Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Tue Sep 26 2023

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

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
FLORIDA AND THE MIDWEST...

...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 resuit...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 2
Valid 12Z Wed Sep 27 2023 - 12Z Thu Sep 28 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND OHIO VALLEY...

...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.

...Southeast...

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.

Bann


Day 3
Valid 12Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
FLORIDA...

Once again...a moist airmass will be in place across the Florida
peninsula with a quasi-stationary surface front and some dynamics
aloft helping to trigger late day and early evening convection
over portions of the peninsula. The guidance tends to show most
the convection off-shore.  However...thinking is that there should
be enough instability to support locally intense rainfall rates
which leads to isolated flooding in urbanized communities and poor
drainage areas. 

Bann



Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt