To prognosticate the next big desert bloom is difficult because why?

Weather patterns are extremely random and complex. A desert bloom is a phenomenon that is contingent upon a series of climatic and biological conditions that do not occur very often, notably rainfall. If it rains too little there will not be enough humidity and water for plants to grow and if it rains too much the seeds and other dormant roots may be drowned or overwhelmed and unable to bloom.

we would need to be able to read the passage in question to determine which factor allowed david believe he could defeat goliath. post the passage, and i'd love to out.