yungchin,
While you are hearing them on 800 MHz per se, that's usually because they are patched to the local trunking system (from their typical P25 VHF frequencies), which is what I imagine the case is with the TxWARN system, as it is with the GATRRS and other P25 trunking systems. That's not to say they don't also have another radio in their car to respond to the TxWARN side (besides their VHF radio, which they all have). It is known that with most of the large trunking systems, at least a few of the troopers in the area have an extra radio to access that system (so you can see they may very well be responding on 800 MHz).
I think the OP was looking for information about the rumored statewide system (which I believe was a fleeting dream) which was supposedly 800 MHz.