From NWS chat below (for KDFW)... earlier they said that 3pm yesterday to 3pm today will be the period to watch:
(10:17 AM) nws-bianca.garcia: Yes, it looks like we are at #4 for the greatest 24 hour period precip totals. For the top 3 stats: The all time highest is 9.57" (September 4-5, 1932), #2 is 8.81" (April 24-25, 1922), and #3 is 8.11" (Sept 21-22, 2018).