Generating Quantitative Precipitation Forecasts for River Modeling

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GeneratingQuantitative Precipitation Forecasts
for River Modeling

• Mike Ekern, Sr. HAS Forecaster
• NOAA, National Weather Service
• California Nevada River Forecast Center

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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.
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Operational Flood Forecasting
NWSRFS SAC-SMA SNOW-17
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NWSRFS Forecast Inputs
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

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Influence of Terrain on Precipitation
Annual Precipitation
Topography
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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 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

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Numerical Weather Prediction
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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.
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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 basin that
contributes to runoff
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NWSRFS Sensitivity to Melt LevelsMid-winter soil
moisture conditions
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NWSRFS Sensitivity to changes in snow levels
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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... lt SHASTA ABOVE SHASTA DAM -
SHDC1 gt
STRDA BEG-END QPF SLVL FRZGLVL 700DIR 6
19 16-22 .00 35 5.1 253-299
WINDRH WK SSE-NNW PRDIF 12 19 22- 4
.04 28 4.3 299-257 WINDRH 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 WINDRH WK SSE-NNW PRDIF 36
20 22- 4 .00 30 4.6 109- 49 WINDRH
WK SSE-NNW PRDIF MODIFIED TOTS 04-04 .38
MOD-FAC .85 700mbWD gt344 or lt155 DEG 42
21 4-10 .00 38 5.4 49- 15
WINDRH WK SSE-NNW PRDIF 48 21 10-16
.00 51 6.6 15- 14 WINDRH WK SSE-NNW
PRDIF 54 21 16-22 .00 57 7.3
14- 7 WINDRH WK SSE-NNW PRDIF 60 21 22-
4 .00 56 7.1 7-303 WINDRH WK
SSE-NNW PRDIF MODIFIED TOTS 04-04 .00
MOD-FAC .85 700mbWD gt344 or lt155 DEG
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Rhea Orographic Aid Basins
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