We'll get into what roof leaks are all about and how you find, fix, and prevent them in Diagnosing a Roof Problem and Fixing a Roof Problem. Right now, if there's water coming into your house, you've got an immediate problem to contend with, so let's look at what you need to do now.
Around the House
Roof leaks can be deceiving as water can run quite a distance from the actual leak. For this reason, it doesn't do you much good to try to find a leak during a storm. Nor is it the safest approach. Wait until after the storm passes or the snow melts.
In the Nick of Time
Water than comes through your ceilng and flows down toward a wall can be hard to contain. You can divert it by nailing a small block of wood flat against the ceiling in the water trickle. This will direct the water into a bucket.
Hail, wind, and torrential downpours can all cause roof leaks, either from the elements themselves or what they do to other things, such as causing tree limbs to fall. If the storm is still raging, your goal is to minimize interior damage while you're waiting for the storm to end. Your plan of attack will depend on how much damage you see:
If you notice brown or reddish-tinged discoloration on the ceiling or walls that you haven't seen before, keep on eye on things. If the discoloration starts to bulge, it means that water is gathering behindthe plaster, wallboard, or ceiling tiles. To mini-mize damage, locate the center of the bulge and break it open by driving a nail or an awlinto the center of the stain area. Place a bucketunder the leak. (For tips on how to repair discoloration and water damage from roof leaks, see Repairing Water-Damaged Interiors.)
If water is coming into your house from a specific spot in the ceiling, place a bucket under the leak. Keep an eye on things, and empty the bucket as necessary.
If water is coming in from lots of places, position containers under as many spots as possible. Or cover the area where the leaks are dripping with a plastic tarp. If you don't have a tarp, garbage or leaf bags will work in a pinch.
If a ceiling is beginning to sag or a wall is starting to buckle inward, significant structural damage is a possibility. There's nothing you can do about it until the storm is over. If it's in a part of your house that poses a threat to you and your family, find somewhere else to stay until the storm passes, at which time you can have an expert (an inspector or contractor) come to assess the damage.