Model “spin up” is the time a model takes to combine initial conditions at the start of the model into a state of thermal and mass-velocity equilibrium. Basically the first few forecast hours of a new model run may not be accurate because of inadequate spin up time.
Models are very complex mathematical constructs that incorporate starting conditions and observations, in a process called “initialization”. Following initialization, spin up time is needed to accurately simulate and blend (and not conflict) with its past simulation so that it maximally captures ‘reality’. Short range weather models can take 2-3 hours to spin-up. Major global climate models can take years to “spin up”.
Some new models being developed by NOAA have a built in ‘cold start’, followed by an intentional 6 hour spin up.
Most current models do not ‘cold start’ but still have a spin up time to ‘mesh’ past runs into equilibrium with new observations. Example: The 00z models in the evening all have new upper air measurements that need to blend into equilibrium with the previous run. Thus, spin up time is needed.
If heavy rain is falling in our area at the start of initialization, it can throw off the first few forecast hours of a new model precipitation forecasts until equilibrium is reached.
Basically, one would think that the latest model is the best. Not true, especially if you’re looking at the forecast time just a few hours out.
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