Why? Density, and moving masses of air.
Start with warmer, wetter air weighing less than colder, drier air. The heavier air rushes in towards the lighter air and pushes it up, creating Low pressure which can suck in more air. So air rushes in towards the Low.
The air rushing in to the low rotates against the clock, because of the rotation of the Earth (the equator and the air above it are moving eastward faster than air farther from the equator because they have farther to go in every 24 hours.)
Now we have masses of air which are not only moving eastward, but are rotating counter-clockwise into Low pressure areas. You can see that on this map.
On Saturday night, warm, moist air from the Gulf of Mexico was spun up over the dry air above us. That warm, moist air floating on top of the colder, heavier air, cooled, and dropped its water on us. And dropped its water on us. And some more. Warm air moves slowly. The red line shows the Front of the warmer air.
Tonight, some colder, drier air from the northwest will be sucked up into the low. That's the blue line. The heavier dry air will throw the warmer air we have now up into the sky with great violence. There will be some serious storms tonight.
On Tuesday, the drier, colder air will be over us and the rain will stop.
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