Storms could impact flooding
Posted on: April 12, 2009
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A storm system could bring additional precipitation as cities experience a second flood crest, according to the National Weather Service.
Mark Ewens, data manager at the Grand Forks office, said the computer models used to determine weather forecasts ?have been in agreement the last several days? that a system is heading toward the region.
That storm most likely will get here sometime between April 16 and 18, which is when many cities along the Red River Valley will be nearing their second crest this spring.
But the amount of precipitation it may bring or where it will impact the most remains too far away to determine. ?We can?t really predict precipitation with any accuracy beyond four days,? Ewens said. ?You can see it, but you?re not quite sure.?
He added that the accuracy of predictions diminishes the further away the event actually is, but the computer models are pointing to a high likelihood of the storm.
There is a slight chance of regional rain showers this Sunday as well, but Ewens said that would have a minimal impact on flooding.
Mid- to late April historically is a time with heavy rains, he said, and the region could see a stormy latter half of the month. The La Nina system over the Pacific Ocean will also likely affect weather in the Midwest, and traditionally, this system means cooler and wetter springs than average.
Ewens said that trend is already noticeable with below-average temperatures and above-average precipitation almost consistently since September.
He also pointed out that much of the southern valley had major snowstorms at the end of last April, so there are several reasons to be concerned about additional precipitation that could affect how high the water gets the second time around.
What it means
NWS officials held a meeting Wednesday in Grand Forks to explain what their different flood forecasts mean and how the different numbers actually work together to provide a broad overview of what could happen.
Ewens said there has been some confusion on the difference between their deterministic forecasts, which are for the next 7 to 14 days, and the long-range probabilistic outlooks which are good for several months.
The NWS released a new deterministic forecast Tuesday that said a second crest of 48 to 50 feet is expected in Grand Forks between April 18 and 20. Fargo likely will see a crest of 38 to 40 feet between April 16 and 18, according to the new forecast.
But the latest probabilistic forecast released Friday said there is a 50 percent chance the second crest would hit 41.7 feet in Fargo and 51.7 feet in Grand Forks.
Ewens explained that Tuesday?s crest range predictions assume no additional precipitation over the next several days and basically are what would happen under ideal conditions.
The long-range flood outlook becomes useful at this point because it gives communities an idea of what will happen if more moisture is added to the system. They are based on the flooding experienced during the past 58 years and provide a historical framework for the predictions.
The water level at the 50 percent probability mark indicates what would happen if the region had average additional precipitation, and the predictions toward the most unlikely end tell officials what to plan for if above-average rain or snow occurs.
For example, the 20 percent chance of Grand Forks reaching 53.8 feet or Fargo getting to 43.3 feet could happen if there were heavy rains or thunderstorms before or during the next crest.
?We want people to be aware there is a very real possibility of the river going higher than what is out there,? Ewens said. ?We need to start looking at these other curves ? those represent what would happen if heavy rains come.?
The short-term predictions will become more certain as the crests approach, he added, but the outlook probabilities help for now because of the inherent uncertainty of predicting weather-related events.
?Anything can happen in meteorology,? Ewens said. ?The weather in North Dakota can change, boom, right now.?
Unusual season
Flooding this year has proved to be unusual for many reasons. Many communities are noticing very slow recessions of water levels after reaching their first crest.
Ewens said the Red River typically rises and falls at a similar rate, but this year is not acting that way.
Another unusual characteristic is the approaching second crest. In Grand Forks, this has happened only five times in the past 58 years, most recently in 1999.
Three of those double-crest years saw river levels break 36 feet. In each of these events, heavy rain near the time of the second crest affected flooding and caused higher water levels.
One historical example of this was in 1950, with an April 25 crest of 42.2 feet and a level of 45.6 feet recorded May 12.
Another unusual factor was the early spring melt that caused rivers to rise last month. ?This is an extremely unusual event to have a flood at this magnitude this early,? Ewens said.
Below-average temperatures have kept much of the soil moisture frozen in the ground, but as the month progresses and temperatures rise, all that water will join the new snowmelt and cause another wave of flooding. A recent band of heavy snowfall in the southern valley ?basically reloaded the gun, as it were,? he said.
?To have spring floods like this back-to-back is just an unfortunate series of events that have come along to plague us this spring,? he said. ?We?re wanting people to understand that this is a potentially serious problem.?
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