The wiggly graphs below, and here, are the GFS ensembles for the four latitude-longitude points bordering the Three Valleys. These readings correspond approximately with Annecy (to the North-West), Martigny (North-East), Bourg d’Oisans (South-West) and Sestriere (South-East).
The Global Forecast System (GFS) is a numerical weather prediction system, run four times a day and creating forecasts for up to 16 days. However, the forecast gets increasingly inaccurate, particularly beyond three or four days.
The model is run four times a day at “00z”, “06z”, “12z” and “18z” (Z for Zulu, the same as GMT), and the GFS, like all the major numerical models, is executed several times for each run, each with slight changes to the original data. Each output from the model is represented by one of the coloured lines, or “wigglies” on the graph. The thick red line is the 30-year average, the thick blue line is the most probable model run and the grey line is the average of all the model runs.
Looking at the ensembles, you can judge the reliability of the forecast given by the main (control) run of the model being correct (the thick, blue line); if a large number of the ensemble runs are very similar to it, it is a good indication that confidence is reasonably high of the outcome.
There are two sets of lines in the forecast graphs: the top set represents temperatures at 850 Hpa (The 850mb pressure level is at about 1450 meters altitude). The lower set represents precipitation. For snowfall, we would want to see low temperatures in conjunction with precipitation. From this information we can calculate snowfall knowing that each 100m gain is a 0,6ºC temperature drop and that snow begins to appear at around 2 to 3ºC.