Similarities
1) Both require the first condition of excess vapour in the atmosphere to obtain.
2) Both happen after extended periods of global warming.
3) They both result in excessive amounts of rainfall leading to widespread flooding with far reaching consequences including deaths of humans and animals, displacement of populations, and significant damage to property especially infrastructure.
Differences
1) Flash floods affect specific regions at a time while El Ninos are global with increased rainfall across the world even in traditionally dry areas.
2) Flash floods are more predictable because they happen during normal rainy seasons though may be occasional off-season due to wind action. El Ninos set in at any time after major volcanic eruptions.
3) Flash floods are less durable lasting sometimes minutes, hours, days or even weeks. El Ninos last several months sometimes exceeding one year.
4) Rainfall patterns are also different with flash floods happening intermittently with sunny hours earlier in the day but heavy downfalls later in the evening. El Nino rains fall continuously sometimes throughout the day and night for several months with continuous cloud cover.

El Ninos and flash floods, is distinction necessary?
While both lead to inundation of land, deaths and destruction, some distinction is necessary. Imagine a scenario where you are required to evacuate your premises (home or business) to return after a week and the contingent preparations needed for the same. Would such preparations remain the same if one is to evacuate to return after several months or even a year? Preparing for El Nino requires long term thinking and planning because regions prone to flooding will remain under water longer.

Credibility of science
Terming every flooding event El Nino may make governments and populations assume that El Ninos are regular events they are familiar with and take a lot of things for granted. Putting dates to occurrence of El Ninos mean that in case what is experienced does not appear or turn out to be extraordinary then scientific predictions loose credibility. In more religious societies people start saying how natural phenomena are God’s plans and that humans cannot foretell or even prepare adequately for the same.

Preparing for El Nino
We constantly get reports of record breaking warm temperatures meaning we are in foreign territory as far as matters global warming is concerned. There are many places that were once under water but have since been inhabited because water receded or dried up. It is important to map out low lying grounds and ask ourselves whether or not we still want such places to remain human settlements and if so, what needs to be done.
Preparations for El Ninos should be a long term issue. Governments and their populations must start considering man-made rivers and lakes to provide easy flow and accommodation for excess water while using the soil removed from such rivers and lakes to raise the surrounding grounds to mitigate effects of flooding. It is equally important to remember that the worst is yet to come because when heat and pressure inside the planet overwhelme the crust causing it to crack open, then an ice age may set in. If the ice age is minor some regions of the world will survive but if it is major then we will all become extinct like the dinosaurs.