National Weather Service California Nevada River Forecast Center Department of Commerce National Oceanic and Atmospheric Administration National Weather Service

HEFS - A Few Words of Caution

In using HEFS products and information, it is very important to recognize that all of the variability in the streamflow traces comes from the precipitation and temperature traces. In reality, uncertainty is introduced from a whole host of sources.

Ensemble Challenges

The ultimate challenge is to appropriately consider all sources of uncertainty in developing accurate ensembles from which accurate statements and effective decisions can be made. Currently, the state of the operational science is limited to the uncertainty introduced by future temperature and precipitation. In other words, the errors (uncertainty) introduced by observations, model states, the modeling system, model parameters, and human input are not currently considered.

Is that at big deal? If you’re only interested in longer range forecasts and with parameters aggregated over time, it probably isn’t a big deal. The uncertainty associated with future weather pretty much dwarfs everything else. As one moves towards the near term (hours to weeks), the dominance of the weather forcing is much less. Additionally, if you’re interested in specifics of flow (instantaneous minimums and maximums), the uncertainty of the modeling system and its parameters can play a significant role. Work is currently underway to address this short-coming, but it is not ready. In the meantime, users are advised that the system is likely to underestimate the uncertainty in near-term (a few weeks) and critical level forecasts.

In addition to considering only a portion of the total variability, one must remember that the HEFS analysis is based on a limited number of samples (30 to 50 years of data). As such the precision is limited. The uncertainty in the uncertainties is very real and users are advised to consider the values derived as estimates. Work is currently underway to develop and provide verification and validation information that will allow users to assess the ability of the system to provide accurate information for each location.

Regulated versus Unregulated Streamflow Forecasts

All results provided on the CNRFC HEFS Trace Analysis web page are for “full natural streamflow” unless otherwise specifically stated. Actual measured can and will be different if there are reservoirs and/or diversions upstream from the location of interest. The vast majority of river gages in the Western U.S. have some sort of regulation or diversion upstream. Until the NWS has the operational ability to simulate the future operation of these reservoirs and diversions in an ensemble environment, they cannot be reasonably reflected in our products.