Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Wed Jun 23 2021
 
Day 2
Valid 12Z Thu Jun 24 2021 - 12Z Fri Jun 25 2021 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAIN OVER PORTIONS OF THE 
UPPER MIDWEST...

...Central Plains/Upper Midwest...
Ongoing convection across portions of the Central Plains and Upper 
Midwest early Thursday morning will leave boundaries draped across 
the region that will set the stage for additional activity to 
develop by late afternoon.  With a shortwave exiting the region 
and the low level jet wanning at the start of the forecast period, 
anticipate the convection to diminish through the morning hours. 
However, additional shortwaves will ride along the northern extent 
of a weak ridge with heights falling through the second half of 
the forecast period ahead of an approaching trough.  Therefore, 
the next wave of convection will be in response to a developing 
surface low emerging out of the Rockies by Thursday 
evening/overnight. As deep layer moisture and instability surges 
northward, precipitable water values will climb over 2.25 inches 
(which is over 2 standard deviations above the mean) aided by 
southerly low level flow and MUCAPE values exceeding 3000 J/kg.  
With freezing levels above 15000ft, anticipate warm rain processes 
leading to very efficient rain rates of 2+ inches per hour. Storm 
motion is a bit harder to pin point, but there is the potential 
for multiple rounds of convection and training.  Activity could 
also be enhanced by a strengthening low level jet during the 
overnight hours into early Friday. 

While it is clear that the key ingredients are present for heavy 
rain, there is still quite a bit of uncertainty with respect to 
the placement of convective, evolution and overall propagation.  
As mentioned earlier, the morning convection will produce outflow 
boundaries that will contribute to the second round of convection 
by the evening hours. Between these two main rounds of convection, 
average precipitation within this region ranges between 2-4+ 
inches with higher amounts expected. There is enough overlap in 
the aforementioned ingredients among most of the guidance (most 
notably over eastern IA, northern MO and western IL) to highlight 
the potential for scattered flash flooding.   Given much of this 
region has observed 200-400+ percent of normal precipitation over 
the past week, which has lowered flash flood guidance, the soils 
have become more vulnerable to heavy rain.  As models come into 
better agreement, fully anticipate the need to adjust the risk 
areas with the potential need for an upgrade.   

Pagano
 
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt