If you are after Salmon or Sea Trout it is useful to know when there is a decent head of water on the river.
This is relevant on the Dee but even more so on other rivers in the area which generally fish better when they are in spate.
To check river levels before venturing out an ideal site is The Environment Agency site. Click on the blue dot on the part of the river you want to visit. This brings up a pop-up box. Click on the link within this box to view the river station details at that spot. You will be able to view a graph of the latest recorded river levels. Below the graph is a facility to change the period of the data.
For weather updates try this BBC site.