598
FOUS30 KWBC 262006
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Sun May 26 2024

Day 1
Valid 16Z Sun May 26 2024 - 12Z Mon May 27 2024

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS MUCH OF
SOUTHERN KENTUCKY TODAY...

16Z Update...
Extended the Moderate Risk area westward across much of southern
Kentucky as several waves of heavy rainfall work their way eastward
through late morning and early afternoon...followed later today by
the potential for a second round of convection. The HREF
probabilities for 24 hr amounts in excess of 3 inches are in excess
of 45 pct across the outlook region with spotty coverage of
probability of 24 hour rainfall exceeding 5 inches embedded within
that area. These amounts are certainly reachable given some
observed values and MRMS depicted amounts that have exceeded 2.3
inches in under an hour. After a lull later today...high
resolution CAM guidance tends to be in pretty good agreement that
a new round of convection will develop over southeast Missouri or
western Kentucky that tracks eastwards...with a pretty good chance
of traversing at least some of the heavy rainfall footprint left
behind by the storms this morning (even though the placement of
storms in round 2 differs a bit from high res model to high res
model). This same area depicted by the expanded Moderate risk area
was highlighted by the National Water Model rapid onset graphic.
Elsewhere...changes tended to be made based on trends seen in mid-
to late-morning satellite and radar imagery.

Bann

...0830Z 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"),
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...

...2030Z Update...
A deep cyclone for the month of May will continue to lift into
southeast Canada. Circulation around the low will continue to draw
moisture northward over portions of the eastern U.S.. The 12Z run
of the models were not showing quite as much instability and
somewhat lower precipitable water values...but still mesoscale
forcing to keep the Slight Risk with just a couple minor
adjustments. Elsewhere...the placement of the Marginal still
adequately covers the uncertainty of where haavy rainfall may occur
given the instability and heard-to-time shortwave energy.

Bann


...0830Z 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 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...

...2030Z Update...
The 12Z suite of guidance continues to favor portions of Texas and
southern Oklahoma for another round of convection capable of
producing heavy to potentially excessive rainfall. Return flow
draws moisture back into the region...setting the stage for
convection to develop late Tuesday afternoon and Tuesday evening as
a mid-level shortwave trough crosses Texas from west to east.
Model agreement was still fairly good allowing the Slight Risk to
stay in place without too many adjustments.

Bann


...0830Z Excessive Rainfall Discussion...

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