From the NWS:

January 24-25, 2012 Severe Storms and Flood Event

A severe storm and flood event, more typical for early spring, occurred from midday on January 24 until midday on January 25, 2012. There were three confirmed tornadoes, widespread minor wind damage (especially east of the I-35 corridor, more than a dozen hail reports (up to golf ball in size), and widespread 2-5 inches of rain east of U.S. 281, including a bull’s-eye of 5-10 inches of rain over eastern Hays, Caldwell and Bastrop counties, resulting in major flooding along a number of creeks and rivers.

The primary ingredients producing the event included an upper level trough that moved from northern Mexico across Texas. Meanwhile, a cold front that had moved into South Texas on Monday stalled and began moving back north as a warm front, which allowed moist Gulf air (dew points in the 60s) to return across the area. The general lifting of air from the approaching trough and increasing instability of cool, dry air aloft over warm, moist air near the ground was enough for thunderstorms to develop. Moderate to strong wind shear then increased the severity of the storms and brought the potential for hail, damaging straight-line winds, and tornadoes. A few severe storms developed Tuesday afternoon and produced hail over portions of the Austin Metro area. The more significant storms with damaging winds and tornadoes were part of a Mesoscale Convective System (MCS), and swept across the entire area, mainly from 10 pm to 8 am.

In addition to the severe storms, the amount of water vapor (i.e., precipitable water) measured by weather balloon at Corpus Christi Tuesday evening was 1.57 inches, which is slightly more than two standard deviations above the mean value for January. This strongly indicates the potential for heavy rain and flash flooding, assuming thunderstorms develop. The resulting rainfall amounts were heavy along and east of the I-35 Corridor, including peak amounts of 9.35 inches 2.4 miles west-northwest of Bastrop and 9.29 inches 4.4 miles southeast of Uhland. New daily records for January 25th were set at Austin Camp Mabry (3.14 inches) and Austin Bergstrom (5.66 inches), and San Antonio (2.94 inches).

Click here for  selected graphics, pictures, and tables from the event.