Forecasting Streamflow with the UW Hydrometeorological Forecast System Snoqualmie River 1995 flood – peaks between 10 and 20 year recurrence The most recent Presidentially declared flood disasters were the November 1995 and February 1996 floods when storm fronts and melting snowpack caused massive flooding throughout Washington and Oregon. More than $3 million in direct cost to King county for repairs, total damages ~$16 million Cedar R.: The November 1990- March 1991 floods were also a Presidentially declared flood disaster. During these floods, all river systems in King County reached flood stage with the majority of damage occurring in the Snoqualmie and Cedar river valleys. Flooding in early November had swelled major rivers throughout Western Washington. "Then on November 23, a heavy downpour of warm rain -- referred to by local weather forecasters as the ‘Pineapple Express’ because of its origins in the southern Pacific Ocean -- began to fall on a recent snowfall in the Cascades. The resulting runoff from melting snow and rain hit the already saturated floodplains on the next day, leading to the highest flows ever recorded on most of the rivers and streams draining the western slopes of the mountains". In some cases the flows were so high that stream gages reached their maximum height, unable to record any additional flow. More than $15 million in damage was done in King County alone. Nearly 900 homes were damaged or destroyed and two men drowned. ~150-200 million total damages. Photos from: www.metrokc.gov Dennis P. Lettenmaier Department of Civil and Environmental Engineering, University of Washington GAPP Mountain Studies Workshop July 24, 2003 presentation prepared by Ed Maurer, Dept of Atmospheric Sciences, UW

MM5-DHSVM Streamflow Forecast System UW Real-time MM5 DHSVM Distributed-Hydrology-Soil-Vegetation Model Completely automated In use since WY 1998 For details: Westrick, K.J., P. Storck, and C.F. Mass, Description and Evaluation of a Hydrometeorological Forecast System for Mountainous Watersheds, Weather and Forecasting 17: 250-262, 2002. Streamflow and other forecasts

Penn State/NCAR Mesoscale Model MM5 Used throughout the world for both research and operational forecasting 48-hour (and some 72-hour and longer) forecasts run twice daily at the University of Washington High-resolution model (4-km) capable of capturing the complex orography of the region, including lee shading and windward precipitation enhancement FOR MORE INFO... http://www.atmos.washington.edu/mm5rt/

DHSVM land surface hydrology model Physically-based, distributed model Solves a water balance at each grid cell at each time step Horizontal scales typically 30m to 150m Designed for and extensively tested in complex terrain Details on DHSVM at: http://www.hydro.washington.edu/

DHSVM Calibration Calibration at 2 sites in Snohomish River Basin Used all available meteorological observations (50sites), 1987-1991 Used flow observations at two USGS gauges: Skykomish R. near Gold Bar Snoqualmie R. at Carnation Snoqualmie R. at Carnation Peaks flows and average water balance are well simulated by DHSVM when forced by observed meteorology

UW Hydromet Domain - 2003 26 basins ~60 USGS Gauge Locations 48,896 km2 2,173,155 pixels DHSVM @ 150 m resolution MM5 @ 4 & 12 km http://hydromet.atmos.washington.edu

Performance of Hydromet System Sauk Snoqualmie Observed MM5-DHSVM NWRFC

Using the Hydromet system for MM5 diagnosis Of, course, not all forecasts were so bad… One exceptionally bad forecast for the Cedar R., events from January 25 to Feb 4, 2003 Second peak: Forecast:1200 cfs Observed: 3700 cfs Flood stages above bankfull occurred, and were not forecast

Representative Meteorological Station – Mt. Gardner Avg. Precipitation from 1/24 - 2/7: Observed: 1.0 mm/h Simulated: 0.7 mm/h Total difference: ~100 mm Precip Average Temperature: Observed: +2.1C Predicted*: -0.1C SWE: Observed: -50 mm Predicted*: +100 mm MM5 biases in P and T combine to produce large underestimation in runoff Temp SWE Qualification: Simulated values vary with the ensemble, time period, and forecast

Opportunity for Improving UW Hydromet Forecasts 1 – Precipitation/Temperature Bias Correction Remove systematic biases in P, T, at land surface 2 – IMPROVE-2 Take advantage of the IMPROVE-2 experiment to examine the interplay between observation density and bias correction performance 3 – Initial State Updating Assimilation of snow and soil moisture information from an observationally constrained data set. This applies at least to the 13-24 hour forecast.

Snow State Updating with Observations Use ground observations (SNOTEL sites) to adjust the basin snow state Challenge: 45-50 snow water observations for 48,000 km2 domain – low density places high dependence on interpolation assumptions

Summary UW Hydrometeorological Forecast System provides accurate streamflow and snowpack predictions when forced with accurate meteorology and when properly initialized Improvements in both initialization and meteorological forecasts are ongoing, by analyzing current flood events and retrospective analysis The capabilities of the system are being expanded to include both probabilistic forecasts using ensembles, and to include landslide hazard evaluation

Mountain hydrology issues for GAPP Hydrologic prediction is essentially a one-way forced problem in these environments. Do the (atmospheric) models get the processes right, especially the interaction of topography and model physics? What can GAPP do to help improve the models? How important are weather scale variations in precipitation and temperature to S/I predictability, and at what (spatial) scales?

Are models able to capture observed long-term changes in snow/rain partitioning within the transient snow zone of the western mountains? Is there a rationale for a field campaign associated with cold season orographic processes in the west? If so, what needs to be done, particularly in light of results from PACJET, IMPROVE, etc.?