1. 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 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. ~ million total damages.
Photos from:
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

2. 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: , 2002.
Streamflow and
other forecasts

3. 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...

4. 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:

5. DHSVM Calibration
Calibration at 2 sites in Snohomish River Basin
Used all available meteorological observations (50sites),
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

6. UW Hydromet Domain - 2003 26 basins ~60 USGS Gauge Locations
48,896 km2
2,173,155 pixels
150 m resolution
4 & 12 km

7. Performance of Hydromet System
Sauk
Snoqualmie
Observed
MM5-DHSVM
NWRFC

8. 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

9. 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

10. 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 hour forecast.

11. 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

12. 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

13. 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?

14. 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.?