  Lattice quantum chromodynamics has proven to be an indispensable method to
determine nonperturbative strong contributions to weak decay processes. In this
white paper for the Snowmass community planning process we highlight
achievements and future avenues of research for lattice calculations of weak
$b$ and $c$ quark decays, and point out how these calculations will help to
address the anomalies currently in the spotlight of the particle physics
community. With future increases in computational resources and algorithmic
improvements, percent level (and below) lattice determinations will play a
central role in constraining the standard model or identifying new physics.
