Takeaway: HydroForecast’s model architecture allows it to parse the many interrelated variables at play in rain on snow events and accurately predict how much precipitation will be stored as SWE or produce runoff and when that runoff will occur.
Event Description: Predicting the amount of runoff that will occur when rain falls on top of an existing snowpack and temperatures fluctuate near freezing is one of the most challenging aspects of streamflow forecasting. Temperatures moving slightly above or below freezing can change whether the precipitation lands as rain - and runs off immediately - or snow - when it is stored as SWE. Further, any liquid precipitation can accelerate runoff from the snowpack, and it is difficult to quantify the extent to which that will happen.
In this event, significant precipitation occurred in the basin on March 15 and March 18-19. Temperatures fluctuated between -10℃ and 10℃, with most of March 18 and 19 sitting almost exactly at 0℃. Nonetheless, HydroForecast very accurately predicted that the March 16 precipitation would not cause a significant increase in inflow, then closely tracked a rise from 150 CFS to 250 CFS on March 18. HydroForecast’s predictions identified this increase in flow as early as March 16 and accurately forecasted the timing and magnitude of the increase.