Abstract - From a recent CPC blog post:  "We recommend that you track the verification skill scores over time (not just for one forecast), as they give you a feel for whether you should trust them or not.  Any individual forecast could hit the mark or be way off".  Below are the CPC seasonal forecasts for winter for the past 20 years.

Comments - This page summarizes the NOAA Climate Prediction Center (CPC) winter (December, January, February) seasonal forecasts and verification as published by CPC for 1995-96 through 2014-15.  The CPC verification methodology are a Heidke Skill Score (HSS) and are calculated for all of the United States and also for only the areas that did not have an EC (Equal chances) forecast.  Higher scores are better forecasts with 100 a "perfect forecast", zero a forecast no better tahn random chance a -50 a worst possible forecast.  A score of 50 means that approximately 2/3 of the verification (geographical) boxes verified.

It can be argued that the CPC seasonal forecasts only give a probabilistic characterization of whether a given area will be above or below but from my personal interactions with users for the past two decades that is not how the product is interpreted or used.  It should also be noted that a skill score like HSS do not quantify a good or bad a forecast is within it's category, but only if it is within the right category (i.e.,  above average or below average).  It is also unclear from the CPC website whether there have been changes in methodolgy over the past 20 years, but there are striking changes in the year-to-year performances of the high temperature scores before 2000 and then the much lower scores 2000-2003. Likewise there's  a significant dropoff in the precipitation scores from 2000-2006.

CPC Heidke Skill Scores All Winter Temperatures (DJF) 1995-96 to 2014-15	CPC Heidke Skill Scores All Winter Precipitation (DJF) 1995-96 to 2014-15
CPC Heidke Skill Scores Non-EC only Winter Temperatures (DJF) 1995-96 to 2014-15	CPC Heidke Skill Scores Non-EC only Winter Precipitation (DJF) 1995-96 to 2014-15
CPC Explanation of Heidke Skill Score
2014-15
2013-14
2012-13
2011-12
2010-11
2009-10
2008-09
2007-08
2006-07
2005-06
2004-05
2003-04
2002-03
2001-02
2000-01
1999-00
1998-99
1997-98
1996-97
1995-96

Source:   http://www.cpc.ncep.noaa.gov/products/predictions/long_range/tools/briefing/seas_veri.grid.php