wonlon,

Here is the link to Russian dictionaries: Словари и справочники русского языка для скачивания

you can download them for free and there are 2 additional links below when it says и многие другие словари и справочники. )

some of these dictionaries may seem hard to use for non-Native speakers, but some are good for any level (I actually downloaded all of them, it's like a small e-library with professional and very reliable sources)

your word Поутру is described very well in Скворцов Л.И. Большой толковый словарь правильной русской речи (book #21 in this list, see page 649), with several good usage examples from literature.