to flip-flop," he said. "Individual models would have high pressure over you on Day 6 and on the next run 24 hours later, you'd have a trough at the same time."

Now the higher-speed computers can crunch data faster, improving accuracy.

And lead times, or how many hours in advance the public gets warned, have improved to 20 hours this winter — giving people nearly a full day to, well, get depressed.

Highly accurate, not perfect

Schmit said the decision to trigger a winter storm warning from a watch "has to do with our confidence." When they're 60 percent sure "an event" is going to happen, the warning goes out.

And 92 percent of the times this winter when they've warned a county, at least 6 inches has indeed fallen.

So go ahead and bet they'll be wrong this time. Your optimism has an 8 percent chance.

"It's getting better, but it's never going to be perfect," Paul Douglas said. "And no matter how good we get, people tend to remember the times we blow it."

Curt Brown • 612-673-4767