Best I can interpret is that the allday number is the probability that it'll be raining at any randomlyselected point during the day. Thus, if it were known with 100% certainty that 100% of the area would be dry all morning, and rain from the stroke of noon to the end of the day, then the daily number would be 50%.
But then, that interpretation is inconsistent with the numbers in post #8.
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 As You Like It, III:ii:328
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