Rain Pursuit is an all-inclusive map displaying radar precipitation totals, rain and stream gauges, watershed boundaries, and correlations to local runs. Developed by Knoxville paddler Alex Zendel, this site makes chasing rain easier than ever before and is an excellent first stop when deciding where to paddle after a big weather event.
The quantitative preciptation forecast (QPF) which is issued daily by the NOAA is a famously fickle beast. These maps display the total predicted rainfall over the following 7 days, and can be further broken down into 1-, 2-, or 3-day totals. The predictions can change wildly in the span of seven days, so a big rain forecast doesn't mean guaranteed good flow, but they can be a good indicator if you should start complaining about the sniffles to get a few days off work later in the week.
For those interested in a no-nonsense site with just the data, the USGS rain gauges display recent rainfall data for 34 sites in Alabama at the time of writing. While this data is collected and reported elsewhere, for instance on the Rain Pursuit map, some prefer the old-school way of looking it up themselves.
It's hard to beat good old-fashioned radar for getting up-to-the-minute data on where the rain is falling. Most weather sites have a free radar map, and most of these get their data from the same sources, so just pick the one whose interface you like the best and get out there.