Many technical questions arise:
• How does one correct for mean error? The answer is by using harmonic climatologies for both forecasts (lead dependent) and verification over the period 1999-2010.
• Does this correction method have beneficial impacts on the independent data for 2011-15 forecasts generated in real time? The answer is yes. The calibration is applicable in spite of some unavoidable changes in the ‘system' since 2011.
CFSv2 has some modicum of skill for November – May, but almost none in NH summer. Among all the tools one can apply, the ensemble mean may be the most efficient in increasing the signal to noise ratio. Therefore 32 ensemble members (2-day lagged) may be best in the current operational set-up. For some variables, taking a 14 day mean is also helpful to reduce noise.
For 500hPa geopotential, the skill from November thru May appears related to 1 or 2 EOF modes, most notably the (N)AO. In many months, these modes can be predicted for week3&4 at 0.6 anomaly correlation, or better. The skill over the operational period 2011-2015 appears higher than for the retrospective period 1999-2010, perhaps because the initial states are getting better, or because nature projects more (by accident) onto these most predictable modes in recent years. Verification of observed/analysed surface fields (such as CPC ‘daily' temperature) is more complicated, but has been attempted. Plans for some extension of CFSv2 forecast menu (like more members from 0, 6, 12Z cycles) will be presented.
Saha et al (2010) The NCEP climate forecast system reanalysis. Bull Am Meteor Soc 91:1015–1057. doi:10.1175/2010BAMS3001.1
Saha et al (2014) The NCEP climate forecast system version 2. J Clim 27:2185–2208. doi:10.1175/JCLI-D-12-00823.1