Using "such" + article form

When we use the word 'such', is there a rule to dictate whether we need to add article a/an behind? For example:

I did not expect such cold weather.

and

I did not expect such a cold weather.

Another example I found is

She has such a marvellous voice!

All three examples are in the such + adj + noun form, only difference is the article 'a'. I would like to find out if there are any rules or is merely a usage issue?

The difference is between mass nouns and count nouns. Singular count nouns, like 'voice' or 'dog', require articles; mass nouns, like 'weather' or 'sand', do not take indefinite articles like 'a' or 'an'. It has nothing to do with 'such' or with the presence of the adjective. For example:

I prefer calm weather

and

I prefer a calm dog

If the count noun is plural, you no longer need the indefinite article. For example:

They had such marvellous voices