Comment: the IC carries three timesteps (T-12h, T-6h, T0)

predict.py

@@ -51,10 +51,10 @@ OUTPUT_UNITS = {"temperature_2m": "K"}
 
 # Index of T+0 on the IC's time axis.
 #
-# The IC carries T-6h and T+0 in that order. Persistence forecasts from the most
-# recent analysis, so it is the last element, not the first. Index 0 would
-# forecast from six hours before the init and silently lose that much skill at
-# every lead.
+# The IC carries T-12h, T-6h and T+0 in that order. Persistence forecasts from
+# the most recent analysis, so it is the last element, not the first. Index 0
+# would forecast from twelve hours before the init and silently lose that much
+# skill at every lead.
 T_ZERO = -1
 
 # 0.25 -> 1.0 degrees. Both axes, because the grid is square in degrees even