GeneratingGeneratingQuantitative Precipitation ForecastsQuantitative Precipitation Forecasts

for River Modelingfor River Modeling

Mike Ekern, Sr. HAS Forecaster

NOAA, National Weather Service

California Nevada River Forecast Center

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

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

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

Lose detail in the terrain!

AnnualPrecipitation Topography

Influence of Terrain on Precipitation

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 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

Numerical Weather Prediction

QPF Process SequenceNational QPF – Hydrometeorological Prediction Center

QPF data are converted from contours topoint data using bilinear interpolation and sent to RFCs.

HPC

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

Effectively reduces the size of the basinthat contributes to runoff

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

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

• 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

Rhea Orographic Aid Basins