From the Spaceflight Meteorology Group:

The National Weather Service has issued a Flash Flood Watch that includes JSC and vicinity until 7AM CDT Thursday morning. Numerous showers and thunderstorms were located well west of Houston at 9AM this morning with some lighter showers developing across southeast Texas.  Showers and thunderstorms, a few possibly severe may develop near JSC this morning and afternoon but the most intense rainfall is expected this evening and overnight.  Storms this afternoon could produce up to 0.5 inch of rainfall and overnight JSC could see and additional 1 to 3 inches.

Impacts at JSC from the forecast rainfall is usually limited to some minor street flooding and ponding of water. Severe thunderstorms with hail, strong winds and possibly a tornado are possible overnight as well in southeast Texas.