Excellent! You'll be in good shape as far as the simulcast sites go.
Find the TxWARN system in Sentinel, scroll down to the departments/talkgroups you want, and append them to your Favorites List. Once you've done that, either avoid the sites you don't want or set Location Control to "on".
Go to the RR page for TxWARN, scroll down to the sites, and click on the "County" column; that will sort the sites alphabetically by county. Scroll down to Harris County to see all sites in the county. Then if you click on the site name, it will take you to that site's page, which includes a Google Map with an (approximate) representation of that site's coverage area. If you're not within (and not planning to take the scanner out of) that site's circle, don't scan it.