If it rains ...
Last time I checked, rain doesn't slow me down. My 10k PR last year was set in the rain; I've had a handful of other races that went well in the rain (and snow). If it rains on Sunday at the Shamrock Half ... so what?
Unless it's windy and pouring rain, showers (which is what is in the forecast ... "showers") won't make or break what happens. In fact, some light rain might even help. It certainly takes away the mental games that happen.
With temperatures in the low 40s and basically no chance of climbing out of the mid-40s, the weather is almost perfect. Rain has never chopped anyone's legs off, so there's no reason to keep thinking and talking about it.