This is exactly what you are looking for. It says outdated, currently still works with the newest build. Hopefully someone will take over it before it stops working for people.
Temporal Storm Timer
This is a simple mod with the objective of helping prepare for temporal storms. The warning bar (brown) is shown when a temporal storm is approaching, and the active bar (teal) is shown when a temporal storm is active.
Note: The author is on indefinite break. If you would like to continue this mod, feel free to use the included source code.