How the forecast has moved
valid two days out
issued 12Z 4 Sep
One moment — 12Z 6 Sep, about 48 hours out — as every stored GFS run has drawn it.
Methodology
Step through the runs and the pressure pattern moves, and the precipitation under it moves with it: that motion is the forecast changing its mind, which is what this site exists to show. Same chart grammar as the surface analysis — the same reduction, the same interval, the same frame; only the issuing run varies.
Every map on this page is drawn from model output we store ourselves, not from anyone's picture of it. The per-city read of the same day — which cities the models are fighting about, and what the discussion makes of it — is on the national page.