From the Spaceflight Meteorology Group:

The National Weather Service has issued a Flash Flood Watch that includes JSC and vicinity beginning at 7PM this evening lasting until 7AM Tuesday morning.

Warm moist Gulf air is flowing into the JSC vicinity this morning bringing with it some occasional light rain off and on this morning.  A break in rain chances is likely this afternoon and evening. Thunderstorms will form in central Texas later this afternoon and move slowly eastward overnight. Best time for strong and possibly severe thunderstorms at JSC appears to be between 4AM and 9AM Tuesday. Rainfall amounts around JSC should average 1 to 2 inches but a few isolated spots nearby could see 2 to 4 inches in stronger storms.