Potent storm to raise floodwaters to historic levels over parts of central US
By Alex Sosnowski, AccuWeather senior meteorologist
April 18, 2019, 3:04:16 AM EDT
An intense spring storm will bring enough rain to worsen existent flooding in the central United States.
The same storm that is forecast to bring another severe weather and tornado outbreak to the South and part of the Midwest through Friday will also bring areas of heavy rain to the Mississippi Valley, the Tennessee Valley and the upper Great Lakes region into Thursday.
"The large and slow-moving nature of the storm this week is a problem for flooding concerns," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.
As feared, heavy rain has been pulled farther north with this week's storm, when compared to that of recent weeks. The region has many weeks to go before the "all clear" can be sounded for small stream to large river flooding this spring.
Rainfall on the order of 1-2 inches with local amounts between 3 and 5 inches can occur in two main zones with the event.
One zone will overlap the severe weather threat area mainly south of the Ohio and lower Missouri rivers.
A second area will likely focus farther north from portions of Iowa and southeastern Minnesota to Michigan.
With this amount of rainfall on saturated ground, there is an immediate concern for urban and small stream flooding in both of these zones.
In the southern part, the rainfall may come in a couple of short bursts lasting a few hours or less. Where 1-2 inches of rain falls in as many hours on saturated ground there is a classic recipe for flash flooding.
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During the first half of April alone, some areas have received two to five times the average rainfall for the two-week period in parts of the South Central states.
In the northern tier, there remains a significant amount of snow on the ground. Within that snow cover, there are 1 to 5 inches of water locked up.
While heavy rain from the big storm last week did not coincide with the existing snow cover, the same storm deposited up to a couple of feet of new snow in some areas.
Some of that snow may remain long enough to be washed away by the new storm this week. This may have the effect of doubling the amount of water released from the storm.
The existing snow cover and heavy rain lies over the headwaters of the Mississippi River and is likely to have a long-term impact on the waterway in the weeks ahead, where a new surge and crest are likely to progress farther downstream.
Later this month, at Mitchell, South Dakota, the James River is forecast by National Weather Service hydrologists to crest just shy of the record level during April 2001.
Interests along some of the secondary, short-run rivers in Minnesota, Wisconsin and the Upper Peninsula of Michigan should be on the lookout for significant flooding this week.
Enough rain may fall on river systems in Iowa and eastern Nebraska to be of concern with the storm as well this week. At the very least, rivers will be on the rise but short of record crests.
Big rain from this week's storm may bypass much of the upper Missouri and Big Sioux basins. Levels on these rivers remain at major flood stage, set into motion by prior melting snow. However, melting snow to come farther upstream could lead to additional rising along these rivers.
"On a positive note, we expect storms over the Central states to trend less intense in general during the last week in April," Pastelok said.
"Less-intense storms tend to move along at a faster pace and tend to reduce the amount of rain that falls along their path."
Even though storms may not be as strong and may move along more swiftly heading into May, the high state of water levels in the Central states will remain a loaded gun.
It may only take one big rogue storm to cause major trouble in these basins this spring.
It may not take a large storm to cause flooding at the local level.
Large meandering rivers such as the Mississippi and Missouri usually take many weeks for floodwaters to crest and then return to their banks after a heavy rain and rapid snowmelt event.
When rounds of heavy rain continue, they complicate and can greatly extend the period of flooding. That remains the case this spring in the Central states.
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