According to Jeff Passan's article - very.

But what's transpired so far might pale in comparison to what happens this summer:

Money quote:

For every 10 degrees gained in air temperature, [Hit Tracker's Greg] Rybarczyk said, four feet of distance are added onto home runs. The average temperature in the Bronx from June through September is nearly 78 degrees ? about 17 degrees higher than April and May. Thus, Rybarczyk believes the ball could travel another seven feet on average. And applying his previous rule about the home runs that barely clear the fence, Rybarczyk said the home run rate could increase by 21 percent should the weather hold this summer.

Keep in mind the current home run pace at Yankee Stadium is 306. The major-league record was 303, set at Coors Field in 1999.