|Audio Only Briefing (mp3)|
As the flood crest nears and waters start to rise, attention shifts from the probabilistic outlooks to the determinstic forecast. This forecast utilizes the forecasted weather during the next 7 days that is then utilized by the NWS River Forecast Model. This means that within seven days, the historical data has no impact on the forecasts. As a result, the forecasts are given with stage heights rather than probabilities. Of course, there are literally hundreds of variables influencing the forecast heights. This means that a change in the temperature forecast of only 5 degrees five days from now could significantly change the river forecast. As a result, sometimes forecasters need to give ranges of possibilities in the river forecasts, especially when the crest is several days out. The crest height is refined slightly as it nears and confidence increases. This does sometimes result in some adjustments to the expected crest. However, in the end it is the current conditions that influence the eventual river heights. It is important to note that the actual crest at a river point may occur beyond the 7 day valid time of the forecast.