Not yet reviewed by Pith; the record is open.

We investigate the role of the chiral anomaly in hydrodynamic and crossover regimes of transport in a Weyl or Dirac semimetal film. We show that the magnetic-field-dependent part of the electric conductivity in the direction of the magnetic field develops an unusual nonmonotonic dependence on temperature dubbed the anomalous Gurzhi effect. This effect is realized in sufficiently clean semimetals subject to a classically weak magnetic field where the electron-electron scattering dominates and the relaxation of the valley-imbalance charge density happens at the boundaries. Moreover, the conditions for the realization of the conventional Gurzhi effect in hydrodynamic and crossover regimes of transport in three-dimensional Dirac and Weyl semimetals are determined.