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The contours on the map show the total probability (%) of two
categories, above, indicated by the letter "A", and below, indicated by
the letter "B". At any point on the map, the sum of the probabilities
of these two categories is 100%.
For any particular location, and season, these two categories are
defined from the 30 observations from 1981-2010. The coldest or driest
1/2 (15 years) define the B category, the warmest or wettest 1/2 (15
years) defines the A category.
When the forecasters decide that one of the extreme categories, say
above (A), is the most likely one, they assign probabilities which
exceed 50% to that category. This means that the chance of the opposite
category is the remaining part of the total (100%). In regions where
the forecasters have no indications favoring either A, or B, the chance
of these two categories is defined to be 50% each, and the region is
labeled "EC", which stands for equal chances.
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www.nws.noaa.gov 
