We're already experiencing minor flooding on the Boulder River near Big Timber, Montana and the warning will continue through Wednesday.

According to the National Weather Service:

The Flood Warning continues for
  the Boulder river at Big Timber affecting Sweet Grass County.
* Until Wednesday afternoon... or until the warning is cancelled.
* Flood stage is 7.0 feet.
* Minor flooding is expected.
* Forecast... the Boulder river at Big Timber is expected to crest
at 7.1 feet at 6 am MDT on Wednesday... then fall below flood stage
by Wednesday afternoon.