Probabilistic Forecasts of Extreme Precipitation Events for the U.S. Hazards Assessment

Kenneth Pelman

32nd Climate Diagnostics Workshop

Tallahassee, Florida

Outline

• Current Hazards Assessment

• Motivation for Probabilistic Forecast

• Details of Objective Probabilistic Tool

• Verification Results

• Conclusions

• Future Work

Current CPC Hazards Assessment

• Made each Monday-Friday and covers Days 3-14

• Designed to take current state of climate and predict hazardous weather conditions in support of CPC’s mission

• Hazards include heavy rain (a proxy for flooding), severe weather, extreme heat and cold, and severe drought

• Hazard forecasts are subjective and deterministic

Motivation for a Probabilistic Hazards Assessment

• Looking to improve on subjective scores

• Probabilistic forecasts provide more information about uncertainty to users

• Puts Hazards Assessment in same format as other popular CPC products, such as 6-10/8-14 day, monthly, and seasonal forecasts

Objective Probabilistic Heavy Precipitation Tool

• Uses 0z, 6z, and 12z GFS ensemble members to forecast for 881 grid points across the CONUS

• Rainfall totals not bias corrected or calibrated

• 1,2,and 3-day Hazards are forecast out to 384 hours

•A Hazard is defined as the greater of 1 inch/day or the 95th percentile value

•Climatology (1971-2000) derived from U.S. precipitation dataset (Higgins et. al. 2000)Forecast probability contours in solid green, hazard

thresholds in dashed black (in mm)

Reliability Diagrams (Using Total Precipitation in Period)

Reliability Diagrams (Using only 1 Day in Period)

1-Day Event Contingency Table Scores (10% threshold)

POD

a/(a+c)

Threat Score

a/(a+b+c)

FAR

b/(a+b)

Bias

a+b/a+c

Day 3 0.32 0.14 0.80 1.62

Day 4 0.26 0.12 0.81 1.45

Day 5 0.21 0.10 0.85 1.38

Day 6 0.14 0.07 0.87 1.10

Day 7 0.09 0.05 0.88 0.83

Day 8 0.05 0.04 0.90 0.55

Assessment 0.12 0.08 0.79 0.57

a b

c d

2- Day Event Contingency Table Scores (10% threshold)

1 Day in Period POD Threat Score FAR Bias

Days 2+3 0.13 0.09 0.76 0.55

Days 3+4 0.12 0.08 0.77 0.52

Days 4+5 0.10 0.07 0.78 0.48

Days 5+6 0.06 0.05 0.81 0.35

Assessment 0.12 0.08 0.79 0.57

Total Precipitation in Period

POD

a/(a+c)

Threat Score

a/(a+b+c)

FAR

b/(a+b)

Bias

(a+b)/(a+c)

Days 2+3 0.21 0.10 0.85 1.38

Days 3+4 0.17 0.08 0.86 1.18

Days 4+5 0.16 0.08 0.85 1.19

Days 5+6 0.15 0.08 0.86 1.09

Assessment 0.12 0.08 0.79 0.57

a b

c d

3-Day Event Contingency Table Scores (10 % threshold)

Total Precipitation in Period

POD

a/(a+c)

Threat Score

a/(a+b+c)

FAR

b/(a+b)

Bias

(a+b)/(a+c)

Days 1+2+3 0.08 0.05 0.91 0.95

Days 2+3+4 0.09 0.05 0.90 0.91

Days 3+4+5 0.08 0.05 0.88 0.67

Assessment 0.12 0.08 0.79 0.57

1 Day in Period POD Threat Score FAR Bias

Days 1+2+3 0.05 0.04 0.80 0.24

Days 2+3+4 0.04 0.04 0.81 0.22

Days 3+4+5 0.03 0.03 0.82 0.17

Assessment 0.12 0.08 0.79 0.57

a b

c d

ROC Diagram

Conclusions

• Limited usefullness of this tool in a strict probabilistic sense

• 1-Day Hazard forecasts show the best improvement over human-made Assessments

• Tool can immediately be used by forecasters as a first guess

• There is validity in converting the Hazards Assessments into a probabilistic forecast

Future Work

• Determine best definition of a hazard

• Generate contingency tables using different thresholds

• Use calibrated precipitation forecasts and calibrated probabilities.

• Use more ensembles in forecast (e.g. CAN, ECMWF, CFS)

Reference

• Higgins, R.W., W. Shi, E. Yarosh and R. Joyce, 2000: Improved United States Precipitation Quality Control System and Analysis. NCEP/Climate Prediction Center Atlas No. 7.