CPC gridded verification

[nevildev]

I understand this is a vague question, but is anyone familiar with the CPC gridded seasonal verication at http://www.cpc.ncep.noaa.gov/products/predictions/long_range/tools/briefing/seas_veri.grid.php?  They are using a Heidke skill score which they describe as "CPC uses the Heidke skill score, which is a measure of how well a forecast did relative to a randomly selected forecast.".  When they say random forecast, are they literally creating a random forecast above and below the NCDC 30-year normal or are they using the 30-year normal?  I'm definitely not a statistics expert, and am trying to wrap my head around the entire process.  Any insight small or large would be appreciated.

 

Thanks,

Evan