  We present findings of 3D filamentary structures in the Smith Cloud, a
high-velocity cloud (HVC) located at $l=38^{\circ}$, $b=-13^{\circ}$. We use
data from the Galactic Arecibo L-Band Feed Array \ion{H}{i} (GALFA-\ion{H}{i})
along with our new filament detection algorithm, \texttt{fil3d}, to
characterize these structures. In this paper, we also discuss how different
input parameters affect the output of \texttt{fil3d}. We study filaments in the
local ISM and compare them to those found in the Smith Cloud. Based on thermal
linewidth estimations we find supporting evidence that the Smith Cloud
filaments are part of its warm neutral medium. We also find a relationship
between thermal linewidth and the $v_{LSR}$ of the filaments. We study the
plane-of-sky magnetic field as traced by Planck 353 GHz polarized dust emission
along the line of sight and find the HI filaments in this region are not
aligned with the magnetic field. This is likely related to their location close
to dynamic processes in the Galactic Plane and/or the low column density of the
filaments relative to emission in the Plane. The results show the HI filaments
are found in a wide range of Galactic environments and form through multiple
processes.
