I have been here 5.5 years so only have that much history but my neighbor, a farmer, says we are in a permanent summer drought. The farmer warned be about the Great Valley and how it will rob near obvious rain every summer and he was pretty spot on. See map below. The rain shadow effect as you call it. My closest NWS Station does average about 9" LESS of precip per year than MDT does but I have found I almost always outpace them in the winter because of the high propensity of non-convection-based systems pushing WAA in from the SW. Being on the windward side of the mountain, the rain/snow is wrung out over my rapidly rising elevation. The Great Valley has much less of an effect on non-convection forcing qpf. So, it is fairly rare when we get a WAA event from the SW that I get less qpf than MDT. Usually, it is more. I think overall I average about 2-3" less qpf than MDT over a year....more in the winter and less in the summer.