  We discuss a fluid model with inclusion of the complete pressure tensor
dynamics for the description of Weibel type instabilities in a counterstreaming
beams configuration. Differently from the case recently studied in Sarrat et
al. 2016, where perturbations perpendicular to the beams were considered, here
we focus only on modes propagating along the beams. Such a configuration is
responsible for the growth of two kind of instabilities, the Two-Stream
Instability and the Weibel instability, which in this geometry becomes
"time-resonant", i.e. propagative. This fluid description agrees with the
kinetic one and makes it possible e.g. to identify the transition between
non-propagative and propagative Weibel modes, already evidenced by Lazar et al.
2009 as a "slope-breaking" of the growth rate, in terms of a merger of two non
propagative Weibel modes.
