Why does the AC blow warm air at idle?

It IS low on refrigerant. Please prove me wrong by posting STATIC PRESSURES, of both high and low sides. That means with the gauges hooked up and open, with the ENGINE/A/C turned OFF for at least 5 minutes.post those pressures AND the AMBIENT temperature at that time.

Hello, you are saying you checked the high and low side pressures of the air conditioning system? Typically, when the air gets warmer when you stop, it's because of a low charge -- OR -- no air flow at the condenser. This is most likely because you could have a fan issue. When you are stopped, there is no natural air going through the condenser, so you need air from electric fans to blow and keep the system operating properly. Check for this first.