The winter of 2019-20 has been warm and wet in much of the north-central U.S. North of Interstate 80, much of that precipitation has come in the form of snow; snow that, when it melts, is likely to flow straight into swollen rivers and streams and cause more flooding when spring 2020 rolls around. That was the message in a monthly forecast update webinar presented by the NOAA Central Region forecast office Thursday, Feb. 20.
Forecasters and climatologists with NOAA and USDA noted snowpack in the interior U.S. has a wide variance. The snow is deeper and heavier in cover in northern states, but is actually below average in the southern part of the central U.S. "Across much of the southern Midwest, we don't have any snowpack right now," said Illinois State Climatologist Trent Ford. Farther west and north, it's a different story; the Missouri River basin snowpack is near to above average. "Even out west, we are seeing that headwater snowpack is near to above normal," Ford said.
A review of snowpack in the northern U.S. from Montana to the Great Lakes shows that in only a portion of the region, western North Dakota and northeastern Montana, is the snowpack below average. In most other areas north of Interstate 80, above to much above average snowpack is in place. Of special note is an area from central South Dakota east to southern Minnesota and south through northern and central Iowa, where snowpack amounts are from 250% to 750% of average.
From this point on, the rate of snow melt will be a key feature in determining the severity of flooding. "Just because the snowpack is high doesn't mean that flooding will occur; it depends on how the snow melt occurs," said Trent Ford. But, when snow melt does occur, Ford has no doubt that soaking in -- infiltration -- will be minimal, due to saturated soils. "Depictions of abundantly wet soils are accurate," he said. "We have an incredible amount of water. And, there will still be a lot of moisture around coming out of winter into spring."
Portions of the central U.S. have had some reduction in soil moisture compared with a year ago. But Ford said that does not mean drier conditions have developed. "We're comparing near-record or record levels last year and not as much of a record category this year," he noted.
The three major rivers in the central region of the U.S. -- the Mississippi, Missouri and Ohio -- are all in an elevated risk for flooding. Ford noted the risk of major flooding is much above normal in both the Mississippi and the Missouri river basins. "Long-duration flooding is a possibility. The rate of snow melt, additional snow and how many heavy spring rains we have will influence that (severity)," he said. He noted the Ohio Valley is slightly drier than a year ago. However, "there are 43 gauges on the Ohio with an elevated risk of flooding," Ford said.
Bryce Anderson can be reached at Bryce.firstname.lastname@example.org
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