Ah, the perils of mesoscale forecasting. "Mesoscale" is the time-space step below synpotic, which generally is the size of the Continental United States and lasts for a day. Mesoscale would be approximately a state to half a state wide (or two Rhode Islands if you're in the Northeast). My first guess is that forecasters relied too heavily on the computer models and not try to factor in their own experience in forecasting for the Piedmont.
I'm not saying that they would have been more correct if they weren't using the models just as a guidance, but with a $2 billion dollar price tag on those Supercomputers out in Colorado, forecasters are kind of forced to go with the models unless they are damn sure the models are absolutely wrong.
(And why didn't it rain in the Piedmont? The Appalachian mountains, probably. Generally, rain on the windward side of the mountain loses its punch as it moves up the mountain due to colder air and a bit of friction. It's why Nevada and Eastern Oregon is pretty dry most of the time, even in a non-drought period. I'll stop now.)
Models like the "high-res" NMM (4km resolution) should be well capable of modeling the effects of orography (the rain shadow you speak of). I'm not sure why the forecasts went wrong, and it may be a resolution issue, but it could just as well be initialization uncertainty or model physics errors. And as far as the cost of supercomputers goes, it is generally agreed in the atmospheric science community that the US doesn't spend enough on supercomputers (in particular when compared with the Europeans at ECMWF). It would be nice to see forecasters do a breakdown of how this particular forecast was made and some guesses of what might have gone wrong.
That's very true that the resolution levels should have caught the orography effect, but it didn't. And I'd be interested to hear as well why it missed that. But the NMM is notorious for having extreme biases, and it wouldn't surprise me that its "summer mode" would have a wet bias.
Sounds like a good time to hit Lake Norman for the day and then have dinner at Big Daddy's?