Because weather models are run in different parts of the world and also cover different parts of the world, a universal time concept is preferred.
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Normal is climatology and is traditionally considered to be the National Weather Service standard 30-year normal, which is currently 1981-2010. This updates every ten years at the turn of the decade (next one: 2021). We normally get this climatology from the NOAA reanalysis data set; however, long-lead modeling like the CFS and European weeklies must use a special “hindcast climatology” to help it account for model drift errors and provide more realistic solutions.