  We apply the vertical Jeans equation to the Milky Way disk, in order to study
non-axisymmetric variations in the thin disk surface density. We divide the
disk plane into area cells with a 100 pc grid spacing, and use four separate
subsets of the Gaia DR3 stars, defined by cuts in absolute magnitude, reaching
distances up to 3 kpc. The vertical Jeans equation is informed by the stellar
number density field and the vertical velocity field; for the former, we use
maps produced via Gaussian Process regression; for the latter, we use Bayesian
Neural Network radial velocity predictions, allowing us to utilize the full
power of the Gaia DR3 proper motion sample. For the first time, we find
evidence of a spiral arm in the form of an over-density in the dynamically
measured disk surface density, detected in all four data samples, which also
agrees very well with the spiral arm as traced by stellar age and chemistry. We
fit a simple spiral arm model to this feature, and infer a relative
over-density of roughly 20 % and a width of roughly 400 pc. We also infer a
thin disk surface density scale length of 3.3--4.2 kpc, when restricting the
analysis to stars within a distance of 2 kpc.
