In Iowa, voters who support candidates who have under 15% have to go support someone else (or leave). If I combine the latest RCP average with the latest Emerson second-choice data for Iowa and treat it sort of like ranked-choice voting, that implies this result:
Round 1
Sanders 26.7%
Biden 23.5%
Buttigieg 18.2%
Warren 14.4%
Klobuchar 9.1%
Yang 3.2%
Steyer 3.0%
Gabbard 1.9%
Round 2
Sanders 27.3%
Biden 23.9%
Buttigieg 18.6%
Warren 14.8%
Klobuchar 9.2%
Yang 3.2%
Steyer 3.0%
Round 3
Sanders 29.0%
Biden 24.6%
Buttigieg 18.7%
Warren 15.0%
Klobuchar 9.5%
Yang 3.2%
Round 4
Sanders 30.6%
Biden 25.1%
Buttigieg 19.1%
Warren 15.2%
Klobuchar 10.0%
Round 5
Sanders 32.0%
Biden 29.2%
Buttigieg 22.2%
Warren 16.6%
(It doesn't actually work in rounds like this, and it's done at a per-district level, so in some districts even Sanders or Biden will be eliminated due to the 15% threshold. The above is just a way of getting a very rough estimate.)
It's looking good for Sanders in Iowa, though with the sheer confusion of the caucus process, Biden's more experienced supporters and caucus managers might push him over the top. If Sanders wins both Iowa and New Hampshire, his overall victory looks much more likely, and he'll at least go far. A contested convention is also very possible in that case, where Sanders would have a huge disadvantage due to superdelegates (who still get to vote if there is no majority). Can you imagine the chaos if Sanders wins a plurality of the delegates but loses the nomination?
My preferred outcomes are, in order:
1. Sanders wins and loses to Trump.
2. Sanders wins and wins against Trump.
3. Sanders has the nomination stolen from him at the convention, and the Democrats implode.
4. Biden wins and loses to Trump.
Those are all decent* outcomes IMO, and it's looking likely that one of them will happen. I can't wait to see the Iowa results.
* By decent I mean about as best as could be hoped for in today's world. Trump is a warmonger, Sanders would seek to utterly strangle the economy, and both are authoritarians.