1. January-March 2009 T&P anomalies vs. trends & La Niña: How did the CPC forecast pan out? Klaus Wolter (NOAA-ESRL) - 14 may09 What was the CPC forecast for late winter 2009? What happened in January-March 2009? Were the anomalies consistent with the tools employed (La Ni ñ a + trend)? What role did the snowcover play in ‘ tweaking ’ the temperature outcome?

2. CPC forecasts for January-March 2009: Temperature Unambiguous weak-to- moderate La Ni ñ a conditions (right) + OCN were main tools used in CPC forecast (top right: gridded version)

3. CPC forecasts for January-March 2009: Precipitation Unambiguous weak-to- moderate La Ni ñ a conditions (right) + OCN were main tools used in CPC forecast (top right: gridded version)

4. Verification for January-March 2009: Temperature Non-EC forecasts: 16.6 (2008: -5.9) All: 10.1 (2008: -4.7) Coverage: 61% (81%)

5. Verification for January-March 2009: Temperature Non-EC forecasts: 16.6 (2008: -5.9) All: 10.1 (2008: -4.7) Coverage: 61% (81%) Two-category errors on both sides! Whoops!

6. Verification for January-March 2009: Precipitation Non-EC forecasts: 41.2 (2008: 31.5) All: 13.1 (2008: 20.3) Coverage: 32% (64%) - big drop in coverage!

7. Verification for January-March 2009: Precipitation Non-EC forecasts: 41.2 (2008: 31.5) All: 13.1 (2008: 20.3) Coverage: 32% (64%) - big drop in coverage! Whoops! Two-category errors on dry side!

8. U.S. temperature anomalies based on OCN (10yr) Out of 344 climate divisions, 97 have an average standardized temperature anomaly for ‘99-08 JFM that exceeds +0.43 sigma (~upper tercile), none are below -0.43 sigma. Most of these ‘warming’ divisions reside in the Western U.S. (TX, AZ, NM, CO+).

9. U.S. temperature anomalies based on OCN (10yr) In January-March 2009 (below), 59 out of 97 flagged climate divisions end up in the upper tercile which translates into a non-EC Heidke Skill Score of +41 for OCN-based temperature forecasts. Total national HSS: +12

10. U.S. temperature anomalies based on GHG-based warming only Out of 344 climate divisions, 80 have an average standardized temperature anomaly from GHG-based forcing that exceeds 0.43 standard deviations, none are below -0.43 sigma. Most of these ‘warming’ divisions reside in the western third of the U.S., with a handful in the northeast quadrant.

11. U.S. temperature anomalies based on GHG-based warming only In January-March 2009 (below), 27 out of 80 flagged climate divisions end up in the upper tercile which translates into a Heidke Skill Score of +01 for GHG- based temperature forecasts. Total national HSS: +00

12. U.S. temperature anomalies in La Ni ñ a winters Ten strongest La Ni ñ a cases (left): out of 344 climate divisions, 184 (or 53%) show composite anomalies in excess of 0.43 standard deviations (33% tails) vs. 18 below -.43. Warmth overlaps with OCN-flagged area in southwestern U.S. Moderate La Ni ñ a cases (MEI ranks 6-15; right): out of 344 climate divisions, 107 show composite anomalies above 0.43 standard deviations (33% tails) vs. 21 below -.43. Pattern correlation with strong La Ni ñ a cases: +0.96 - similar anomalies, just weaker.

13. U.S. temperature anomalies in La Ni ñ a winters In January-March 2009 (top right), 63 out of 184 strong La Ni ñ a-flagged climate divisions (top left) and 37 out of 107 such divisions for weak-to- moderate La Ni ñ as (left) end up in the upper tercile, while 12 out of 18 strong (11 out of 21 weak) La Ni ñ a cold divisions end up in the lower tercile. This translates into a Heidke Skill Score of +06 for both La Ni ñ a-based temperature forecasts. Total HSS: +3 (2)

14. 2007-08 impact of anomalous snowpack JFM’08 was cold in the Intermountain West and Midwest with a warmish pocket in between (MT+). Except for New England, temperature anomalies match snowdepth anomalies quite well. Snowdepth anomaly map for 6z, 22dec07 - Source: National Operational Hydrologic Remote Sensing Center (NOHRSC)

15. 2008-09 impact of anomalous snowpack Cold winter from northeastern MT eastward across ND into WI and further east matches snowpack influence, but not La Ni ñ a nor OCN composites! Conversely, lack of snow in southern MT and WY helps OCN to ‘ win ’ against La Ni ñ a impacts. On the other hand, southern UT and southwestern CO lost their snowcover and cold air to end up warm in JFM ’ 09 despite high snowpack early on. Conclusions from two winters: southern (<40N?) and coastal snowpack anomalies need ‘ help ’ to persist&exert influence! Snowdepth anomaly map for 6z, 22dec08 - same scale as previous year! Source: National Operational Hydrologic Remote Sensing Center (NOHRSC)

16. U.S. precipitation anomalies based on OCN (15yr) Out of 344 climate divisions, 17 have an average standardized precipitation anomaly for JFM ‘94-08 that exceeds +0.43 sigma (~upper tercile), 28 are below -0.43 sigma. Most coherent wet (dry) trend is found in northeast (southeast) U.S.

17. U.S. precipitation anomalies based on OCN (15yr) In January-March 2009 (below), only 2 of the 17 flagged wet-trending climate divisions ended up in the upper tercile, but 20 of the 28 flagged dry-trending divisions verified in lower tercile. Total OCN-based skill score: +23. Total national HSS: +3

18. U.S. precipitation anomalies in La Ni ñ a winters Ten strongest La Ni ñ a cases (left): out of 344 climate divisions, 119 show composite anomalies in excess of 0.43 standard deviations (33% tails) vs. 67 below -.43. Dryness overlaps with OCN-flagged area in southeastern U.S. Moderate La Ni ñ a cases (MEI ranks 6-15; right): out of 344 climate divisions, 65 show composite anomalies above 0.43 standard deviations (33% tails) vs. 51 below -.43. Pattern correlation with strong La Ni ñ a cases: +0.80 - similar anomalies, just weaker.

19. U.S. precipitation anomalies in La Ni ñ a winters In January-March 2009 (top right), 21 out of 119 strong La Ni ñ a-flagged climate divisions (top left) and 12 out of 65 such divisions for weak-to-moderate La Ni ñ as (left) end up in the upper tercile, while 62 out of 67 strong (46 out of 51 weak) La Ni ñ a dry divisions end up in the lower tercile. This translates into HSS: +17 (+25) for top 10 (ranked 6-15) La Ni ñ a-based precipitation forecasts. Total HSS: +9 (8)

20. U.S. temperature and precipitation anomalies in models driven by global SST (AMIP) in JFM’09 Modeled temperature anomalies (left) covered 190 climate divisions with anomalies in excess of +0.43 sigma (13 below -0.43). Observed anomalies matches the positive forecasts in 70 divisions (low skill), but 12 out of 13 negative divisions, yielding an overall non-EC HSS of +11 (national score: +6). For precipitation (right), only 20 climate divisions reached the negative 0.43 sigma threshold (none above +0.43), and matched the observations 18 times (HSS: +85; national score: +5).

21. Summary CPC’s winter forecast scored better than last year - a combination of less ambition (smaller area coverage) and good use of OCN and La Ni ñ a information. Compared to the previous winter, warming trends re-asserted themselves in much of the country, leaving OCN as the best temperature forecast tool this time around. GHG-based temperature projections for the U.S. were less well matched than either La Ni ñ a- or OCN-based forecasts. La Ni ñ a was not as strong as in the previous winter, but provided for good guidance for cold temperatures and low precipitation anomalies. In a rare almost-repeat of the previous year, fairly extreme snowdepth anomalies (both positive and negative) could have been used to tweak temperature forecasts in the right direction and avoid 2-category forecast misses which may be the most damaging to reputation. Hindcast model forecasts for U.S. temperature and precipitation based on global SST did a very competitive job in predicting both fields.