This type of short writing question is present in all Spanish AS and A-level papers and the ideal answer for such a question is typically 2-3 sides long. The main goal is to creatively try and show as many grammatical structures within these paragraphs as possible without many errors. To do so it may be easier to structure this letter into 3/4 main areas: paragraph one describing the present situation to make use of various structures within the present tense, paragraphs 2 and 3 to describe in the past tense what happened the moment the incident occurred and the final paragraph to describe what you are hoping will happen in the future making use of future and conditional structures. Throughout it is best to use a range of interesting and niche vocabulary and with a limited range of topics generally you can learn specific ones for each question interesting examples for a natural disaster could be something like a huayco (specific to Incan civilisations in South America). It is also recommended to fit in as much use of subjunctive (to demonstrate your understanding of this) set rules may be applied and this can fulfil your marks on this section and if you are unsure generally you can use only these set examples and avoid those which are unclear to you. Idioms that are less specific and can be applied generally are good to learn for these essay type questions e.g. quedarse de piedra is to be shocked and could be used in this question or in so many other styles of questions. If dialogue is included, then slang specific to the country/region you are speaking of is very impressive to include.
N.B for these questions it would be best if a student could prepare an attempt beforehand and then we will work with that. The creative side must come from the student, but can be developed and channelled into a more impressive piece of writing through tutoring. The lessons on these questions will also have a substantial amount of time in introducing new grammar structures and using them to express what the student may have written in a lengthier/less-fluent structure.