The classification of conditional sentences into numbered "types" is not normally used by native speakers. It is a simplifying device used by ESL teachers, and only by such teachers. The numbered types capture the most commonly used conditional forms, but far from all valid forms. Many valid conditional sentences do not fit any of the numbered "types".
I advise against using this division of conditionals into types when communicating with a native speaker who is not familiar with ESL teaching methods. Many will have no idea what they mean.
The proposed sentences:
(1) If it rains tomorrow, it will flood.
(2) If it rained tomorrow, it would flood.
are both valid, and carry much the same meaning, the difference is small and subtle. Neither implies that the speaker is certain that it will rain tomorrow. Both imply that the speaker is reasonably certain that flooding would accompany any rain. Sentence (1) suggests that the speaker thinks that there is a significant likelihood of rain tomorrow. Sentence (2) suggests that the speaker is less convinced that there will be rain on the next day. Sentence (2) also suggests a greater possibility that rain could occur without flooding.
But this distinction is often not carefully observed by fluent speakers. Many fluent speakers may treat (1) and (2) as interchangeable, and treat the suggested differences in meaning as unimportant.
(3) If it rained tomorrow, it could flood.
(4) If it rains tomorrow, it could flood.
Sentences (3) and (4) are also both grammatically valid and natural. A fluent speaker might use either. Sentence (3) suggests less assurance on the part of the speaker that flooding will following rain, if rain occurs, than sentence (2) does. Sentence (4) makes the corresponding change to (1).
Again the distinction between (3) and (4) may not be carefully noted by many fluent speakers. Indeed many speakers may treat all four sentences (1)-(4) as interchangeable.
