FARGO — The risk of a severe spring flood has ratcheted up after back-to-back winter storms delivered heavy moisture that could produce a crest that rivals or even exceeds the record 2009 flood.
Forecasters warn that a "Top 10" spring runoff awaits the Red River Valley.
Fargo-Moorhead now has a 5 percent risk of confronting a 41.4-foot Red River flood crest, which would top the record 2009 flood, and there's a 10 percent chance of nearly equaling the record, the National Weather Service said in a new forecast issued Friday, March 15.
A 41.4-foot crest is approximately the level of the 100-year flood, as defined by the Army Corps of Engineers, and the level the $2.75 billion diversion project is designed to protect against once it's built. The 100-year flood ranges from 41.4 feet to 42.1 feet, assuming levees hold.
The updated forecast follows two recent wet and snowy storms that raked the region, adding to an already deep snowpack.
"Red River Basin runoff risk has jumped markedly," the weather service said in a statement. "Moderate to major flooding is nearly guaranteed, especially along and near the main stem Red. Significant overland and rural flooding is also likely."
Some good news: Forecasters expect the weather over the next two weeks to bring a gradual spring thaw.
"Fortunately, I don't have any winter storms to talk about at this time, at least none coming in on the horizon," said Greg Gust, a weather service meteorologist.
But that is tempered. "There's no guarantee that the end of March and beginning of April will cooperate," he said. In fact, Gust added, weather forecasting models show the possibility of a cold dip in late March or early April, and the possibility of a stormy pattern.
Significant overland runoff and rural flooding is now considered likely in most areas, with less of a chance in northeast North Dakota.
"Significant snow melt flooding is likely, and runs from above to much above long-term historical averages across the Red River and Devils Lake Basins," the weather service flood outlook report said.
The ingredients are building for a major flood: high fall moisture, normal stream flows, deep frost, very high snowpack, very high snow water content and a spring thaw that is running late. A late thaw increases the risk of a rapid melt, Gust said, and could coincide with spring rains.
Over the last week, which included two winter storms, the Fargo area received 1.5 to 2 inches of rain, with areas nearby pushing 3 inches.
Fargo has received 59.1 inches of snow so far — compared to the normal season total of 50 inches — but moisture in the last storm fell as rain and freezing rain.
Since Oct. 1, Fargo-Moorhead has received 8.38 inches of moisture. That's 3.7 inches more than long-term normal. By comparison, in the near-record 1997 flood and record 2009 flood, many areas were running from four to seven or more inches above normal, Gust said.
"We are nowhere near those amounts yet in the basin," he said, but acknowledged the moisture levels have been rising. "Hopefully, these next two weeks will be gentle to us."
The spring thaw likely will come in the first or second week of April, which is typical, Gust said.
"There's a lot of snow to melt," he said. Recent rains have frozen into ice. "It's going to take a while to ripen that back up. So it's going to take a bit of time."
Most likely, the valley could see floods in the 25 to 50 percent risk range. Fargo has a 50 percent chance of a 37.9-foot flood and a 25 percent risk of a 38.9-foot flood. Those floods are in the neighborhood of the 2011 flood, which crested at 38.81 feet.
Floods in that range would force bridge closures. At 37 feet, the convent bridge at 52nd Avenue South in Fargo closes. At 36.3 feet, the river reaches the clearance height of the First Avenue South bridge. The Northern Pacific Avenue/Center Avenue bridge clearance is 34.4 feet, while the 12th Avenue North bridge closes at a flood level of 28 feet.
Fargo has a 95 percent chance of a flood of 33.8 feet. That would be a bit higher than the 2013 flood, which crested at 33.21 feet.
Major flood stage in Fargo-Moorhead starts at 30 feet. The record 2009 flood was 40.84 feet.
Once the Red reaches a level of about 38 feet, some sandbagging and emergency clay levees are required, Nathan Boerboom, a Fargo city engineer, said in a recent briefing at City Hall.
Every foot above 39 feet requires "exponentially increased work" to build emergency flood defenses, Boerboom said.
Officials are dealing with probabilistic flood forecasts until the thaw is well along and water actually starts moving. Then the weather service is able to issue what it calls its deterministic forecasts, which predict the level and timing of the flood crest.
Once water starts moving, flood fighters have about a week to deploy emergency measures. The Red River rises at an average rate of 2 to 3 feet per day, Boerboom said.
The cities of Fargo and Moorhead are much better protected than they were in 2009. Miles of permanent levees and flood walls have been built, and hundreds of homes have been removed from difficult-to-defend, flood-prone areas.
"Unfortunately," Boerboom said, "we're still in the sandbag business" if forced to fight a severe flood.
Outside Fargo-Moorhead, two potential trouble spots stand out in the latest spring flood outlook.
Harwood, N.D., located near the confluence of the Red and Sheyenne rivers, likely will see a flood crest on the Sheyenne of around 92 feet, with a 5 percent chance of reaching 92.4 feet. Because the Sheyenne runs into a hydrological "traffic jam," as its waters back up while trying to enter the swollen Red, flooding there often is exacerbated.
In Minnesota along the Wild Rice River, Hendrum has a 50 percent chance of a level of 33.3 feet and a 5 percent chance of reaching 35.6 feet.
When the Red rises: Flood impacts at certain river levels
45 feet ... Top of VA Medical Center's flood wall (Fargo).
43 feet ... Top elevation of Fourth Street permanent levee (Fargo).
42 feet ... Top of emergency clay levee (Fargo, 2013).
40.84 feet ... RECORD FLOOD: March 28, 2009.
40 feet ... River is lapping at the base of the Hjemkomst Center (Moorhead). Top of emergency sandbag levee (Fargo, 2013).
37 feet ... Convent Bridge at 52nd Avenue South is closed (Fargo).
36.29 feet ... Clearance height of First Avenue bridge (Fargo).
36 feet ... Sandbagging starts if the river is forecast to rise above 38 feet (Fargo).
35 feet ... Actions taken to prevent storm sewers from backing up (Moorhead).
34.4 feet ... NP Avenue (Fargo)/Center Avenue (Moorhead) bridge clearance.
33 feet ... Begin diking to protect the Hjemkomst Center (Moorhead).
32.5 feet ... First Avenue North bridge closed for construction of temporary dikes.
31 feet ... First Avenue North underpass is closed (Moorhead).
30 feet ... MAJOR FLOOD: Flooding at Second and Third Avenue North; Second Street South closed between Main Avenue and Fourth Street; temporary dike constructed on Oak Street dike from 87th Avenue North to 11th Avenue North (Fargo). Water crosses entrance road to Gooseberry Mound Park (Moorhead).
28 feet ... Fargo-Moorhead's former toll bridge closed. Construction begins on the dike to protect sewage treatment plant if forecast is above 34 feet (Fargo).
25 feet ... MODERATE FLOOD: City parks and recreation areas along the river begin to flood (Fargo).
24 feet ... Second Street North closes between First and Sixth Avenue North for construction of temporary dike that protects City Hall if the river is forecast to rise above 31 feet (Fargo).
23 feet ... North Broadway bridge floods (Fargo).
22 feet ... North Broadway bridge is closed (Fargo).
19 feet ... Low-level dam south of Main Avenue is underwater (Fargo).
18 feet ... MINOR FLOOD: Elm Street between 14th and 15th Avenue North is closed (Fargo).
17.7 feet ... Sanitary sewers affected; pump station at Island Park is started (Fargo).
17 feet ... ACTION STAGE: Water rises to the edge of the bike path at El Zagal bowl along Elm Street North between 14th and 15th Avenue (Fargo).
Source: National Weather Service
Top 10 Red River crests in Fargo-Moorhead
Date of crest ... River depth
1. March 28, 2009 ... 40.84 feet
2. April 18, 1997 ... 39.72 feet
3. April 7, 1897... 39.10 feet
4. April 9, 2011 ... 38.81 feet
5. April 15, 1969 ... 37.34 feet
6. April 5, 2006 ... 37.13 feet
7. March 21, 2010 ... 36.99 feet
8. April 14, 2001 ... 36.69 feet
9. April 9, 1989 ... 35.39 feet
10. April 19, 1979 ... 34.93 feet
Source: National Weather Service