Hydro-Meteorological Assessment-Updated October 4th 2011: Overall, conditions have improved slightly since the assessment issued in early August. Much of west central and portions of northwest Minnesota have become much drier than during the summer 2011. As of the end of September 2011, the wettest surface soil conditions were generally along and west of the Red River Valley, including much of the Sheyenne River and Devils Lake Basins. East of the North Dakota/Minnesota border, surface soil conditions range from normal to slightly drier than normal. The following is an updated assessment of current hydro-meteorological conditions and the possible implications for next spring. Although this summer had been quite wet, the past 30 days have seen a drastic decrease in the frequency and intensity of excessive rainfalls.
It is very important to remember the most significant factors in the severity of a Red River Valley spring flood are the rate of snow melt, amount of available snow water and precipitation at the time of melt. Those obviously are not known until we get much closer to the melt. The extremely wet weather of the recent decade, while troublesome, is only one of the 5 main indicators we look at for flood potential. Calendar year 2010 was the wettest year on record for the North Dakota and Minnesota Climate Divisions (C.D.) which comprise the Red River Valley and Devils Lake basin. Ten of the top 20 wettest years in the 5 C.D. region have occurred since 1991, with the average annual precipitation 22.08 inches, well above the historical 17.38 inches that would be normal.
It is very important to note this is not yet an outlook or forecast for flooding in the spring of 2012. Those will be issued beginning later in the mid winter period (in coordination with the needs of our partner agencies and community officials). The National Weather Service partners with federal, state and local agencies such as the U.S. Geologic Survey, Army Corps of Engineers and Emergency Management Community. As such, we are tasked with providing assessments of current and historical meteorological conditions, especially as they relate to flood potential. Therefore, the information below is provided with the intent to meet that requirement by providing a historical framework for the current excessive wet weather.
Rainfall: During July and August 2011, the Wild Rice River (ND), Bois de Sioux, upper Sheyenne, Devils Lake, Ottertail, upper Red River, upper Red Lake River and Two Rivers basins all had well above normal rainfall. In excess of 20 inches had fallen in some spots, with large areas of 10 to 20 inches in the above mentioned area. Drier than normal conditions had existed from the Pembina and Langdon areas of northeast North Dakota and over to the Roseau and Thief River Falls regions of northern Minnesota. An early September rainstorm helped saturate the top soil in much of northeast North Dakota and far northwest Minnesota; otherwise areas generally south of the U.S. 200 corridor have been drier than average.
As of October 1st, Fargo has had 22.39 inches of precipitation this year, while normal precipitation through October 1st is 18.60 inches. (Normal annual precipitation is 22.69 inches). In Grand Forks, 19.64 inches of precipitation has fallen, while normal precipitation through October 1st is 18.15 inches. (Normal annual precipitation is 21.52 inches.) Based on preliminary data, the southern Red River Valley had a very dry September. In fact, September 2011 was the 4th driest on record for Fargo. When combined with the mild temperatures, surface soils have dried considerably. Meanwhile, heavy rainfall occurred across much of northeast North Dakota, including the Devils Lake Basin, and small portions of northwest Minnesota, including the Two Rivers River Basin. (See images below)
Current Hydrologic Conditions: Based on the data from the U.S. Geological Survey (USGS), there has been a general decrease in base flows throughout the Red River Valley and associated tributaries since late August as compared to last year, which is reflective of the recent dry spell. As of October 4th 2011 above average flows continue on most river points along the main-stem Red. Most of the Red River tributairies of eastern North Dakota are still experiencing above normal flows, with Minnesota tributaries and streams experiencing more normal flows. In the Roseau River Basin, below normals flows are now occurring due to the recent drier weather. Surface soil moisture ranges from well above normal in the Wild Rice (ND), Sheyenne and southern Red River Basins to below normal in the Red Lake and northern Red River Basin. While surface soil moisture in much of the Devils Lake Basin remain higher than normal, that region too has seen a slight decrease in surface soil moisture as compared to last year.
Climate Outlook: Based on information provided by the Climate Prediction Center (CPC) the October - December time period has equal chances for below normal, normal or above normal precipitation. Typically during the October time frame, the threat for heavy rainfalls is statistically low. With the redevelopment of a weak La Niña, generally dry weather is perhaps more likely until late in the fall, precipitation increasing again through the winter season. According to the CPC, La Niña conditions developed in August and will persist through the 2011/2012 winter season. Typical La Niña winter temperature impacts may be viewed here, with precipitation impacts here.
As of October 4th, the CPC indicates enhanced probabilities that the upcoming winter will see below normal temperatures, with the tendency for colder than average conditions extending into the late winter and early spring of 2012. The winter precipitation signal is less robust, with the CPC suggesting equal chances for normal, above or below normal precipitation. Based on the CPC data, the risk for above normal precipitation increases later in the winter and into the spring over eastern North Dakota and northwest Minnesota. Based on the CPC data, the statistical risk for above normal precipitation, including above average snowfall, increases during the later part of the season.
The very large scale ocean-atmosphere patterns that influence the current wet regime remain similar to those of the 2010/2011 winter season. The positive phase of the Atlantic Multidecadal Oscillation,(AMO), and negative phase of the Pacific Decadal Oscillation (PDO) are considered by some climate scientists to be influential. These two very large scale ocean patterns of warm and cold pools of water, as the names imply, will tend to create atmospheric conditions conducive to above average precipitation, perhaps for several more years.
Hydrologic Implications: Soils range from near total saturation in some areas to normal or below normal levels in increasingly larger areas. With the recent drier than normal weather, base flows are decreasing or have decreased significantly. With the drier soils and lack of widespread, significant rains in northwest and west central Minnesota, base flows have dropped to more normal values. As there has been more rainfall on the North Dakota side of the Red Basin, stream flows are still running above normal there, although fewer locations are experiencing record daily flows. On balance, the recent dry weather in the southern Red River Valley and adjacent portions of Minnesota and North Dakota is very good news, a welcome respite from the wet cycle.
The persistence and duration of current high water levels this year are rather unusual. There have been major negative impacts to the agricultural industry in the basin and to parts of our infrastructure, such as rural roads and drainage systems. The recent relative dry spell is expected to continue through the fall freeze-up. This will allow continued percolation of surface water into the sub-surface, and a general decrease in top soil moisture levels. We are less likely to be going into the winter with deeply saturated soils than we were two months ago. This would decrease the risk that a deep layer of “concrete” frost, such as that of the winter of 2008-2009, will develop. Should soils continue to dry out ahead of the freeze up, this could lead to a further improvement of the spring snow-melt run off, allowing a larger portion to percolate into the soil. As is the case every fall precipitation through freeze-up will be crucial to helping determine spring flood potential.
Surface storage conditions have improved in portions of the southern Red River Valley. When viewed in a longer term historical sense, there is still a relative lack of [surface and sub-surface water] storage, and the recent climate signals suggest the overall wet regime may continue for the foreseeable future. IF significant rains occur prior to freeze up and IF a classic La Niña winter follows, THEN we will have concerns for an unprecedented 4th consecutive major spring flood threat in 2012. That's a lot of IF's at this time. We therefore advise our partners to be ready for the potential of another severe winter season and the attendant risk of major spring flooding.
Below is a table of the top ten wettest Climate Division Annual Precipitation. Annual data through 2010. Data courtesy High Plains Regional Climate Center.
Below are some images indicating how much above normal the precipitation is. These images are courtesy of the High Plains Regional Climate Center in Lincoln Nebraska.
Above shows the precipitation total from January 1 through September 30th 2011 for Grand Forks NWS, compared to the wettest year and 2008. Courtesy High Plains Regional Climate Center
Above shows the precipitation total from January 1 through September 30th 2011 for Fargo, compared to the wettest year and 2008. Courtesy High Plains Regional Climate Center
September 2011 precipitation totals for the region. RADAR estimate and surface reports used. Courtesy NOAA/NCRFC
September 2011 preliminary departure from normal precipitation (mosaic). Click for larger images. Data Courtesy High Plains Regional Climate Center
Soil Moisture Multimodel Surface Water Monitor - courtesy University of Washington. Dark blues show saturated soils within the Red River Basin, yet the white in west central Minnesota represents drying there. As of September 28th 2011.