DULUTH — Every additional inch of snow on the ground across the Northand is increasing the risk of spring flooding as a near-record snowpack sits waiting for the inevitable melt.
The National Weather Service in Duluth on Thursday updated its spring flood outlook to say the risk of floods has again increased in recent weeks as snowfall piles up.
Meteorologists warned that if you've had issues with spring flooding in the past, it's likely it will occur again this year.
“Most river basins have an extreme, near-record, above-normal amount of water in the snowpack, increasing the risk for spring flooding,’’ the Weather Service reported Thursday. “Snow amounts and snow water equivalent continues to increase across the area.”
Much of the region is now covered by snow equaling 4-14 inches of rain.
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The deepest snowpack — snow still on the ground — remains in the Arrowhead region, especially along higher elevations inland from Lake Superior, where snow is 4 feet deep in some areas. Those areas will be prone to flash-flooding as North Shore streams tumble fast into Lake Superior.
In other areas, the St. Louis River has a slightly elevated risk of flooding near Duluth, while the St. Croix and Chippewa rivers in Wisconsin are likely to see flooding.
The Mississippi River south of Grand Rapids, especially near Aitkin and downstream, has a greatly elevated risk of flooding.
The Weather Service says there’s a 60% chance of moderate flooding along the St. Louis River near Scanlon and a 95% chance of moderate flooding along the Mississippi River near Aitkin.
Because of lesser snowpack in much of its watershed, the Weather Service says the risk of flooding along the Rainy River system — from the Boundary Waters west to Lake of the Woods — is only slightly above normal. Much of the water from last year’s flood came from areas of Ontario where less snow is on the ground this year.
The Weather Service says any flooding will depend on how fast the current blanket of snow melts and whether that melt comes with more water from spring rains. That’s what happened last year when a late melt coincided with near-record spring rains, causing flash flooding along the North Shore, wiping out bridges and culverts and causing devastating, record, long-term flooding along the Rainy River watershed.
A slow, gradual melt with little rain could alleviate most of the flooding risk, with nighttime low temperatures a key factor. If lows continue to dip below freezing, the melt slows down. If lows stay above freezing, the melt speeds up.
Another mitigating factor is frost depth. With the thick blanket of snow insulating the ground, there is much less frost than normal in the ground, which should allow more water to soak in, lessening flooding.
Most of Minnesota is in the 80 percentile for snow depth, meaning only 20 years out of 100 have this much snow. Parts of Northeastern Minnesota are in the top 1 percentile — near-record snow depth.
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Duluth has officially received 125.3 inches of snow this winter, already the sixth-snowiest on record and just 10 inches shy of the all-time record. Areas to the northeast of Duluth, and along the South Shore snowbelt, have received even more snow.
Brainerd with nearly 80 inches is just an inch shy of its record-snowiest winter, while Spooner, Wisconsin, at 91 inches, is at the fourth-snowiest on record.
“We have so much snow water on the ground … that we think flooding is likely,’’ said Joe Moore, National Weather Service meteorologist in Duluth. “All this water sitting on the ground in snowpack has to go somewhere.”
The heavy snowpack has virtually eliminated an increasing drought that was creeping into the region last fall, meteorologists noted Thursday.
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