It is/was heavily raining here. Or 2. It is/was raining heavily here.
In the examples, it is important to keep the continuous form of the verb as the position of the adverb is semantically significant.
3 “The meal was quickly cooking.” = the meal was cooking without delay / The meal was soon cooking.
This can also be expressed as
3a “Quickly, ...
To be a bit more precise, englishgrammar explains:
The structure as…as is used to compare things that are of similar proportion.
In this case the first as acts as an adverb modifying the adjective or adverb that goes after it. The second as can act as a preposition or conjunction. If it is used as a preposition, it will be followed by a noun or pronoun. If ...