A High risk of for excessive rainfall has been issued by the Weather Prediction Center for parts of extreme Southern IL from 6a.m. Saturday through 6a.m. Sunday. Much of the region has already experienced rainfall totals of 2-4 inches over this past week. The cumulative effect of several rounds of heavy rainfall has set the stage for a high risk of flash flooding.
With a warm front lifting north overnight tonight, anomalously high precipitable water (PW) values associated with deep moisture will move into the area ahead of the developing low across the Great Plains. PW values are expected to approach climatological maximums at or above 1.5″. Additionally, sounding analysis for tomorrow shows MUCAPE (Most Unstable Convective Available Potential Energy) values reaching 500 J/kg below the favored region for ice production (this simply means the available instability will be very efficient at producing rainfall). Because of this, rainfall rates during this period will likely be higher than what they were throughout the week. A training band of heavy rain and thunderstorms should set up along the nose of the LLJ (low-level jet) allowing for a period of significant rainfall rates that could range between 1-2″ per hour which will exacerbate already over-saturated conditions leading to life-threatening flash flooding conditions. Be prepared for rapid rises in rivers and streams.
The heaviest axis of rainfall should exist in extreme southern IL (as seen by the NMM/Euro above) along the nose of a stout low-level jet, generally along and south of Carbondale and Harrisburg. In this region, it will be possible to see 2-3″. in addition to what has already fallen. Further north, heavy rainfall and dangerous flash flooding conditions will still exist although total rainfall should average 1-2″. Locally higher amounts may be observed.
In addition to the significant flash flooding threat, parts of southern IL will also face the potential for severe weather tomorrow as well. An Enhanced Risk exists for extreme southern IL and a Slight Risk extends north to near Mount Vernon. During the afternoon hours, surface initiation will occur along the cold front in Arkansas and southeast Missouri. This will eventually develop into a line given the strong forcing along the front and will begin marching east into southern IL during the evening/overnight hours. Given the background conditions already discussed above concerning the flooding threat in concert with dew points reaching the upper 50s and favorable low-level storm relative helicity values bowing segments and LEWP (line echo wave patterns) with embedded supercells will be possible. I expect the main hazards for this to be damaging winds and potentially an embedded tornado or two with the greatest concern existing south of Mount Vernon (more specifically south of Carbondale/Harrisburg).
Remember, if you see water crossing a road TURN AROUND, DON’T DROWN.
For information regarding river and creek levels, please use the link below. We will continue to monitor this situation and post updates as needed.