The rains were so intense through the afternoon and into the early evening causing flooding throughout the downtown district.
The rains were so intense through the afternoon and into the early evening as to cause flooding throughout the downtown district.
The rains were so intense through the afternoon and into the early evening causing flooding throughout the downtown district.
The rains were so intense through the afternoon and into the early evening as to cause flooding throughout the downtown district.
To show the cause of an event, the correct idiom is "so [adjective] as to."