The emergency alert system on your EVO won't give you common severe thunderstorm warnings, flash flood warnings, tornado warnings, etc. The system is there for particularly dangerous situations, for example the National Weather Service sees a possible tornado outbreak and issues a particularly dangerous situation tornado watch/warning (2011 super outbreak) has to be issued by FEMA or your state emergency management agency. Other examples a possible hazmat situation that may threaten a city, township, county, nuclear emergencies, hurricane evacuations.
Don't rely on this feature if you see that severe weather is coming and feel that you may need to take the necessary precautions to keep you and your family safe.