Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
205 PM EDT Tue Sep 26 2023
 
Day 1
Valid 1757Z Tue Sep 26 2023 - 12Z Wed Sep 27 2023 

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

...18Z Update...

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. 

Kleebauer

...16Z Update...

There is very little change to the previous D1 ERO. The overall 
synoptic evolution is still on track with the ULL progression over 
the Midwest and the surface trough positioning across the Gulf 
Coast. The main mesoscale factor(s) will continue to be outflow 
and sea breeze propagation within the afternoon and early evening 
time frame across FL. 

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". 

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. This was sufficient for 
maintenance of the MRGL risk with little need to deviate from the 
previous forecast.  

Kleebauer   

...Florida...
Given the 2 to 2.25 inch precipitable water values over 
Florida...a quasi-stationary surface front draped across the 
region and a mid- and upper-level trough over the Gulf of 
Mexico...the most active convection that forms should be able to 
take advantage of nearly 2000 J per kg of MLCAPE to produce some 2 
or 3 inch per hour rainfall rates.  With those kind of rainfall 
rates...instances of flash or urban flooding from this afternoon 
into early this evening.  The previously issued Marginal Risk 
still appears to capture the area well.

...Midwest...
Areas of moderate to locally heavy rainfall will continue to 
spread south- and eastward today in association with an upper 
level closed low.  As a result...maintained the Marginal risk area 
from parts of the Upper Mississippi Valley to the western/southern 
Great Lakes.  The above-mentioned low pressure system will 
continue to funnel 850mb moisture flux on the eastern flank of its 
circulation with PWS rising to around 1.25". MLCAPE is expected to 
range between 500-1,000 J/kg and mean 1000-500mb RH values are 
likely to average >80%. It is the slow progression of the low that 
could allow for training convection and elevated rainfall amounts 
given slower storm motions. The 26/00Z HREF still showed 
probabilities for 3-hr QPF > 3-hr FFGs as high as 40-45% in parts 
of northern IL this afternoon. Some soils are a little more 
sensitive than others, particularly north-central IL where AHPS 
7-day rainfall totals have been as high as 400-600% of normal. 
Given these reasons, have chosen to introduce a Marginal Risk for 
parts of the western Great Lakes region this forecast cycle.

Bann

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