A number of factors came together to produce a heavy rain event across South Texas on October 24. Abundant gulf moisture was already in place as deep onshore flow had been occurring for several days, pulling in very moist gulf air. Additionally, Hurricane Patricia made landfall along the Pacific Mexican coast on the 23rd...and its remnant moisture quickly pushed across Mexico and into South Texas. The third ingredient was a cold front dropping down through the central U.S. The combination of all these elements were ideal for widespread heavy rain.