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

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.

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

NWSRFS Forecast Inputs H 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 72-120 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

Lose detail in the terrain!

Influence of Terrain on Precipitation Annual Precipitation Topography

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

Mountain Mapper Concept PRISM

Mountain Mapper Concepts

Mean Areal Precipitation

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

Numerical Weather Prediction

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.

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

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

NWSRFS Sensitivity to Melt Levels Mid-winter soil moisture conditions

NWSRFS Sensitivity to changes in snow levels 80 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 )

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 6 19 16-22 .00 35 5.1 253-299 WIND&RH WK SSE-NNW PRDIF 12 19 22- 4 .04 28 4.3 299-257 WIND&RH WK SSE-NNW PRDIF 18 20 4-10 .13 26 4.1 257-228 RH ONLY NORMAL PGRAD 24 20 10-16 .17 27 4.3 228-210 RH ONLY NORMAL PGRAD 30 20 16-22 .09 28 4.4 210-109 WIND&RH WK SSE-NNW PRDIF 36 20 22- 4 .00* 30 4.6 109- 49 WIND&RH WK SSE-NNW PRDIF MODIFIED TOTS 04-04 .38 MOD-FAC = .85 * = 700mbWD >344 or <155 DEG 42 21 4-10 .00* 38 5.4 49- 15 WIND&RH WK SSE-NNW PRDIF 48 21 10-16 .00* 51 6.6 15- 14 WIND&RH WK SSE-NNW PRDIF 54 21 16-22 .00* 57 7.3 14- 7 WIND&RH WK SSE-NNW PRDIF 60 21 22- 4 .00 56 7.1 7-303 WIND&RH WK SSE-NNW PRDIF MODIFIED TOTS 04-04 .00 MOD-FAC = .85 * = 700mbWD >344 or <155 DEG

Rhea Orographic Aid Basins