An Example Based On The Northwest Wisconsin Spring Snowmelt Flooding
By Sarah Marquardt, National Weather Service
River forecasts are among the many useful resources the National Weather Service (NWS) provides. The spring snowmelt flooding in northwest Wisconsin is a fitting example to demonstrate what goes into developing a forecast.
If you recall, in early April, a lot of snow across the northern half of the state melted in a hurry. This resulted in widespread standing and flowing water, which affected roads and caused basement flooding, increasing lake levels, and washed out culverts. Creeks also approached the bottom of bridges, and rivers rose into flood stage, mainly across the northwest quadrant of the state.
The NWS was concerned about this during the late winter as the snowpack continued to build. Snow depths on the ground in northwest Wisconsin were some of the highest values that have ever occurred that late in the season. The snow water content of the snowpack was 5 to 10 inches north of Eau Claire and Wausau, with values up to 15 inches in Bayfield County. This was a large factor in the spring flood risk because there was more water than normal on the landscape.
Several things that play into river forecasts from the NWS. The forecasts use a model to simulate the current conditions, ingest forecast parameters, and output a river forecast. Forecasters consider current conditions, like the amount of snow on the ground, how saturated soils are from recent precipitation or melting snow, whether the ground is frozen or not, and how much water is flowing through the rivers. This acts as a starting point for a forecast, then forecast temperatures and precipitation are taken into account.
If soils are saturated from recent rainfall, less will soak into the ground, and more will run off into rivers, resulting in high water levels. This will be a big player going forward across much of Wisconsin, except for the far southeast, because rivers were running high at the end of April due to the copious amount of melted snow and rain. As of the end of April, streamflow values were 200 to 500 times normal values for this time of year for much of Wisconsin. In southeast Wisconsin, they were close to normal. It will take some time for river levels to resume to their normal levels. Additional rain could prolong the high flows and make an area more prone to river flooding.
Another key factor in the flooding in northwest Wisconsin was that the ground was still frozen when all the snow melted. This results in most of the water ponding on the ground and running off into creeks and rivers, resulting in flooding. As soils start to thaw, more water will be able to soak in, helping to reduce the flood risk. Also, vegetation was still dormant. Farm crops and grass can help to soak up moisture once it comes out of dormancy, helping reduce the flood threat.
Temperatures played a big role in this year’s snowmelt. Most of the snow in northwest Wisconsin melted in a few days when highs were in the 70s and 80s. These were unusually warm temperatures which resulted in a very quick and impactful snowmelt. The rate of snowmelt is one of the more challenging things to forecast. This is accounted for in the river forecasts based on the forecast temperatures. The forecast temperature drives how much snow is melted in the forecast model and the resulting amount of water that ends up in the river. If there is snow in the forecast, that amount of precipitation will stay on the ground and not end up in the river until temperatures are forecast to be warm enough to melt that snow.
Precipitation also plays a role in determining river forecasts and is one of the more challenging things to forecast. If the placement of precipitation ends up shifting in or out of a river basin to a neighboring basin, river levels can end up being higher or lower than expected. For small river basins, that leaves little margin for error. The second part is forecasting the magnitude of precipitation. Some of this uncertainty is accounted in river forecasts by using the average precipitation amount of various forecast scenarios to determine the best guess river response.
A second round of river flooding occurred in mid-April in northwest Wisconsin. Most of the rest of the snow melted along with a period of rain. The rain was widespread in this case which makes it a little easier to predict river response than if it were say, scattered thunderstorms where there is uncertainty in which river basin the rain will fall. This exacerbated ongoing flooding and resulted in a second but lower crest on many rivers. Other slower responding rivers were still rising from the original melt. By now, the rivers have returned to normal or even below normal levels as a result of lack of rainfall. Our spring experience this year, however, was a good reminder to be prepared because, if conditions are right, a flood condition can develop quickly.
There are a lot of factors that play into the NWS river forecasts. Forecasters can tweak the forecast temperatures and precipitation to assess the river response and make an educated guess on the most likely river response.
NWS river forecasts are available online.
|
