An extremely bright flash could indicate a nuclear detonation. Many nuclear explosions have a "double flash" effect, and some can glare for several seconds. Looking at a nuclear flash even momentarily could cause blinding burns.
Immediately "duck and cover": that is, get your head down and try to find something to hide under that may protect you -- or at least your head -- from collapsing walls and falling objects. Don't wait for the sound of an explosion; the blast wave of an atomic detonation expands at the speed of sound, and you may only have seconds to react.
The blast wave could take up to a minute to reach you. Wait twice this long before assuming you are safe from the blast.
Maintain your cover for several seconds after you feel an impact. The blast may effect your location more than once as it reflects off of hills or structures, and powerful wind gusts may be blow back towards the explosion or swirl unpredictably.
Your next concern should be fire. A nuclear flash can ignite fires even in areas far enough away to escape the blast. Dark, flammable surfaces facing the fireball are especially vulnerable, but all damaged structures carry their own fire risks, especially from broken electrical wiring or natural gas leaks. If you cannot be certain your structure is not on fire or filling with gas, follow the evacuation procedure for your location and get out immediately.
If this may have been a nuclear blast, you need to think carefully about where you want to be during the next few minutes, hours, and days.
If you believe this attack might be part of a nuclear war, and you are in a major city or militarily important location, it is very likely that more attacks will occur shortly. Stay in the safest place you can reach within seconds, and remain there for at least a few minutes. If you need to move outside, be constantly on the lookout for potential shelter in the event of another attack.
Your next concern is fallout: highly radioactive dust or flakes that will fall to the earth after the blast. This can start landing within minutes and continue falling for hours, and can spread over many miles. It will fall mostly in the immediate vicinity of the blast and in the areas downwind of it. It may affect your area even if you felt little or nothing of the blast, and can remain deadly for days or weeks. As soon as safely possible, you will want to determine where you are in relation to the blast and the direction of the wind.
A mushroom cloud is an obvious clue to where a blast occurred, but dust, smoke, or structures may obscure your view. In these cases, look to fallen poles, trees, or signage. Most will have fallen in the direction opposite the blast. You may also look for flash burn damage, which would occur mostly on surfaces that had faced the explosion.
Look to clouds and high trees to determine wind direction. If no wind direction is apparent, assume the prevailing wind pattern for your location. If you do not know this information, assume that the wind blows west to east if you are in the middle latitudes of the northern hemisphere (i.e. North America), and east to west if you are in the middle latitudes of the southern hemisphere.
If you are downwind of the blast, do not attempt to outrun the fallout by moving farther downwind. Instead, plan to move in a direction perpendicular to the wind and thereby sidestep most of the fallout. If you are lucky enough to receive official instructions regarding the fallout from this attack, follow them. Otherwise, gather any available supplies and leave as soon as possible. Try to put several miles between you and the likely fallout path.
The pattern of possible fallout expands with distance. The farther you are downwind, the farther you will have to go before you can be sure you are out of harm's way. Travel after a nuclear attack could prove slow and dangerous, and you should weigh your options carefully before deciding to move.
Whether you have moved or not, you can add an extra measure of safety by creating a "fallout room" at your location. The ideal fallout room is a small interior room that will put as much distance and mass as possible between yourself and the outside of the structure. Cellars and basements are good choices. Take any measures you can to thicken the walls and ceiling of this room by moving furniture and other items. Plan to spend as much time as possible in this room for the next few days, and to a lesser extent the next few weeks. Take all possible steps to avoid breathing, consuming or even touching outside dust during your stay.
Whatever your plans, keep in mind that electronic-dependent devices in the vicinity, including most motor vehicles, may have been rendered inoperable by an electromagnetic pulse accompanying the nuclear explosion.
In addition to these nuclear-specific measures, standard disaster response plans should be followed in the aftermath of a nuclear attack. Consult related resources for additional information.