1. Geoff Roest MO 590/790

2.  Home builder in Southern California San Gabriel Mountains  Using unpaved access roads while building Concerned about washouts  Wants to know Are news reports regarding PDO and rainfall true? Consultant’s views on these issues

4.  Pacific Decadal Oscillation (PDO) – variation in sea surface temperature in North Pacific  Prominent report – LaDochy et al  Claims correlation between PDO and rainfall in Southern California is statistically significant

5.  I calculated the correlation using seasonal rainfall (July 1 to June 30) in Los Angeles for period of 1948 to 2003 Correlation coefficient = 0.08 Explains 0.64% of variance For sample size of 57 years – 45% confidence level  Correlation between PDO and seasonal rainfall in Los Angeles is not statistically significant

7.  Southern Oscillation Index (SOI) – based on sea level pressure at Darwin and Tahiti  Correlation between seasonal rainfall in Southern California and SOI is > -0.5 Explains > 25% of variability Meets 99.9% confidence level  Correlation between seasonal rainfall in Southern California and SOI is statistically significant

9.  Nino 3 – based on sea surface temperature anomalies  Correlation between seasonal rainfall in Southern California and Nino 3 index is > 0.5 Explains > 25% of variability Meets 99.9% confidence level  Correlation between seasonal rainfall in Southern California and Nino 3 index is statistically significant

12.  Negative PDO is not good indicator of rainfall during next 5 years Correlation not statistically significant  ENSO not good indicator Well correlated, but not well forecasted  My suggestion: identify threshold rainfall recurrence rate that the temporary access roads should withstand i.e. 100 year event (1% chance of occurring each year)  If roads are well constructed, more likely to avoid washouts