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FOUS30 KWBC 250751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Sat May 25 2024

Day 1
Valid 12Z Sat May 25 2024 - 12Z Sun May 26 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR THE
KANSAS/MISSOURI BORDER...

...Midwest/Portions of the MS Valley...
The Marginal Risk is a little more distended into the Southeast,
and some small changes made to the Slight Risk area. A cyclone
emerging into the central Plains will bring precipitable water
values of 1.5"+ across the region on the heels of sufficiently
strong low level inflow/effective bulk shear. The region lies
within a general instability gradient, with MU/ML CAPE expected to
rise to 5000+ J/kg within an increasingly capped atmosphere across
the southern Plains. Pockets of moderate to heavy rainfall are
forecast within this region. Given the ingredients, hourly rain
totals to 2.5" with local amounts to 5" are possible where
mesocyclones form and/or cells manage to train.


...Portions of the East...
Near western PA and WV, showers and thunderstorms along a
progressive front could drop locally heavy rainfall within an area
of modest flash flood guidance. Hourly rain totals to 1.5" with
local amounts to 3" are possible where short bouts of cell training
or cell mergers manage to occur. Since the flash flood threat
appears to be isolated, added a Marginal Risk. Down south in the
Carolinas, higher moisture pools near the coast as a front slowly
moves through. With MU CAPE as high as 3000 J/kg and effective bulk
shear expected to be sufficient to organize convection to some
degree, added a Marginal Risk within a regime that can lead to
isolated flash flooding during the warm season.

Roth


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...

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...

...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