1. Generating Quantitative Precipitation Forecasts for River Modeling
Mike Ekern, Sr. HAS Forecaster
NOAA, National Weather Service
California Nevada River Forecast Center

2. Definitions QPF Snow Levels
the total amount of expected liquid precipitation (in hundredths of inches) in a future specified time period
Snow Levels
the elevation above which frozen precipitation occurs and does not contribute immediately to runoff.

3. Operational Flood Forecasting
model
guidance
Hydrologist
hydrologic
expertise &
judgment H A S
Precip (QPF)
Temperature
Snow Levels
Forecast
Inputs
Flood Forecast
Guidance
Bulletins
Graphics
NWSRFS
SAC-SMA
SNOW-17
Observing
Systems
data
parameters
Model
Calibration

4. NWSRFS Forecast InputsH A S
Precip (QPF)
Temperature
Snow Levels
QPF
Single FMAP (Forecast Mean Areal Precipitation) per basin or sub-basin (some basins are subdivided into lower, middle, and upper)
6 hour time steps – NWSRFS assumes a uniform areal and temporal distribution of rainfall
0-72 hours – HAS Forecaster
hours – Rhea Orographic QPF Aid
Typically ranges from zero to 3+ inches in a 6-hour period
Snow Levels
Input is the average of the instantaneous snow level values at the beginning and end of the 6-hour period.
Snow Level = Model Freezing Level – 1,000 feet

5. Lose detail in the terrain!

6. Influence of Terrain on Precipitation
Annual
Precipitation
Topography

7. Mountain Mapper Software
Graphical suite of software developed at the CBRFC (Salt Lake City)
Specify (QPF) - uses PRISM monthly climatology to distribute point QPF data to a 4km grid using inverse distance squared weighting
DailyQC (QPE) – uses PRISM monthly climatological to quality control observed precipitation.
Verify (QPF – QPE)
Currently used by 3 western RFCs

8. Mountain Mapper Concept
PRISM

9. Mountain Mapper Concepts

11. Mean Areal Precipitation

12. Forecasting Methodolgy
0-12 Hours
Rain gage and weather observations
WSR-88D Radar coverage and movement
Satellite trends
Forecaster skill/experience
Numerical weather prediction models
12-72 Hours or more
Pattern recognition
Ensemble prediction

13. Numerical Weather Prediction

14. QPF Process Sequence HPC QPF data are converted from contours to
National QPF – Hydrometeorological Prediction Center
HPC
QPF data are converted from contours to
point data using bilinear interpolation and sent to RFCs.

24. Strengths/Weaknesses
Simplifies QPF in complex terrain
Easily converts from gridded QPF to FMAP for NWSRFS
Similar technique used to QC observed precipitation
Weaknesses
Short duration QPF does not exhibit monthly PRISM distributions
Careful selection of QPF points is required – generally mid-high elevation sites work best

25. Snow Levels Effectively reduces the size of the basin
that contributes to runoff

26. NWSRFS Sensitivity to Melt Levels Mid-winter soil moisture conditions

27. NWSRFS Sensitivity to changes in snow levels80
NWSRFS
Sensitivity
to changes
in snow levels 1 6 4 3 7 9 8 70 7 3 5 1 8 9 6 60 K l a m a t h R i v e r ) S m i t h R i v e r 3 - T r i n i t y R i v e r 1 50 T r u c k e e R i v e r x s f c ( e t a 40 r 2 1 7 4 9 8 w o f l k 30 a e P 20 1 6 5 9 8 7 10
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000 M e l t i n g l e v e l ( f t )

28. Rhea Orographic Aid Objective tool Outputs 6-hour orographic QPF
Input - NCEP gridded datasets from AWIPS
Eta
GFS
Performed well during large-scale rain events in California (1986, 1997)
Mesoscale resolution
Sample output...
==< SHASTA ABOVE SHASTA DAM - SHDC1 >====================================
STRDA BEG-END QPF SLVL FRZGLVL 700DIR
WIND&RH WK SSE-NNW PRDIF
WIND&RH WK SSE-NNW PRDIF
RH ONLY NORMAL PGRAD
RH ONLY NORMAL PGRAD
WIND&RH WK SSE-NNW PRDIF
* WIND&RH WK SSE-NNW PRDIF
MODIFIED TOTS MOD-FAC = .85 * = 700mbWD >344 or <155 DEG
* WIND&RH WK SSE-NNW PRDIF
* WIND&RH WK SSE-NNW PRDIF
* WIND&RH WK SSE-NNW PRDIF
WIND&RH WK SSE-NNW PRDIF
MODIFIED TOTS MOD-FAC = .85 * = 700mbWD >344 or <155 DEG

29. Rhea Orographic Aid Basins