That's a big watershed, but at least it leans NW, away from the storm's current position.
You know about a lot of stuff!
Your right, the trinity basin swings NW and W to Dallas...most of the heavy rain is tending NE.
Hydrometeorological Prediction Center
Six-Hourly QPF Forecasts
http://www.hpc.ncep.noaa.gov/qpf/6hrqpfall.html
Looks like Arkansas is going to get a lot of rain also.