When it is raining outside and outside there is only so much you can do at the moment to stop the leak. However, there are some steps that you can take to mitigate the damage as well as reduce the total cost of repairs. Since you are not prepared for the leak, below are some of the things that you can do before a professional arrives.
Minimize interior damage
In most cases, the leak will most probably collect into a pool of water at the ceiling as it looks for an escape route. The water will find a penetration point, it can even be at a light fixture. With time it will find multiple escape routes. This could make the situation even worse and cause much more damage in the interior. You should check where there is a bulge. Poke the area and drain the water into a bucket. This not only minimizes interior damage but also minimizes the damage to the ceiling. After doing this call a professional.
Locate the entry point
This can be quite difficult while it is still raining. You need to start inspecting the inside such as from the attic. When looking for the leak, keep in mind that the leak in the roof will not always align with the leak in the ceiling. Water will generally travel down with gravity. Once you have located the point of entry you can temporarily repair it before an expert comes. You can use tarp or roofing tape. You can easily purchase these items from a convenience store.
It is important to stress how important time is when it comes to a roof leak. You cannot just wait it out since the leak will not fix itself. However, it will just keep getting worse with time. When the damage is too large you might end up replacing the entire roof. Ensure that the leak is fixed by a professional.