I'm reading Dirac's "Principles of quantum mechanics" right now, being a little confused about the following part: (Chapter V$\S$31: "The Motion of Wave Packets").
He's making the following ansatz:
"Suppose that the time-dependent wave function is of the form $$\psi(\vec{q},t) = Ae^{iS/\hbar}\tag{35},$$ where $A$ and $S$ are real functions of $\vec{q}$ and $t$."
Schrödingers Wave equation gives us $$i\hbar\frac{\partial}{\partial t}Ae^{iS/\hbar}=H(\vec{q},\vec{p})Ae^{iS/\hbar}.$$
Then letting $\hbar \rightarrow 0$, and setting $\vec{p} = \nabla_{\vec{q}} S$, and assuming $\hbar\nabla_{\vec{q}}A \ll A \nabla_{\vec{q}} S$, he derives that
$\frac{d\vec{p}}{dt} = - \nabla_{\vec{q}}H(\vec{q},\vec{p})$.
My question is: How can i make sense of defining $\vec{p} = \nabla_{\vec{q}} S$, dealing with wave packets. I see that in operator terms $<\vec{p}> = \int (A^2\nabla_{\vec{q}} S)dq$, but im wondering if there's some way to interpret $\vec{p}$ for example as a pointwise momentum of the wave packet?
Generally speaking: Is there some other way to talk about momentum or velocity of a wave packet besides the mean value described above?