Why is Fantasy Island, Fantasy Island? What prevents models from looking further...

hatrickpatrick

...than about 5 days ahead with accuracy?

I mean presuming weather is basically a continuing chain of cause and effect, action and reaction, presumably involving solar output, Earth's position in space, its angle, and then earthly factors regarding air pressure which lead on from one another - all of which you'd imagine would be at least relatively mathematically predictable - why exactly can't we run a model which accurately forecasts weeks if not months into the future?

Is it because there are variables which are simply too unpredictable and random which influence our weather?
Is it that we don't actually know what all the variables are? That there are some things which influence weather which we don't understand or processes we haven't quite figured out the mechanisms of?
Or is it more that most of it is predictable, but we either don't have the equipment capacity to collect all that data or the computing power to analyze it all?

Sorry if it seems like a silly question. This is the product of going out for more than a few drinks with your mates at a time when the weather is even more prominent in casual conversation than it usually is :pac:

maquiladora

I was going to write something but wikipedia sums it up nicely.

Factors affecting the accuracy of numerical predictions include the density and quality of observations used as input to the forecasts, along with deficiencies in the numerical models themselves. Although post-processing techniques such as model output statistics (MOS) have been developed to improve the handling of errors in numerical predictions, a more fundamental problem lies in the chaotic nature of the partial differential equations used to simulate the atmosphere. It is impossible to solve these equations exactly, and small errors grow with time (doubling about every five days). In addition, the partial differential equations used in the model need to be supplemented with parameterizations for solar radiation, moist processes (clouds and precipitation), heat exchange, soil, vegetation, surface water, and the effects of terrain.