Flood forecasters struggle to handle 'uncharted territory'
FARGO - Forecasters will issue a new flood prediction today to give flood fighters guidance about the expected magnitude and timing of the impending Red River flood.
The new outlook, the first new crest predictions in nearly four weeks, will be based on possible weather scenarios over the next two weeks, given expected temperature and precipitation ranges.
Forecasters are calling it a "bridge outlook." It is meant to span the gap between the probability-based outlooks, in which winter snowpack conditions are a major factor, and actual crest predictions, which do not come until that snow is melted and flowing.
The unusual forecasting step comes as the valley gears up amid what the National Weather Service on Tuesday called an "unparalleled situation," poised for what will be the latest Red River spring flood on record.
The most recent outlook, issued March 21, predicted a 50 percent chance of a crest higher than 38.1 feet, a 25 percent chance of more than 39.1 feet and a 5 percent chance of topping 41.3 feet.
A 38.1-foot flood would be the fifth-highest recorded in Fargo. The highest crest ever recorded in the city was in 2009 at 40.84 feet.
Since March 1, precipitation has been "substantially" above normal, with sub-basins that flow into Fargo-Moorhead picking up about 3 inches during the period, or roughly double what is normal, according to the weather service.
Also, little moisture was lost through evaporation or absorption in soils, since the ground remained largely frozen, with air temperatures well below normal.
As of Monday, the Red River and most of its tributaries remained frozen, with the notable exception coming from the rivers that converge near Wahpeton-Breckenridge, Minnesota's Bois de Sioux and Otter Tail.
The rivers, which form the Red River, are flowing from reservoir releases.
Faced with such a complicated and unprecedented set of factors, hydrologists and meteorologists have struggled to provide guidance for flood fighters.
The historical record doesn't fit well with current conditions.
For example, the record high temperature in Fargo for April 21 was 100 degrees, set in 1980. But what, if anything, does that meteorological fluke mean to the prospects for the flood this spring?
That's the sort of question that Steve Buan, service coordination hydrologist, and his colleagues at the North Central River Forecast Center have been asking themselves.
With the valley still cloaked in heavy snow, a spike of extreme hot weather seems highly improbable in the near future. Yet probabilities gleaned from weather records dating back 40 or 60 years form important components for the computer models used to make flood predictions.
The North Central River Forecast Center has run four prediction scenarios trying to come up with a useful forecast, but the results were so divergent that forecasters must devise new methods for today's "bridge forecast."
"This is uncharted territory," Buan said. "Our traditional methods we just can't apply."
Sooner or later, as spring belatedly takes decisive hold, the cold, wet weather pattern will change.
The critical questions: How abruptly? And will the melt coincide with heavy rains?
River forecasters can't make an actual flood crest prediction until water from the spring melt begins flowing widely throughout the basin.
If the snowmelt progresses as expected - with high temperatures in the low 40s later this week - initial crest forecasts could come this weekend or early next week.
Today and Thursday, another storm could bring 2 to 5 inches of snow across southeastern North Dakota and west-central Minnesota, the weather service warned in a statement Tuesday afternoon.
But a shift in the track of the storm could raise or lower expected snow totals.
A snowstorm Sunday and Monday essentially replenished the water content of the snowpack that had been lost during the past month, said Mark Ewens, data acquisition program manager for the weather service in Grand Forks.
As a result of the snowstorm, the Sheyenne, Wild Rice and Maple rivers in North Dakota now will play a more significant role in the Red River flood, Ewens said.
"That's a game changer," he said. "Now all of a sudden we're looking at more of a basin-wide event."
Before the latest snowstorm, Ewens said, the snowpack in the Minnesota side of the basin appeared to contribute most of the moisture for the coming flood.
That sets up a situation in which multiple tributaries could be flowing into the Red River simultaneously.
That scenario could produce a higher flood crest.
Under ideal conditions, the tributaries would flow into the Red River at staggered intervals, producing a lower crest.
"Now we're at that time of the year when the odds of having everything warming up simultaneously are much greater," Ewens said.
"The odds," he added, "are starting to go against us."