It is important to understand a few things here. Firstly, the view presented by Aquinas is one view that is permitted by the Church, but which is not taught explcitly. Official Catholic teaching concerning predestination goes something like this:
Predestination is a truth of the faith. That is for certain. What exactly predestination is and how it functions, however, is not certain. The Church is very humble in this regard insofar as that she does not claim to know what she does not. As concerns predestination, little is known, so little is taught authoritatively. What is known allows the Church to declare that there are two extremes, each of which are acceptable to believe along with the spectrum of views lying in between. On the one side is Thomism, the belief which the original post explained, and on the other is Molinism. The Church affirms that either of these views might be correct, as may any number of views which fall in between, but that the Church does not possess the revelation to declare with certainty what the proper understanding is. What is taught with certainty is that all have the possibility of salvation, and that no beliefs outside of this spectrum are correct.
My understanding (which is relatively close to Molinism) is based upon a reading of Romans chapter 8.
28And we know that in all things God works for the good of those who love him, who have been called according to his purpose. 29For those God foreknew he also predestined to be conformed to the likeness of his Son, that he might be the firstborn among many brothers. 30And those he predestined, he also called; those he called, he also justified; those he justified, he also glorified.
To begin, note that the text indicates that God predestined those whom He foreknew. In other words, first her foreknew them, then He predestined them. This indicates that God in His omniscience knew who would accept his call, and therefore predestined these individuals to Heaven. I point to the text in the Gospels wherein Christ says that he is preparing a room for each in Heaven. Is He literally preparing a room? This is doubtful, for even if it were true He could accomplish this in an instant. It makes sense then to compare Christ’s statement that He is preparing rooms to St. Paul’s indication regarding predestination. It seems appropriate to roughly equate this preparation with predestination. In other words, God, knowing whom would accept His call, predestined them in the sense that He “prepared this room” for them while not “wasting His time” preparing rooms for those who would not come.
Continuing in the passage we see that those whom God predestined he “also called.” This indicates to me that God, knowing whom would accept and whom would reject Him, called those whom would accept Him. It may mean that He called only those, but this is not certain and somewhat problematic when we consider those whom accept God’s call only to reject it later in life. What makes more sense is the idea this passage teaches that God at least calls those whom will accept Him. In other words, for those God foreknew would accept Him, there is a special guarantee of a call. In this sense, Aquinas’ belief that God bestows upon some a special predestination to Heaven (as well as the Catechism’s similar statement) is resoundingly true. What seems to be the key, however, is that St. Paul indicates that foreknowledge precedes predestination. In other words, it is our own free will by which we choose to accept or reject God, and predestination is dependant on this. It is the clear teaching of the Church that God does not interfere with our free will, and that each and all are free to reject or to accept God’s offer of salvation. What the Church does not claim to know is exactly how this happens.