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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 2011Z Sep 26, 2023)
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 PM EDT Tue Sep 26 2023

Day 1
Valid 1757Z Tue Sep 26 2023 - 12Z Wed Sep 27 2023


...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.


...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. 


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.

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.


Day 2
Valid 12Z Wed Sep 27 2023 - 12Z Thu Sep 28 2023


...2030Z Update...

The previous MRGL risks in place continue to have merit with only
some minor adjustments made in coordination with the latest
guidance trends and convective potential. Marginal Risk over Ohio
Valley is in place thanks to our meandering upper trough/low that
will exit the central Midwest and wander eastward into IN/MI
through the D2 period. Large scale ascent will be maximized over a
zone of increasing PWATs across KY/OH with convective signatures
being depicted on hi-res deterministic along a ribbon of elevated
MLCAPE between 1000-1500 J/kg. 12z HREF mean has come up from
previous run with ensemble-bias corrected QPF and NBM also
indicating a slightly more robust signature for locally heavy
rainfall. Expansion of the MRGL to the north was in part to the
increasing mid-upper level VV's focused over northern OH down into
eastern KY. This aligns well with the increased QPF footprint
within those zones and is backed by both global deterministic and
HREF blended mean QPF.

Shifted the northern and southern ends of the MRGL risk over FL to
reflect the trends in highest PWAT anomalies shifting a bit
further south, aligning with the latest ensemble-bias corrected
QPF field and NAEFS PWAT anomalies. Also wanted to outline the 12z
HREF probability for 2-3"/hr exceeding 30% or greater within the
D2 period.


...Ohio Valley...

Maintained the Marginal Risk over portions of the Ohio Valley as a
slow moving cutoff low meanders eastward on Tuesday.  Afternoon
and evening convection embedded within an airmass characterized
with PWATs around 1.25 inches will develop ahead of/east of the
cutoff low. The slow movement of the forcing will support
slow-moving/training storms.   As mentioned before...even though
the area has been much drier than normal, the storms will have the
potential to cause high enough rainfall rates and/or locally heavy
rain amounts over an urban center to make flash flooding possible,
albeit in isolated instances.


A near carbon-copy forecast from today as there will still be a
pool of 2-2.5" PWs to go along with a quasi-stationary front
draped over Florida and an upper trough over the Gulf of Mexico.
Wednesday features a little more instability across the Gulf side
of the Florida rather than the Atlantic for now, maintained the
focus for Excessive Rainfall from southwest Florida on north to
southern Georgia and southern Alabama. As much as 500-1,000 J/kg
of MLCAPE will be present across central and northern Florida, and
with such a tropical air-mass in place, rainfall rates could
approach 3"/hr in the strongest storms. A Marginal Risk for
Excessive Rainfall remains in place as urbanized communities and
poor drainage areas are most vulnerable to possible flash flooding.


Day 3

The Day 3 outlook will be updated by 2030Z.

Day 1 threat area:
Day 2 threat area:
Day 3 threat area: