Bob Livezey and Marina Timofeyeva Diagnosis of Skill Variability of CPC Long-Lead Seasonal Forecasts: Implications for Production and Use Bob Livezey and Marina Timofeyeva NOAA/NWS/OCWWS/Climate Services Division Climate Diagnostics and Prediction Workshop October 21, 2004 Madison WI

Outline Introduction Skill Stratifications Results Conclusions and Lessons

Introduction Users should only care about the performance of forecasts that can potentially benefit their decision process Livezey (1990): There are non-random subsets of seasonal forecasts that were skillful enough to be useful

Seasonal Temperature Forecast Skill 1960s to 80s All Seasons 8.3 Winter 12.6 Spring 8.6 Summer 9.3 Fall 2.8 Seasonal Temperature Forecast Skill 1960s to 80s All Seasons 8.3 Winter 12.6 Spring 8.6 Summer 9.3 Fall 2.8

Introduction (Cont.) This talk will make the point : That was made by Livezey (1990) That there are many non-random subsets of forecasts that do not have useful skill That it is critical for this information to be shared with potential users That skill analyses with different stratifications are highly informative That seasonal forecast production should be dominantly objective That long-lead seasonal forecasts of non-trend signal should only be done on appropriate opportunities, not routinely

Displays and Stratifications CPC Seasonal Forecasts For 3-equally probable temperature and precipitation classes at 102 Climate Divisions Made every month from 1995 to present for 0.5-, 1.5-, …, 12.5 month leads Skill Measure: Modified Heidke Skill Score of Categorized Forecasts 1/3 EC’s scored as ‘hits’ Displays and Stratifications Summed over all forecasts for each lead for three overlapping seasons at a time (DJF to FMA, FMA to AMJ, etc.) Stratified further by strong ENSO years vs. other years Mapped for appropriate combinations of leads

Temperature

Temperature

Temperature

Results Seasonal Temperature Die-Aways: Moderate to high national-scale skill confined to Fall/Winter strong ENSO years at short to medium leads Otherwise skill is dominantly modest and level with lead (derived from biased climatologies, ie long-term trend) Worst forecasts are for Fall/Winter at short to medium leads for non strong-ENSO years: Forecasters subtracting value Summer/Fall at medium to long leads for strong ENSOs: No remedy except science advances Short-lead forecasts are better now than for the 1960s-80s, ~14 vs ~8 overall, ~20 vs ~13 for the winter

Temperature

Temperature

Temperature

Temperature

Results Seasonal Temperature Maps: Without a strong ENSO, justification for OND-DJF forecasts is questionable For certain regions/seasons/situations skill is unambiguously useful even for undisciplined, occasional users

Precipitation

Precipitation

Results Seasonal Precipitation Die-Aways: Barely useable national-scale skill entirely confined to Fall/Winter strong ENSO years in short to medium leads Otherwise skill is statistically indistinguishable from zero Short-lead forecasts overall seem to be no better now than for the 1960s-80s (but at least are now made two-weeks earlier)

Precipitation

Precipitation

Precipitation

Precipitation

Results Seasonal Precipitation Maps: Justification for long-lead forecasts is questionable for at least three of six seasonal groups examined For certain regions/seasons/situations skill is unambiguously useful even for undisciplined, occasional users

Conclusions and Recommendations There are non-random subsets of seasonal forecasts that are skillful enough even for undisciplined, occasional users These skills exclusively reflect ENSO and trend signals There are many non-random subsets of forecasts that do not have useful skill We must share this information with potential users Skill analyses with different stratifications are highly informative Seasonal forecast production should dominantly rely on objective exploitation of ENSO and trend with subjective modification only after rigorous criteria are met Routine production of certain long-lead forecasts should be terminated (can be accommodated by transition to separate trend and high frequency forecasts with latter issued on forecast of opportunity basis only)