Red River and Devils Lake Basin - 2011 Spring Flood Outlook
Talking Points for 3/3/2011
Cutting to the chase - an overview of the numbers:
In general, the risk of major to record scale flooding has increased only slightly across the headwaters areas and the southern Red River Basin, basically Fargo-Moorhead and points south. For most all other areas including the mid to upper Sheyenne sub-basin, the northern Red River basin, and the Devils Lake sub-basin the risk has decreased slightly.
These changes are mostly due to the singular significant snow event, the Washington’s Birthday Holiday Weekend (Feb 20-21) storm, which mainly impacted that extreme southern basin area. Otherwise, the remainder of the basin received only minimal new snowfall.
Also, recent and more extensive aerial and ground snow surveys now indicate a slightly lower snow water equivalent (SWE) across the Devils Lake Basin and the northern Red River Basin than was previously determined, leading to slightly lower inflows expected there.
Key points for the Red River Basin, from south to north:
- All points along the Red River still have a better than 90 percent risk of major flooding.
- Wahpeton and Fargo, both now have a roughly 35 percent risk of exceeding 2009 flood levels. [in 2009 Fargo hit a flood of record at 40.84 ft, Wahpeton hit 3rd place at 17.5 ft]
- The ND Wild Rice at Abercrombie has close to a 20 percent risk of 2009 scale flooding.
- The risk levels along the Sheyenne River from Valley City into Lisbon have been reduced by nearly a foot. Past Lisbon into Kindred, West Fargo, and Harwood the flood risks have remained near 2006 and 2009 levels.
- The mainstem Red River flood risk at Halstad has reduced slightly, while the risk along the MN Wild Rice at Hendrum remains near 20 percent for exceeding 1997’s record [33.85 ft].
- Both Crookston (on the Red Lake River) and Grand Forks (at the confluence of the Red and Red Lake Rivers) have had their flood risk levels reduced around a foot.
- In the north, Drayton and Pembina (on the Red River) have had their flood risks reduced by about half a foot. Grafton (on the Park River) and Roseau (on the Roseau River) have also had their flood risks reduced by about half a foot.
Key points for the Devils Lake Basin: This outlook still indicates that Devils Lake and Stump Lake will likely rise (80 percent probability) to a new record height in excess of a 1454.1 ft elevation, which is two feet above their previous record set last summer. These lakes could rise (20 percent probability) to a height in excess of a 1455.1 feet, or three feet above their previous record. Overall risk factors have been reduced slightly for this update, largely due to below normal snowfall through February. [Creel Bay gauge at 1451.73 on 3/3/11]
What has occurred since Feb 17th:
From one half inch to an inch of moisture was added to the headwaters snowpack. This area received nearly double their normal February precipitation, whereas most of the rest of the basin received roughly one half inch below February normals.
Temperatures have plummeted well below seasonal normals. The average temperature for February was roughly 3 degrees below normal across the area, and well below freezing.
Whatever snowpack warming and ripening (nearing 32F melt temperature) had occurred in early February has been lost, as overall snowpack has now cooled back into the mid-teens.
What will be occurring over the next few days:
The Otter Tail River (below Orwell Dam), the Bois de Sioux River, and the Red River from Wahpeton to Fargo will be rising to near minor flood stage. Reservoir releases at Orwell Dam and White Rock Dam will be at peak levels for the next week or two as these reservoirs are drawn down to winter minimum levels to provide maximum spring snowmelt storage.
The Otter Tail River will likely have some breakout flow in mainly rural areas.
Only slight rises are expected on the Sheyenne River near Valley City, as Baldhill Dam releases are nearly stabilized and Lake Astabula is near its projected winter minimum level.
What is likely to happen from next week into late March:
Temperatures should remain well below seasonal normals through the first three weeks of March. High temperatures could sneak above freezing around the weekend of the 20th or later.
Precipitation this first week of March will remain slight, with light snow chances today (Thursday) and again this weekend.
A more significant winter storm is expected to impact the central and northern plains around this coming Tuesday, March 8th. Current storm tracks suggest that heaviest snowfall could impact South Dakota and southern Minnesota, but brush the southern Red River Basin.
By the third week in March, the NOAA Climate Prediction Center (CPC) outlook projects the storm track to be more distinctly across the Red River Basin, with be low normal temperatures and above normal snow as the most likely outcomes in our area.
The overall CPC climate outlook for February calls for cooler and snowier conditions, while this active storm pattern is expected to persist well into April and possibly May.
How this may affect the spring snowmelt (a climatologically probable scenario):
A favorable thaw cycle is one in which daytime temperatures rise above freezing, but drop back below freezing at night… allowing for a slow but steady melt and runoff.
If a gradual thaw cycle begins around or after March 20th, as longer-range weather projections now suggest, we would expect higher elevation, headwaters areas, to thaw and start to flow in that last week of March, while most mainstem Red River locations would remain cooler with the river still frozen and local runoff slower to begin.
Then, from the end of March into the first week of April, southern basin locations could be opened and flowing with flood crests moving first along southern basin tributaries and then along the mainstem Red from Wahpeton into Fargo.
Local snowmelt would also be increasing in the northern Red River Valley, so that from the first into the second week of April, northern basin tributaries would be opened and sending their flood crests towards the mainstem Red. Around this same time, the flood wave along the mainstem Red would likely be progressing north from Fargo into Grand Forks and points north.
In such a scenario, far northern mainstem sites, from Oslo northward to the Canadian border, would remain in flood from late April into early May.
Meanwhile, throughout April, local runoff would be increasing across the Devils Lake Basin. From late April into early May, inflows into Devils Lake and Stump Lake would be reaching their peak and the lakes would rise most rapidly.
The later into April that such a flood may persist, the increased chance there is for an excessive spring rain event to occur. A warm and heavy rain onto frozen ground or snowcover during the runoff period would speed up the overall process and likely increase the severity of flooding.