Normalized spread is computed by averaging the spread values at every
gridpoint separately for the most recent 30 days, for each lead time. Today's
ensemble spread then is devided by the average spread (for each gridpoint
and lead time separately). Normalized spread values below (above) 1 represent
areas where today's spread is below (above) the typical spread over the
most recent 30 day period. With this normalization the effect of
different lead times, different geographical areas, and different seasons
is eliminated. Consequently the forecasters can identify areas with below
and above average uncertainty in the forecast at any lead time, geographical
location, and season.
For further information, please consult the Reference and Training sections of the EMC global ensemble home page.