I'm not sureI am. Spend some time in the desert southwest, particularly on indian reservations. You'll find that NWS temperatures are regularly off by 10-20°. But that's still better than Weather Underground, which can be off by as much as 50°.
One thing I will give the NWS credit for is the wind forecasts. Those things are spot-on at least 90% of the time.
I've lived in 15 cities in a dozen states, and what I've noticed is that if you're in a large city, or east of a large city, the forecasts are great.
But if you're west of a large city, or in a smaller city, it's hit-or-miss. This makes sense, as the better forecasters tend to end up in the larger markets.