DC appears to have recorded an August 20 high of 76 or lower 16 times in the 125 years between 1900 and 2024. Rainfall was recorded on 10 of those 16 days. So, to answer @mattie g's question, it appears to be an uncommon occurrence on a day with full sun.
This morning's DCA low of 59 ties for the 10th coolest August 21 low since 1900. The last time that a cooler low was recorded was in 1969 (58).