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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

GeneratingQuantitative Precipitation Forecastsfor River Modeling

Mike Ekern, Sr. HAS Forecaster

NOAA, National Weather Service

California Nevada River Forecast Center

definitions
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.
slide3

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

nwsrfs forecast inputs

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
slide6

Influence of Terrain on Precipitation

Annual

Precipitation

Topography

slide7

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
forecasting methodolgy
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
qpf process sequence
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.

strengths weaknesses
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
snow levels
Snow Levels

Effectively reduces the size of the basin

that contributes to runoff

slide27

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NWSRFS

Sensitivity

to changes

in snow levels

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slide28

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