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National Weather Service Predicts Significant Midwest Flooding


Forecasters are predicting significant flooding in the Midwest this spring. To be accurate, they measure ice and snow that will eventually melt into streams and rivers. So the National Weather Service has to rely on pilots who fly low and slow to get that data. Matt Sepic of Minnesota Public Radio went up with them.

MATT SEPIC, BYLINE: It's a clear and very cold morning at a small airport outside Minneapolis as Lieutenant Conor Maginn and Lieutenant Junior Grade Mason Carroll warm up their twin-engine turboprop and run down the preflight checklist.

CONOR MAGINN: Feather, unfeather.


MAGINN: Data backup.


SEPIC: They're among 40 pilots with the National Oceanic and Atmospheric Administration's commissioned officer corps based in central Florida. Besides measuring snow, NOAA pilots handle a wide variety of scientific missions, from counting whales to flying into hurricanes. On today's flight, Carroll sits at the controls while Maginn scans the navigation charts.


SEPIC: We're soon at cruising altitude, but not for long. A half-hour north of Minneapolis, Maginn tells air traffic controllers that we'll be dropping down - way down - to just 500 feet so the instruments on board can accurately measure the snowpack.

MAGINN: Twin Otter NOAA46, blue and white survey aircraft approximately 10 miles south of field setting up for a low-level survey.

SEPIC: We're so close to the ground that trees, snowmobile tracks and the lettering on small-town water towers come into sharp view in the bright sun. Avoiding birds and broadcast towers requires intense concentration. The ride is a bit bouncy as Carroll follows the rolling landscape up and down.

With a large tablet computer, Maginn activates the radiation detector in the plane's belly that's at the heart of the mission. This box, the size of a large suitcase, picks up natural low-level gamma rays emanating from the soil. Because water blocks this radiation, Maginn can determine how much water is on the ground by putting today's measurements side by side with readings taken in the fall.

MAGINN: If we fly over the same lines, like we're doing now, we can compare those two numbers and get an idea of how much water is in the snowpack.

SEPIC: Snow can be fluffy or compact or anything in between, so forecasters rely on a figure called snow water equivalent. That's what's left over when everything melts. After measuring a 9-mile-by-thousand-foot strip of land, Maginn's computer reveals that the snowpack contains about 3 1/2 inches of water. But hydrologists on the ground need much more data, so we fly for another 3 1/2 hours, measuring nine more of the survey areas marked on the map, dipping down low each time. Many of these spots were established 40 years ago before GPS navigation. They're typically near railroad tracks and other easy-to-spot landmarks and away from large population centers. The crew also shoots photos to provide a visual reality check on the numbers. We get halfway to Canada before heading back to Minneapolis.

At the hangar, Maginn hands off a thumb drive to Carrie Olheiser from the local weather service office, who's excited to see the new data. She says it'll help smooth out the bumps in measurements still taken the old-fashioned way, on the ground by volunteers with rulers.

CARRIE OLHEISER: You have observers that take point observations. And you might have someone who is like, hey, I see bare ground. I'm going to go report a zero. And you have another observer that's like, hey, I'm going to go see how deep a snow I can find. This actually takes an average, so it takes into account those extremes.

SEPIC: Soon, Maginn and Carroll will fly up to the Dakotas and over to New England to do similar surveys. Their work is already yielding results and is helping communities along the Mississippi and other rivers prepare for possible flooding this spring.

For NPR News, I'm Matt Sepic in Minneapolis. Transcript provided by NPR, Copyright NPR.

Matt Sepic