So far the fall of 2008 has been extremely wet, as anyone who works outside can tell you. Record or near record setting rainfall has saturated the topsoil, and lead to river flooding in the southern Red River of the North Basin. As a result of the wet weather going into the winter, many folks are concerned as to what this may mean for the spring flood potential. However, spring floods are a combination of several factors, and a wet fall is one of them.
Below is a list of the primary spring flood preconditions must be considered when determining the magnitude of a spring flood. These conditions include, in order of significance:
1. The freeze/melt cycle
2. Early spring rains which increase melting of the snow pack or late spring snow storms adding to the existing snow pack
3. The actual snow pack depth and water equivalency
4. Frost depth
5. Soil moisture content from the previous fall season
6. River ice conditions during the melt cycle
Some of these conditions are known in advance such as frost depth, fall soil moisture content, and river ice conditions. However, the most critical conditions, the first three mentioned on the list, are not fully known until just before or during the spring thaw. A typical spring thaw occurs from the middle of March across southern portions of the basin, to mid or late April across the north.
Naturally, considerable variability in these conditions can exist prior to the spring melt and runoff period. For example, temperature, and precipitation amounts can vary greatly. By considering all the factors, there are numerous scenarios which may or may not cause flooding in the Red River Basin. A wet fall is just one of those factors. To illustrate let's look at two different fall seasons in the Red River Valley and the subsequent spring. One event caused significant flooding and the other did not.
In contrast, the fall of 1900 was, on balance, the wettest on record for eastern North Dakota. However, average winter snowfalls were only 25 to 35 inches, with ideal melt conditions. As a result, little flooding was observed in Grand Forks or Fargo during the spring of 1901.
Based on approximately 120 years of weather data, it is apparent that a wet fall, in and of itself is not a guarantee of significant flooding the following spring. As the list above indicates, heavy winter snows, with rains that occur during the melt process, are the greatest contributors to major flooding. While a wet fall is a sign that the National Weather Service regards as an important signal, we will not have a clear picture of the 2009 spring flood potential until we are well into the 2008/2009 winter season. While climate prediction is not yet as mature a science as short term weather forecasting, present indications are that normal amounts of snow are the most likely scenario for this upcoming winter season. Typically, 40 to 50 inches of snow fall accumulates across the Red River Valley region in an ‘average’ winter.
Your NOAA’s National Weather Service will be closely monitoring the hydrologic situation within the Red River Valley of the North these next several months. The first spring flood outlook is typically issued in early March; sooner if conditions warrant.
Below is a table of the top ten wettest falls on record, with the following seasonal snowfall and following spring crest. Your National Weather Service defines fall as the months of September, October and November. The asterisk indicates some missing data during that period.