Improving Post Processing Of Ensemble Streamflow Forecast For Short To Long Ranges

Improving Post Processing of Ensemble Streamflow Forecast for Short-to-long Ranges

A novel multi-scale post-processor for ensemble streamflow prediction, MS-EnsPost, and a multiscale probability matching (MS-PM) technique for bias correction in streamflow simulation are developed and evaluated. The MS-PM successively applies probability matching (PM) across multiple time scales of aggregation to reduce scale-dependent biases in streamflow simulation.For evaluation of MS-PM, 34 basins in four National Weather Service (NWS) River Forecast Centers (RFC) in the US were used. The results indicate that MS-PM improves over PM for streamflow prediction at a daily time step, and that averaging the empirical cumulative distribution functions to reduce sampling uncertainty marginally improves performance. The performance of MS-PM, however, quickly reaches a limit with the addition of larger temporal scales of aggregation due to the increasingly large sampling uncertainties. MS-EnsPost represents a departure from the PM-based approaches to avoid large sampling uncertainties associated with distribution modeling, and to utilize fully the predictive skill in model-simulated and observed streamflow that may be present over a range of temporal scales.MS-EnsPost uses data-driven correction of magnitude-dependent bias in simulated flow,multiscale regression over a range of temporal aggregation scales, and ensemble generation using parsimonious error modeling. For evaluation of MS-EnsPost, 139 basins in eight RFCs were used. Streamflow predictability in different hydroclimatological regions is assessed and characterized, and gains by MS-EnsPost over the existing streamflow ensemble post processor in the NWS Hydrologic Ensemble Forecast Service, EnsPost, are attributed. The ensemble mean prediction results show that MS-EnsPost reduces the root mean square error of Day-1 to -7 predictions of mean daily flow from EnsPost by 5 to 68 percent, and for most basins, the improvement is due to both bias correction and multiscale regression. The ensemble prediction results show that MS-EnsPost reduces the mean Continuous Ranked Probability Score of Day-1 to -7 predictions of mean daily flow from EnsPost by 2 to 62 percent, and that the improvement is due mostly to improved resolution than reliability. Examination of the mean Continuous Ranked Probability Skill Scores (CRPSS) indicates that, for most basins, the improvement by MS-EnsPost is due to both magnitude-dependent bias correction and full utilization of hydrologic memory through multiscale regression. Comparison of the mean CRPSS results with hydroclimatic indices indicates that the skill of ensemble streamflow prediction with post processing is modulated largely by the fraction of precipitation as snow and, for non-snow-driven basins, mean annual precipitation.The positive impact of MS-EnsPost is particularly significant for a number of basins impacted by flow regulations. Examination of the multiscale regression weights indicates that the multiscale regression procedure is able to capture and reflect the scale-dependent impact of flow regulations on predictive skills of observed and model-predicted flow. One of the motivations for MS-EnsPost is to reduce data requirement so that nonstationarity may be considered.Comparative evaluation of MS-EnsPost with EnsPost indicates that, under reduced data availability, MS-EnsPost generally outperforms EnsPost for those basins exhibiting significant changes in flow regime.

The Modernized End-to-end Forecast Process for Quantitative Precipitation Information

Completing the Forecast

Uncertainty is a fundamental characteristic of weather, seasonal climate, and hydrological prediction, and no forecast is complete without a description of its uncertainty. Effective communication of uncertainty helps people better understand the likelihood of a particular event and improves their ability to make decisions based on the forecast. Nonetheless, for decades, users of these forecasts have been conditioned to receive incomplete information about uncertainty. They have become used to single-valued (deterministic) forecasts (e.g., "the high temperature will be 70 degrees Farenheit 9 days from now") and applied their own experience in determining how much confidence to place in the forecast. Most forecast products from the public and private sectors, including those from the National Oceanographic and Atmospheric Administration's National Weather Service, continue this deterministic legacy. Fortunately, the National Weather Service and others in the prediction community have recognized the need to view uncertainty as a fundamental part of forecasts. By partnering with other segments of the community to understand user needs, generate relevant and rich informational products, and utilize effective communication vehicles, the National Weather Service can take a leading role in the transition to widespread, effective incorporation of uncertainty information into predictions. "Completing the Forecast" makes recommendations to the National Weather Service and the broader prediction community on how to make this transition.