Use a file mask with a time limit and a keyword today:
synchronize local -filemask=">=today" "D:\ftp\OUT" /out
This syntax is supported only in WinSCP 5.15 and later.
In earlier versions of WinSCP, you could use the syntax%TIMESTAMP% to create a limitation today.
synchronize local -filemask=">=%TIMESTAMP#yyyy-mm-dd%" "D:\ftp\OUT" /out
Further reading: