Flash floods are slightly more complex to detect as it depends more on where excessive rainfall is occurring. The severity of rains cannot be determined and flash flooding occurs on a real-time basis based on the location. Another reason is that it is difficult to prepare for it as they can happen suddenly when there are heavy rains, sometimes within a few short hours. 

Further Reading: 

• South Asian Flash Flood Guidance System
• Indian Meteorological Department
• Floods in India – An Overview