As a strong El Nino weakens, the weather across the Lower 48 will change. In much of the East, a hot summer is in store. Warm temperatures lead to a decrease in an air density, which in turns leads to a batted baseball to be able to travel further.
3 Month Forecast from NOAA for May, June and July
Rain and thunderstorms will dominate the pattern in the central and southern Plains.
Rain forecast for the same time period. Notice all the above average in the southern Plains
In general, in summer the wind for much of the East is out out of the southwest. Air from where the rain and thunderstorms fall in the Plains will be pushed into the Big Cities of the Midwest and East where there are many MLB stadiums (Chicago, Detroit, Cincinnati, Cleveland, Washington, Baltimore, Philadelphia, New York City and Boston). Because of this rain, this warm will also contain alot of water vapor, in everyday language we call this air humid.
Not only does warm air lead to low air density, humid air does so as well. This combination of warm and humid air will lead to conditions that favor offensive outbursts. In addition to warmth and humidity, many ballparks in the East and Midwest (Fenway Park comes to mind) are built so that a southwest or south wind blows out and further helps batters.
Another factor to consider is that many of the mentioned teams play more road games than home games early in the season since the MLB schedule makers know the weather is often bad early in the spring. For example, the Cubs have played 15 road games as compared to 10 home games. That means that more home games than road games must be played during the summer in many of these cities. Of course, this is not true for all of these teams. The Red Sox have played 15 home games compared to only 11 on the road.
Already we have seen more home runs hit per game than the previous two seasons. Please see CBS Sports for the exact numbers and possible explanations. Will the warm and humid weather help this pattern continue? I do not think it can hurt!