941
FOUS30 KWBC 260047
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Sat May 25 2024

Day 1
Valid 01Z Sun May 26 2024 - 12Z Sun May 26 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE CENTRAL PLAINS INTO THE MID MISSISSIPPI VALLEY...

A Slight risk stretches from portions of central/eastern KS into
central and southern MO for the overnight hours. An expanding
cluster of convection currently near the KS/OK border should
continue to grow in coverage over the next several hours with an
east northeast motion. At least an isolated flash flood risk will
likely exist with this activity given some cell merging extending
the duration of heavy rainfall rates.

The low level jet and moisture transport really ramps up tonight,
which combined with strong and persistent mid/upper level forcing,
should allow for upscale convective growth. One or more convective
clusters will likely evolve into a more consolidated MCS as the
night progresses bringing a heavy rain and flash flood threat. Cell
motions will be fairly quick...however it will initially be the
cell mergers that will drive some flash flood risk, potentially
evolving into more of a classic convective training setup by later
tonight.

There are indications that the south southwest flank of convection
will take on more of a northwest to southeast orientation by later
tonight, which would be a favorable alignment for some convective
training. Recent HRRR runs have consistently depicted a swath of
2-4" of rainfall across portions of eastern KS into
central/southern KS and this seems reasonable. The 23z HRRR is the
most aggressive, showing localized amounts exceeding 6". While this
is a lower probability outcome, it does still represent a
plausible outcome if training potential ends up being maximized
and/or the same location gets impacted by multiple convective
clusters tonight. Either way an increasing flash flood risk is
expected tonight over portions of KS/MO, with some potential for
localized significant flash flooding over portions of southeast KS
into southwest MO.

Chenard


Day 2
Valid 12Z Sun May 26 2024 - 12Z Mon May 27 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE MISSISSIPPI & OHIO VALLEYS...

...2030Z Update...
Only change made to the existing outlook was to expand the Slight
Risk area a bit southward where there was overlap between somewhat
higher QPF compared to continuity and with modestly lowered Flash
Flood Guidance as low pressure makes its way across the Mississippi
Valley towards the Great Lakes.

Bann

...Previous Excessive Rainfall Discussion...

A cyclone moving across the Midwest and Great Lakes is expected to
draw ample moisture (precipitable water values of 1.50-1.75") into
the region. The area that is expected to see the greatest residency
time with higher moisture and instability is across portions of TN
and KY. However, warm temperatures at 700 hPa appear to be a
limiting factor the farther down its front you go. The best warm
air advection ahead of the low is slated to occur from portions of
IL across OH, where the guidance is more robust in its QPF signal,
with local amounts in the 3-5" range. Considering the progression
of the warm air advection pattern and best moisture across the
region, that degree of heavy rainfall would need to fall quickly;
hourly rainfall potential to 2.5" is possible within the organized
thunderstorms, with embedded mesocyclones, could get them there
given the low- level inflow and effective bulk shear expected.
Maintained the Slight Risk across the region, where flash flood
guidance values are modest.

The overall Marginal Risk was expanded across portions of the East
from continuity. Moisture invading from the Atlantic within an
unstable airmass is capable of producing hourly rain totals to 2"
and local amounts to 4" which is expected to lead to isolated flash
flooding. This would be most problematic in urban centers as well
as portions of VA/WV which have seen heavy rainfall as of late.

Roth


Day 3
Valid 12Z Mon May 27 2024 - 12Z Tue May 28 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE NORTHERN MID-ATLANTIC STATES...

...20Z Update...
Shifted the western periphery of the Marginal Risk area eastward a
bit given the timing shown by the latest guidance. However...the
location of the highest model QPF moved very little and there was
no need to modify the Slight Risk area.

Bann

...Previous Excessive Rainfall Discussion...

...Great Lakes/East...
A cyclone on the deep/strong side of May climatology lifts into
southeast Canada. A broad area of 1.5"+ precipitable water values
advects in ahead of the system and focuses along the system`s
frontal boundaries and within area terrain. The Slight Risk for
portions of PA and NY is slightly expanded from continuity. There
is a 6-12 hour period where Atlantic moisture advects in while the
850 hPa flow remains quasi-stationary and fairly strong, which
could lead to a decent period of moderate to heavy rainfall. In
this area, hourly rain totals to 2.5" and local amounts to 5" are
considered possible where cells merge, train, or where mesocyclones
form.


As 700 hPa temperatures are modest, heavy rainfall potential is
there along its cold front as it sweeps through the Cumberland
Plateau and the Appalachians. There`s even some convective
potential within the system`s comma ahead across portions of MI,
though the guidance shows a wide range of possible outcomes, which
appear to be due to timing differences. Within the Marginal Risk
area, hourly rain totals to 2" and local amounts to 4" are possible
where cells merge, train, or mesocyclones form.

Roth


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