How extended periods of drought can encourage flooding
Although rainfall over the past couple of days in Missouri has aided slightly in lessening dry conditions in the state, drought is still persisting through the majority of the area. As of September 18th, when the last drought monitor was released, areas to the south of I-70 are seeing the most impacts. A moderate drought is in place for areas like southern Boone County, Cooper County, Moniteau County, and northern Cole County. Further south, a severe drought is in place for southern Cole County, Miller County, Maries County, and Gasconade County. The worst conditions reside in the bootheel of Missouri, where extreme drought conditions are in place.
Over the weekend, over an inch of rainfall in total from these events was observed for a good amount of the areas in Central Missouri. Although these events alone will not be enough to make a significant change in the drought monitor, it will at least promote the start of better conditions.
In the meantime, these noticeably dry conditions are still in place, especially in southern portions of Missouri, and could have led to the ground hardening or compacting due to the lack of moisture. If locations are seeing these conditions, rainfall is of course needed, but heavy rainfall events in particular can end up being a negative impact instead of helping.
This is because when the ground gets excessively dry or compact, it acts more like a surface that is not permeable or will not let water be absorbed into it. For example, runoff and excessive rainfall is often seen traveling on concrete or pavement due to the fact that those surfaces can't absorb any of the rain falling on it. So, all that rainfall quickly travels to low lying spots or flood prone areas, increasing the potential of flooding in that area.
Hardened or compacted soils will act in a similar way to the examples above. So, when heavy rainfall events occur over a dried out surface, the speed at which the water travels to those more flood prone spots is increased. Not nearly as much water is being absorbed, so the potential of flooding increases at a more rapid pace compared to if the soil was at a more typical state.
Over the next 24-48 hours, much more rainfall is in the forecast. Another round of potentially heavy rainfall and storms enters the Lake of the Ozarks region by 9-10 am, moving into the I-70 region near Columbia and Jefferson City by 12-1 pm. These rainfall and storm chances persist into Wednesday, leaving much of the area with an inch of rainfall or greater by the end of the event. Far southern Missouri, like Springfield, West Plains, and the bootheel are expected to get even more rainfall, potentially exceeding 3-4" by Thursday morning.
That rainfall will be landing in areas that are currently dealing with noticeable drought conditions. Those expected totals is why there is a threat level 2 for excessive rainfall in the I-70 and southern regions of Missouri for tomorrow. This threat level 2 tells us there is a higher chance of rainfall exceeding flash flood criteria anywhere in or around this area. This paired with the drought conditions means there could be a heightened chance of flooding, especially in low lying or flood prone areas.
It does not take much flooding for impacts and hazards to become noticeable, as flooded roads could have enough rainfall to sweep a vehicle away. A general rule when you come across flood conditions and you are uncertain of the depth of that water: turn around, don't drown.
