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GeneratingGeneratingQuantitative Precipitation ForecastsQuantitative Precipitation Forecasts
for River Modelingfor River Modeling
Mike Ekern, Sr. HAS Forecaster
NOAA, National Weather Service
California Nevada River Forecast Center
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QPFthe total amount of expected liquid precipitation (in hundredths of inches) in a future specified time period
Snow Levelsthe elevation above which frozen precipitation occurs and does not contribute immediately to runoff.
Definitions
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Operational Flood Forecasting
NWSRFSSAC-SMASNOW-17
ObservingSystems
data parameters
ModelCalibration
modelguidance
Hydrologist
hydrologicexpertise &judgment
HAS
Precip (QPF) TemperatureSnow Levels
ForecastInputs Flood Forecast
Guidance
• Bulletins• Graphics
www.cnrfc.noaa.gov
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NWSRFS Forecast Inputs
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
HAS
Precip (QPF) TemperatureSnow Levels
ForecastInputs
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Lose detail in the terrain!
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AnnualPrecipitation Topography
Influence of Terrain on Precipitation
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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
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Mountain Mapper Concept
PRISM
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Mountain Mapper Concepts
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Mean Areal Precipitation
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Forecasting Methodolgy0-12 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
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Numerical Weather Prediction
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QPF Process SequenceNational QPF – Hydrometeorological Prediction Center
QPF data are converted from contours topoint data using bilinear interpolation and sent to RFCs.
HPC
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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
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Snow Levels
Effectively reduces the size of the basinthat contributes to runoff
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NWSRFS Sensitivity to Melt LevelsMid-winter soil moisture conditions
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Rainfall (inches)
12-18 UTC 18-00 UTC 00-06 UTC 06-12 UTC
6-Hour Period
24 Hour QPF
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NWSRFSSensitivityto changes
in snow levels
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000Melting level (ft)
0
10
20
30
40
50
60
70
80
Pe
ak
flo
w r
ate
(c
fs x
10
-3)
Klamath River
Smith River
Trinity River
Truckee River
21
74
98 100
16
43
71
91
98100
1659
8796 100
735
71
89
96
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• 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
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Rhea Orographic Aid Basins
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