Most days, MLB lineups officially get submitted about 2-3 hours before first pitch. There's no hard and fast rule so teams are allowed to wait until right before the game starts. Most days, 95% of lineups get submitted about 2 hours before and they'll get updated above. Any exceptions? Sometimes - for example, when a star player is battling an injury and questionable, teams will often let him continue with warm-ups and wait to lock in a lineup until 30-60 min before first pitch.
Before official lineups get entered, we project lineups based on recent trends and matchups (most managers have different lineups/players for lefty or righty starters).
For the first game of every series, assigned umpire teams are mostly unknown. They usually get announced 2-3 hours before first pitch (similar to the official lineups). For the rest of the games in the series, umpires rotate on a clockwise basis (the next game, first base umpires shift to home plate duty).
Why care? Home plate umpires can have a statistically significant effect on fantasy baseball outcomes. Batter-friendly umps tend to call more borderline pitches as balls. Conversely, pitcher-friendly umpires will lean towards a strike on the exact same pitch. The end result is better or worse counts and, as we all know, it's much easier to consistently barrel balls on a 3-1 count vs 2-2. Be sure to keep on eye on umps with extreme splits and act accordingly.
Hot days with the wind blowing are a recipe for offense. Of course, pitchers still need to miss their spots but long flies are more likely to clear the fences when the air is thinner and they get a boost. In fact, excellent weather conditions can increase the likelihood of home runs by upwards of 50%. We created a metric to quantify exactly that potential - known as HRForce, it gives you a very precise approximation of how many more HRs could be hit in a game. An HRForce reading of 1.5 means 50% more HRs could be seen. Conversely, an HRForce of 0.7 (e.g., in the chilly days of March) means 30% less dingers are likely be hit.