Union County is listed along with several other counties in North Mississippi as being at risk for life threatening flooding. Both Tippah County and Alcorn County are also listed.
From the NWS:
Attention: Flooding south of Interstate 40 has already been bad this week and it is about to get worse. Possibly much worse. This is a serious situation.
Be prepared for numerous rounds of heavy rain that will last through the night and for much of Friday. The ground simply cannot absorb any more rain and this is going to result in significant flooding for some areas. Some roads will likely wash out. Water may get high enough to inundate some homes. If you live in a flood prone area it would not hurt to prepare to evacuate, just in case. Do not even think about driving across flooded roadways tonight and tomorrow. It’s not worth your life.
To be clear, all areas outside of the highlighted region, including Davidson County, are still at risk of seeing flooding tonight and tomorrow. A flash flood watch is in effect for every location you see on the map above. Areas highlighted in yellow are at a particularly serious risk though and could potentially see flooding on levels that haven’t occurred in years.