The -flush option causes wgrib2 to flush the
output buffers after every write. You would use this option
if you were running wgrib2 as an interactive decoder or using
pipes to multitask the processing.
Multitasking wgrib2 by using pipes is well established, so wgrib2
detects when writing to a pipe and enables the flush mode.
If wgrib2 is writing to a disk file and another program is reading
that same file be the file is closed, then you will need to use
the -flush option.