737
FOUS30 KWBC 260743
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
343 AM EDT Sun May 26 2024

Day 1
Valid 12Z Sun May 26 2024 - 12Z Mon May 27 2024

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS NEAR THE CENTRAL
TENNESSEE/KENTUCKY BORDER...

A cyclone moving across the Midwest and Great Lakes is expected to
draw ample moisture (precipitable water values of 1.50-1.75"),
low-level inflow, effective bulk shear, and instability into
portions of the MS, TN, and OH valleys. A convective complex trying
to congeal near the KS/MO border at the present time is expected
to continue moving east to east-southeast across KY and portions
of TN this morning into this afternoon, near and to the north of an
old/weak segment of the polar front. Making matters worse, a
second convective round is expected Sunday evening moving from the
confluence of the OH/MS rivers east to east- southeast across the
same region as a cold front moves through. Rainfall over the past
week across portions of the TN/KY border is 300-600%, so soils
should be saturated. With the specter of hourly rain totals to
2.5", and local amounts of 6", an upgrade to a Moderate Risk was
necessary in this area, despite the 00z guidance spread. This
evolution is supported by the 00z HREF which was on the southern
portion of the guidance spread and since 700 hPa temperatures are
low enough to support a relatively uncapped atmosphere, a more
southern solution as advertised by the HREF makes sense
conceptually. A broad Slight Risk surrounds this region where
similar amounts are possible and soils are less saturated.
Mesocyclones, cell training, and cell mergers are the expected
reasons for excessive rainfall here.

While lower rainfall amounts generally are expected in the East and
the Upper Midwest, the expected low-level inflow, effective bulk
shear, and precipitable water values indicates that an isolated
excessive rainfall threat exists, with hourly rain totals to 2" and
local amounts to 4". The difference between the two regions are
the degree of soil saturation/recent rains and the available
instability. Much of WI has experienced 300-600% of their average
seven day rainfall so soils should be sensitive. However,
instability should be minimal so low- to mid-level frontogenesis
appears to be doing the heavy lifting, heavy rainfall production-
wise. Went ahead and raised a Slight Risk in and near portions of
WI.

Roth

Day 2
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 similar to 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, the Appalachians, and the Southeast. There is some concern
that MCVs from previous activity in the Midwest and Mid-South
could sail into the area to increase divergence aloft, effective
bulk shear, and thunderstorm organization across the Southeast, so
broadened the Marginal Risk in this area. There`s even some
convective potential within the system`s comma ahead across
portions of MI; the guidance shows a narrower range of possible
outcomes compared to this time yesterday, which appears 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 3
Valid 12Z Tue May 28 2024 - 12Z Wed May 29 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
TEXAS AND SOUTHERN OKLAHOMA...

Weak shortwaves rippling through and dampening mid-level ridging
across TX are expected to force a return flow of moisture and
instability back into the western Gulf Coast. Precipitable water
values of 1.5-1.75" are forecast, and low-level inflow and
effective bulk shear should be just sufficient for convective
organization near a front. The 00z NAM advertises MU CAPE of 5000+
J/kg, so there`s instability to spare. While 700 hPa temperatures
are plenty warm, implying a decent mid-level cap, the front should
be an effective focus and allow the cap to break.  Most of the
guidance shows reasonable agreement, with the exception of the 00z
GFS that streaks heavy rainfall eastward. Since it can be prone to
convective feedback which can lead to such an evolution, decided to
account for its solution in the Marginal Risk area by showing some
eastward expansion. The inherited Slight Risk resembles
continuity.

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