A Priori Identification of Skillful Extratropical Subseasonal Forecasts

The current generation of subseasonal operational model forecasts has, on average, low skill for leads beyond 3 weeks. This is likely a fundamental property of the climate system, due to the relative high amplitude of unpredictable weather variability compared to potentially predictable, but generally weaker, climate signals. Thus, for subseasonal forecasts to be useful, their high versus low skill events should be identified at time of forecast. We show that a linear inverse model (LIM), an empirical-dynamical model constructed from covariability statistics of wintertime (December-March) weekly averaged observational analyses, can be used to identify, a priori, the expected extratropical subseasonal surface and midtropospheric forecast skill. The LIM's predicted signal-to-noise ratio identifies the subset (10%-30%) of Weeks 3-6 forecasts-of the LIM and two operational models from the National Centers for Environmental Prediction and the European Centre for Medium-Range Weather Forecasts-with relatively higher skill versus the much larger remainder of forecasts whose skill cannot be distinguished from random chance.