If you thought it was pretty unusual to have a 77-degree temperature on April 12 at Wrigley Field, you’re right.
That was the game-time temp for Wednesday’s Cubs/Mariners game, and in searching baseball-reference’s database, I found just three other games with a temperature that high or higher before April 15:
April 6, 2021, vs. Brewers: 79 degrees
April 14, 2003, vs. Reds: 79 degrees
April 10, 1977, vs. Mets: 82 degrees
Now, I should note that this is not a definitive list. BB-ref’s database doesn’t have game-time temps for every single game, and notably, the 1984 season doesn’t have any listed at all (though I checked the two home games before April 15 that year and it was in the 40s and 50s those dates).
However, before expansion and definitely before divisional play began, the MLB season often didn’t start at all before April 15. So I’m reasonably certain these four are the only games in Wrigley Field history played before April 15 where the game-time temp was 77 or higher, though it’s possible there could be a couple missing.
The average high temperature on April 15 in Chicago is 58 (and obviously, lower before that), so these sorts of temperatures in the first half of April are very unusual. The Cubs should consider themselves very fortunate that only one game so far this year (April 1) has been played in suboptimal baseball conditions.
Unfortunately, based on these advance forecasts (which could possibly change, of course), this might not continue:
Those would cover the first series of the next homestand, April 20-23 against the Dodgers. And based on this, below-normal temps are expected into late April:
Sigh. That’s April in Chicago for you. Hopefully, it’ll just be cool, and not rainy.