After reading another post in Introductory Weather & Chasing, I got to thinking: How does a person go about learning how to piece together numerical models? During my baby steps in forecasting, I used only the GFS. I was aware that NAM and RUC existed, and at times I heard rumors of an ECMWF and GEM and other more esoteric models, but I didn't understand why I needed them. Wasn't one model enough? Go ahead, smile, but remember that others here are presently in that very same place, wondering how to make sense of the model forecast maps. One of the challenges is, of course, understanding how to connect the dots within a given model. But another concern lies in determining which model has the best handle on a given synoptic situation. So I thought I'd start a thread that focuses specifically on the GFS. If it proves helpful, other threads can be started for other models. Here are two simple questions: What strengths and what weaknesses have you encountered with the GFS? What are your own "best practices" in using it? My own knowledge in this area still has a lot of gaps to fill, so I stand plenty to learn in posing the question. However, I'll kick things off with a few of my own basic observations. I welcome correction and/or expansions on the following. And of course, please add your own insights: STRENGTHS: The GFS is the only complete package of long-range model forecast maps available free to the public. It can alert you to possible synoptic systems that may evolve down the road. Look for consistency in a long-range setup, because it just may hold together until it falls within range of NAM corroboration. But don't be surprised if everything falls to pieces once it draws closer. WEAKNESSES: The GFS becomes increasingly unreliable beyond around 5 days. After that, its potential for inaccuracy hugely increases. At 7.5 days out, or 180 hours--after which the GFS moves from 6-hour updates to 12 out to 384 hours--the GFS becomes more a matter of fortune-telling than forecasting. That's why pinning your hopes on a long-range forecast is pointless, let along trying to get specific about it. Three days is the threshold for starting to look more closely at the maps. Beyond that, it's a matter of looking for consistency from run to run without worrying about details. The farther out you go, the more it's just a matter of looking for a general consistency. The GFS tends to be aggressive with its timing. The GFS likes to scoot systems eastward faster than the ECMWF and NAM; then as a system moves to within the 3-day range, it often slows down and agrees with the other models. This is just a generalization, though; there are times when the GFS is accurate in its bullishness. It pays to compare the GFS with the Euro (ECMWF), which is usually more conservative in its timing, and, it's commonly conceded, more accurate overall.