Computer Models forecast based on a certain set of current conditions whenever the model runs. They forecast for 6 hours out, use the 6 hour forecast to forecast for 12 hours, etc. until the model reaches its particular time limit.
Now one problem with this lies in Chaos Theory's "Butterfly Effect." That is...one small error can lead to HUGE errors down the road. A simple example of the butterfly effect is...let's take the numeric value "a" as our initial point. From "a" we need to get the number "j", but to do this we have to go through numbers "b", "c", "d", "f", "g", and "h". If the value of "a" = 28.3, but we use "a" = 28 to make it simpler, and if "b" = 3.1"a", "c" = 3.1"b", etc. then our final estimated number "j" = 77035.3, differing from the actual value of "j" = 77860.7 by 825.4. So the error difference went from .3 to 825.4!
This is what happens in weather computer models, obvious the 6-hour forecast will be correct or very close to correct. The keywords here are "very close". This small error here, and in subsequent forecasts, the following errors, will eventually lead to errors so large that the forecasted weather pattern given by the models could be the exact opposite of what occurs! (It could forecast a trough, when a strong high actually becomes present!)
So now you ask yourselves "How far out should I go before I should ignore the model's output?" Generally anything within 72 hours is good, and anything within 120 hours is "okay." Beyond 120 hours you can pretty much consider bovine fecal matter.
I hope this clears up the misuse of models on these boards in the future and that all of you have learned something new about weather forecasting tools!
