Tim Mayer at Yahoo announced a new weather report late last night:

We will be making changes to the ranking of our index tonight. I would expect that this update will be mild and quick compared to recent ones but will impact the ranking of some sites.

So keep an eye on your Yahoo SERPs.

As an aside, we applaud the glasnost of the Weather Report concept, as pioneered by Yahoo. Google obviously agrees, since Matt Cutts has also been very good lately at mapping out the rollouts of Google algo updates. It appears the major engines have decided (rightly) that announcing updates ahead of time helps webmaster relations and PR (public relations, silly) far more than it compromises any sort of corporate secrecy.