Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
830 PM EDT Tue Sep 26 2023
 
Day 1
Valid 01Z Wed Sep 27 2023 - 12Z Wed Sep 27 2023 

...A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS 
OF ILLINOIS, WISCONSIN, AND NORTHWEST INDIANA...

...01z update...
Trends in RADAR/Satellite and recent Hi-Res CAMs continue to trend 
southward across IND with convective coverage and have adjusted in 
turn.  The largest change was an expansion of the Marginal Risk of 
Excessive Rainfall upstream through the Lower Missouri River 
Valley and into SW & S IL. 

The trailing tail-end of an internal shortwave trof aloft in 
combination with consolidating outflow boundaries with return flow 
across the Ozark Plateau has supported a moderately defined 
boundary across the Missouri River Valley.  The LLJ is likely to 
respond to the slowly wobbling upper-low to the north with 
increasing southerly flow that will initially be oriented 
orthogonal to the sharpening boundary. Initially winds will be 
light, so thunderstorm activity may be initially scattered in 
nature but have ample elevated CAPE to around 1750-2000 J/kg and 
modest moisture up to 1.25" through depth.  As the LLJ strengthens 
toward 25-30kts by 06z, it is expected to veer with fairly broad 
confluence in the 850-700mb depth, likely to increase convective 
coverage.  While moisture is generally lacking for very intense 
rainfall rates of 1-1.5"/hr (with HREF probs reaching 30-50% of 
1"/hr between 06-12z), there is favorable steering/orientation for 
upstream redevelopment to potentially repeat/train, particularly 
across east-central MO into SW IL.  Isolated spots of 1-3" may be 
possible through 12z, so a spot or two of low-end flash flooding 
may be possible overnight, with best potential for increased 
runoff if crossing urban areas, including metro St. Louis. 

Gallina


...Midwest...
Upper low over the Midwest will continue to pivot southeast with 
convective development underneath the closed circulation and 
within the difluent area downstream of the main low. PWATs are 
elevated with 12z sounding out of KDVN indicating moisture 
extending pretty solidly through the boundary layer, up towards 
the tropopause. Strong ascent under the ULL will generate plenty 
of lift within a zone of modest instability to create a scattered 
heavy rainfall risk within the QPF footprint. Hi-res deterministic 
was consistent in the axis of where the heaviest rain would fall, 
even extending a little more into northwest IN and southwestern 
corner of MI. Considering the urbanized area surrounding Lake 
Michigan, this is the primary area of flash flooding potential, 
but thunderstorms across northern IL over into northern IN will 
create isolated flash flood risks as well thanks to hard soils 
from very low soil moisture (0-4%) as depicted by the latest NASA 
SPoRT soil moisture analysis. 12z HREF neighborhood probability of 
2"/3 hrs indicates areas of 15-20% with a max of 30-40% near the 
Quad Cities later this afternoon.

In coordination with the Chicago and Milwaukee WFO's, have 
introduced a SLGT risk for the urban/suburban areas of northeast 
IL and southeast WI as well as northwest IN. Heavy rainfall this 
morning across portions of the above area has created a higher 
risk for flash flooding concerns within the urbanized areas of 
Chicago and Milwaukee. MRMS Multi-Sensor readings have pockets of 
1-2" within the past few hours and considering the complex 
environment around our ULL to the west, there's concern for 
locally heavy rainfall training over the same areas this afternoon 
and evening over places impacted this morning. Local WFO's are in 
agreement for the targeted area of interest, leading to an upgrade 
beginning at the top of the hour.   

...Florida...
12z sounding from KJAX showed a tall, skinny CAPE signature within 
a zone of very high PWATs running between 2.1-2.2" which is within 
the 1.5 deviations above normal climatologically. The primary 
target will be within the urban corridor stretching from Savannah 
down through Jacksonville into northern Daytona Beach. Weak 
ridging to the north will shift winds near the coast out of the 
northeast with sea breeze progression inland after 18z. Surface 
trough to the east, in tandem with a weak shortwave traversing 
overhead has created a conglomeration of heavy rainfall within the 
Panhandle thanks to a solid convergence zone from Tallahassee and 
points southwest. Expecting the convective energy remnants to 
shift eastward and interact with the sea breeze located over 
northeast FL which will generate a second area of very heavy 
rainfall within the aforementioned corridor. 12z HREF neighborhood 
probabilities for both 2" and 3"/hr are incredibly high within the 
northeast FL Panhandle up into southern GA with 2"/hr signals 
exceeding 60-70% for a large chunk of the area and 3"/hr potential 
between 40-50% across Jacksonville and surrounding suburbs. Local 
FFG's are still high considering FL standards, but even so the 
risk will be higher within that corridor today given the expected 
interaction of convergence within a zone of deep tropical moisture 
and modest instability. Urban flooding will the primary focus due 
to the runoff potential, but even some areas within the coastal 
plain will have an opportunity for flash flooding concerns if 
training storms becomes an issue. Only adjustment made for FL was 
to bring in the western fringes of the MRGL to match current radar 
trends and 12z HREF blended mean of anything >1.5". 


Kleebauer   

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