I have usually seen "a + adj + noun" instead of "adj + a + noun". In the sentence below, why the speaker used the latter? Did he want to transfer a special meaning e.g. emphasizing on the adjective, in this case the very long period that no one had expected? The situation was that the speaker went to the US for a meeting and suddenly he had to have an operation and stay a while in a hospital.
It's grand to be home again after so long an absence. [The Crown]