Let's look at the baseball months (average precipitation in inches):
March: BOS 4.33, SEA 3.70
April: BOS 3.74, SEA 2.68
May: BOS 3.50, SEA 1.93
June: BOS 3.66, SEA 1.54
July: BOS 3.43, SEA 0.67
August: BOS 3.35, SEA 0.87
September: BOS 3.43, SEA 1.42
October: BOS 3.94, SEA 3.46
https://www.usclimatedata.com/climate/boston/massachusetts/united-states/usma0046
https://www.usclimatedata.com/climate/seattle/washington/united-states/uswa0844