Can you spot the nested domain boundaries?

Why is there the rectangular outline of the inner domain in the accumulated precipitation?  What has likely happened is the following.

The outer domain has parameterized convection because it can't resolve the open-cellular convection present in this region.  This parameterization results in light precipitation across much of the domain. 

Meanwhile, the inner domain has explicitly resolved convection, which means that the convective instability is handled somewhat differently within the model.  In this domain, grid cells are 4 km by 4 km square, so ascending air in the model must rise in cells of this minimum scale.  This size makes forming shallow boundary-layer convection in this open cells more difficult, leading to less convection than in the outer domain, which is aided by the convective parameterization.

This example shows the reader to always take a large grain of salt when looking at mesoscale model output.