So when this convergence zone set up north of Houston it could only very slowly move southeast toward the gulf which is the way most of these eventually move thanks to the spin of the earth. Allison, Harvey, and Imelda all finally moved southeast out of the area. Same pattern. Here’s last night’s radar from 9pm, then 3am and 6am this morning:

Torrential rain caused flooding Thursday in southeastern Texas and officials in one county were asking residents to leave. A storm system dumped heavy rain in Montgomery County, on the northern edge of Harris County and Houston.