Initially, large hail and damaging straight-line wind gusts will be the primary threats. However, as the activity becomes more organized and the upper level forcing and stronger wind shear overspreads the I-35 corridor area later tonight, the threat of tornadoes will increase as well.
Nighttime severe weather situations are particularly dangerous. Especially given this scenario where a large population area (i.e., the I-35 corridor from San Antonio to Austin) will potentially be affected after Midnight. It is critical that all residents across the area stay alert tonight and be prepared to seek shelter should severe weather threaten their area.
It's not too late to buy a NOAA Weather Radio, which can alert you to severe storms 24-hours per day. You can even program many newer models to only sound the alarm when severe weather threatens your immediate area. Radio Shack, Wal-Mart, Target and other major retailers sell a variety of these devices.