Why can’t I run a back trajectory from 1900 UTC on 31 July, 2001 using downloaded FNL data? I attached my CONTROL file for you.

You are starting your trajectories at 1900UTC on 31 July and you have only defined the July meteorological data in your CONTROL file. Data fields are only every 6 hours (0,6,12,18 UTC) in the FNL archive. Therefore when you start at 1900UTC, the model needs to interpolate data to that time from 1800UTC on 31 July and 0000UTC on 1 August. You need to also define the August data in your control file and have it availble on your computer.

Roland Draxler