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Generating Quantitative Precipitation Forecasts for River Modeling

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

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Generating Quantitative Precipitation Forecasts for River Modeling

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  1. GeneratingQuantitative Precipitation Forecastsfor River Modeling Mike Ekern, Sr. HAS Forecaster NOAA, National Weather Service California Nevada River Forecast Center

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

  4. Forecast Inputs 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

  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

  10. Mean Areal Precipitation

  11. 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 • Numerical weather prediction models • Pattern recognition • Ensemble prediction

  12. Numerical Weather Prediction

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

  14. Strengths/Weaknesses • Strengths • 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

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

  16. NWSRFS Sensitivity to Melt LevelsMid-winter soil moisture conditions

  17. 1 0 0 9 8 9 1 9 6 8 9 7 1 1 0 0 9 8 7 1 4 3 7 4 7 3 5 1 0 0 9 6 8 7 1 6 5 9 1 6 2 1 80 NWSRFS Sensitivity to changes in snow levels 70 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 0 1 T r u c k e e R i v e r 50 x s f c ( e t 40 a r w o l f k 30 a e P 20 10 0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 M e l t i n g l e v e l ( f t )

  18. 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

  19. Rhea Orographic Aid Basins

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