Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Sat Oct 23 2021
 
Day 1
Valid 01Z Sun Oct 24 2021 - 12Z Sun Oct 24 2021 

....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF 
NORTHERN MISSOURI, SOUTHERN IOWA AND WEST CENTRAL ILLINOIS...

...California...
A strong atmospheric river is on track to impact CA later tonight 
into Sunday. Heavy rains should move into northwest CA between 
06-12z tonight...with as much as 2-4" falling by 12z. Rainfall 
impacts through 12z should end up limited in scope...thus the 
Marginal risk should be okay...although any more sensitive burn 
scar areas could experience some runoff issues before 12z. The 
bigger and more widespread impacts still look to be after 
12z...where we have Moderate and High risks of excessive 
rainfall...with significant impacts expected.

...Central Plains into mid MS Valley...
Convection should expand in coverage later this evening from 
northeast KS/southeast NE...east into portions of southern IA, 
central/northern MO into IL. Strong southerly flow will increase 
ahead of the approaching trough, resulting in a strengthening 
corridor of low level moisture convergence. Flow in the upper 
levels will also become gradually more divergent...although the 
real uptick in this looks to hold off until after 12z. 
Nonetheless, the low level convergence, modest upper level 
divergence, and plentiful upstream instability should support an 
expanding region of convection. Corfidi vectors may become quite 
weak overnight pointed towards the south...which should support 
some backbuilding/training of convection given the persistent low 
level convergence and increasing mid/upper level support with 
time. Hourly rainfall approaching 2", and localized swaths of over 
3" in a couple hours seems probable with this setup. It will 
likely be a pretty narrow swath and some lingering uncertainty on 
the exact latitude...with the most probable location somewhere 
from northern MO into far southern IA into west central IL. 

We will go ahead and upgrade this corridor to a Slight risk for 
the remainder of the overnight hours...as some exceedance of the 
1hr and/or 3hr FFG seems probable on an isolated to scattered 
basis. While 2-4" amounts are most likely within the swath, there 
is at least some risk of localized 4-7" totals if training ends up 
a bit more robust/persistent.

Chenard
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt