The graphic below lops through GFS 228, 204, 180, 156, 132, 108, and 84 hour forecasts valid at 0000 UTC 4 December (5 PM MST Sunday). Imagine trying to forecast for Sunday afternoon based on just this single forecast system. Good luck.
The loop also illustrates the folly of forecasting based on model trends, or what forecasters call "DModel/Dt" (i.e., the rate of change of the model forecast with respect to time). Clearly, there is no evidence of a consistent trend in those forecasts. The pattern is too chaotic, leading to a lack of run-to-run consistency, even in the more recent forecasts.
It's fun to talk about DModel/Dt, but studies have found that it has little forecast value for medium-range forecasts. Hamill (2003) examined the value of DModel/Dt and here's what they found:
"Extrapolation of forecast trends was shown to have little forecast value. Also, there was only a small amount of information on forecast accuracy from the amount of discrepancy between short-term lagged forecasts. The lack of validity of this rule of thumb suggest that others should also be carefully scrutinized before use.Let's put this rule of thumb to bed.