Re: there will be a heavy rain tomorrow 'there will be heavy rain tomorrow'. I thought this sentence would be used in weather forecast.
Yes - a statement of fact - come tomorrow and it will rain heavily In spoken English native speakers would say 'there's going to be heavy rain tomorrow'.
Yes. But why are we bothering to say this to someone?
Because the speaker is alerting the person to the consequences of this. What is implied or understood between the speaker and hearer (if plans have been made) is something like: 'There's going to be heavy rain tomorrow (so I won't be able to mow the lawn after all.)"
or
'There's going to be heavy rain tomorrow so be sure to take an umbrella.' 'There's going to be heavy rain tomorrow.' (The two people are on holiday, it's been overcast all day, and now the speaker says this. The listener understands he is not only saying, we won't be able to get out and about much tomorrow particularly not down to the beach for sunbathing, but also, 'this is turning out to be a lousy/rotten holiday because of the bad weather.' |