373
FOUS30 KWBC 252033
QPFERD

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


Day 1
Valid 16Z Sat May 25 2024 - 12Z Sun May 26 2024

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

...20Z Amendment...

Made some minor tweaks to the Marginal risk areas to align with
current observational and model trends. Main adjustment was to
expand the Marginal risk a bit west and southwestward over the
central and Southern Plains to account for convective development
near and just east of the dryline. These cells will rapidly
intensify and will be capable of intense rainfall rates resulting
in isolated flash flooding.

Chenard

...Previous Excessive Rainfall Discussion...

Overall...few changes needed to the previously issued Excessive
Rainfall Outlook other than some minor adjustments based on trends
in radar/satellite imagery. Still expecting development of
convection in proximity todry line in a sheared environment that
destabilizes during the day over portions of the Plains later
today. Chances for excessive rainfall build later tonight as the
areal coverage of convection expands. Elsewhere...larger scale
forcing for excessive rainfall is less well defined but sufficient
instability and low- level boundaries exist that the isolated flash
flooding may still occur over portions of the eastern and
southeastern U.S..

Bann

...12Z Excessive Rainfall Discussion...

...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...

...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