A crest within reach and a story of what if…

Guest post by Aaron Kennedy, assistant professor of atmospheric sciences

In the southern valley, the 2019 flood has crested.  Wahpeton reached a crest of 14.62’ on April 1st, placing it well within the category of ‘major’ flooding. Although impressive, it’s important to note this crest won’t be enough to put it within the top 10 historical floods at this location.

As expected, the crest has moved downstream with Fargo reaching a crest of 35.05’ on April 8th. Once again, these conditions fell well within the category of ‘major’ flooding, although well below the record of 40.8’ set in 2009. This crest will *just* make the top 10 in Fargo, and did provide for a first test for some of the new flood walls downtown.

How does this crest fit within the scope of earlier forecasts? Quite well actually. As seen below, the probabilistic guidance issued on March 11th had the crest occurring during the week of April 8th. The height fell at an exceedance probability of ~70% or so, while at Wahpeton it was ~50%.

With these numbers in mind, we can make an educated guess for the crest in Grand Forks. Current forecasts expect a crest of 48’ later this week. Compared to the probabilistic forecasts created earlier, this matches with an exceedance probability of ~70%. Taking the other locations into consideration, 48’ +/-1’ seems like a reasonable expectation for this year.

The lack of higher crests can be attributed to the near-perfect melt over the past few weeks. The warming trend has been gradual with many nights falling below freezing, allowing for a slower progression of water into the system. Further, there has been no major rain/snowfall events in the area over the past week as seen in our estimate of precipitation (percent of normal) over this period; broad areas of the Red River Valley had ~50% or less of the climatologically expected precipitation.

All of this said, meteorologists love to play what-if scenarios.  The big news this week is a potentially historic snowstorm south of the Red River Valley Thursday into Friday.  Areas of South Dakota along the I-29 corridor may see upwards of 2-3” of precipitation (liquid equivalent) resulting in a mix of rain/snow across the region. If this would have occurred a week prior, displaced one state north, there could have been major ramifications for our crest predictions. This would be seen as crests that would inch higher towards lower exceedance probabilities. The impact of these individual, late-season storm systems will always be a source of uncertainty in flood forecasting. This is why proper forecasts (such as the first plot) should convey probabilistic information.