The Iowa caucus is an exercise in organization and commitment. The people who come out to cast votes in the caucus are likely people most personally engaged and interested in the candidates. Thusly, statewide polling and the coverage by the national punditocracy will likely be meaningless. The winner will more likely be the candidate most able to get committed supporters out to vote.
The candidate that has the largest most well organized team is Senator Cruz, therefor I expect that tomorrow night Ted Cruz will likely win the caucus. Donald Trump leads statewide polling but does not have an organization covering the entire state the way Senator Cruz does.