where r u, dear teachers?

I can't think of a "grammatical" reason (although I'm sure there must be one), but my intuitive reasoning tells me that "this" and "that" would need to refer to a "case" that has actually been defined and is already in existence whereas in your example the "case" of bad weather is still a hypothesis.

Just an idea...

*^^* said:

but why we cant use 'this' and 'that'?[/LEFT]

Click to expand...

Punctuation, that is why.

corum said:

Punctuation, that is why.

Click to expand...

???

A comma does not suffice with 'that' and 'this'.

Compare:

It may not be fine tomorrow. In this case we’ll have to put the trip off.
It may not be fine tomorrow. In that case we’ll have to put the trip off.

to

It may not be fine tomorrow, in which case we’ll have to put the trip off.