452 Precipitation

Northwest Weather = Terrain + Ocean Influence

Annual Precipitation

SW Olympic Slopes-Hoh Rain Forest: 150-170 inches yr-1

Sequim: roughly 15 inches per year.

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Puget Sound Convergence Zone

Bias scores for the (a) 1.33- and (b) 4-km

model simulations for 1400 UTC 13 Dec 2001 through 0800 UTC 14

Dec 2001.

Human Versus Objective Skill for Precipitation Forecasts: NWS Offices Around the US

Prob. Of Precip.– Cool Season(0000/1200 UTC Cycles Combined)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996 1999 2002

Year

Brier Score Improvement over Climate

Guid POPS 24 hr Local POPS 24 hr

Guid POPS 48 hr Local POPS 48 hr

Brier Scores for Precipitation for all stations for the entire study period.

Brier Score for all stations, 1 August 2003 – 1 August 2004. 3-day smoothing is performed on the data.

Brier Score for all stations, 1 August 2003 – 1 August 2004, sorted by geographic region.

Precipitation

Reliability diagrams for period 1 (a), period 2 (b), period 3 (c) and period 4 (d).

More than a day out….Human Forecasters Tend to Overpredict 10-30%--exaggerating the threat of rain when it is not likely.“Its probably not going to rain…but I will throw in 10-30% anyway to cover myself”

And underpredict rain when it is fairly definite. “Well it looks like it will rain…but I am unsure…so I will knock down the probabilities to 60-80% to cover myself”

Rain Psychology

Humans Help with Heavier Precipitation

Convective Precipitation

• Need at least 2-4 km resolution to get most convection even half right.

• If grid spacing is more coarse must have cumulus parameterization--Kain-Fritsch is used for MM5.

• Convection is least skillful precipitation in virtually all models.

• High resolution models can potentially give a heads on type of precipitation a day ahead (squall line, supercell, etc.)

Real-time WRF 4 km BAMEX Forecast

Composite NEXRAD Radar

4 km BAMEX forecast 36 h Reflectivity

4 km BAMEX forecast 12 h Reflectivity

Valid 6/10/03 12Z

Real-time 12 h WRF Reflectivity Forecast

Composite NEXRAD Radar

4 km BAMEX forecast

Valid 6/10/03 12Z

10 km BAMEX forecast

22 km CONUS forecast

Hurricane Isabel Reflectivity at Landfall

Radar Composite

18 Sep 2003 1700 Z

41 h forecast from 4 km WRF

The End