Toward Identifying Subseasonal Forecasts of Opportunity Using North American Weather Regimes

Large-scale atmospheric circulation regime structures are used to diagnose subseasonal forecasts of wintertime geopotential height fields over the North American sector, from the NCEP CFSv2 model. Four large-scale daily circulation regimes derived from reanalysis 500-hPa geopotential height data using K-means clustering are used as a low-dimensional basis for diagnosing the model's forecasts up to 45 days ahead. On average, hindcast skill in regime space is found to be limited to 10-15 days ahead, in terms of anomaly correlation of 5-day averages of regime counts, over the 1999-2010 period. However, skill up to 30 days ahead is identified in individual winters, and intraseasonal episodes of high skill are identified using a forecast-evolution graphical tool. A striking vacillation between the West Coast and Pacific ridge patterns during December-January 2008/09 is shown to be predicted 20-25 days in advance, illustrating the possibility to identify "forecasts of opportunity" when subseasonal forecast skill is much higher than the average. The forecast-evolution tool also provides insight into the poor seasonal forecasts of California precipitation by operational centers during the 2015/16 El Nino winter. The Pacific trough regime is shown to be greatly overpredicted beyond 1-2 weeks in advance during the 2015/16 winter, with weather-scale features dominating the forecast evolution at shorter lead times. A similar though less extreme situation took place during the weaker El Nino of 2009/10, with the Pacific trough overforecast at S2S lead times.