Prog Charts are valid at the time printed on the chart. A Prog Chart that says it is valid at 00Z on December 31 (Valid 00Z Thursday December 31, 2015) is valid at 00:00Z on December 31.
This 60 HR Prog Chart shows a valid time of 00:00Z on December 31.
In contrast, the valid time shown on the 12 HR Probability of Precipitation (PoP) charts is the ending time of the 12-hour valid period.
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