581
FOUS30 KWBC 222026
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Wed May 22 2024

Day 1
Valid 16Z Wed May 22 2024 - 12Z Thu May 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHEAST
TEXAS AND EXTREME SOUTHEAST OKLAHOMA INTO SOUTHERN ARKANSAS AND
NORTHERN LOUISIANA...

...16Z Update...

The latest trends in the guidance have shifted the axis of storm-
total heaviest rainfall about a row of counties south from
inherited, so the Slight was adjusted accordingly. The surrounding
Marginal looks good, and no major adjustments were made other than
small expansions to account for ongoing activity from northwest
Arkansas into south-central Oklahoma. The rapidly progressing
segments are aligning perpendicular to the flow, reducing the time
any one location sees heavy rain. Thus, the Marginal risk for those
areas looks good.

In a few hours this afternoon, additional convection potentially
on the outflow boundary of the southernmost storm near Ardmore, OK
at the time of this writing, is expected to develop. It will likely
be surface based due to peak heating. These storms will form from
both outflow boundaries and leading an advancing cold front. A
nearly stationary southwesterly upper level jet oriented parallel
to the surface cold front will not help to advance the line of
storms. However, any mesoscale upper level disturbances, moving
outflow boundaries, the tendency for storms to backbuild into the
moisture and instability, and the axis of the jet pushing east
should all allow for enough movement of the line to cut back on the
flooding potential. Thus, while areas from the DFW Metroplex
northeast to Texarkana are in a higher-end Slight, no Moderate Risk
was considered at this point.

For the Metroplex, urbanization locally lowering FFGs will
increase the flash flooding threat higher. Meanwhile, further north
and east towards Texarkana, guidance is in reasonably good
agreement that if there is to be an extended period of training
storms, it would occur in this area up to the Red River. For east-
central Texas into northern Louisiana, the southern side of the
Slight, recent heavy rains have kept the soils nearly saturated, so
think the FFGs in this area are likely still a bit high. Towards
Houston, the guidance is in better agreement that by late tonight,
the convection will quickly dissipate as it reaches that area so
for now no risk areas have been drawn in the immediate area.

Further north and east along the cold front, recent heavy rains
have saturated soils into Kentucky and Tennessee. Just like the
ongoing storms, think the convection in this area will also be
fast-moving and only pose an isolated/Marginal flash flooding
threat.

Wegman

...Previous Discussion...

The southern portion of the frontal boundary emanating from the
low moving from the Upper Mississippi Valley into southern Ontario
this morning will slow significantly from the Southern Plains into
the Lower Arkansas and Mid Mississippi Valley during the upcoming day
1 period, becoming aligned more parallel to the southwest flow
aloft. Additional convection likely to form in the axis of PW
values 2+ standard deviations above the mean as shortwave energy
moves east northeastward across the Southern Plains and along this
slowing frontal zone. There has been a significant southward shift
in nearly all of the models with the forecast axis of heaviest
precip along this front. The individual hi res runs and the HREF
mean are the farthest south now, with global models to the north of
the HREF mean, closer to the 18Z axis of the RRFS and 06Z May 21
axis of the RRFSMEAN. The 00Z EC is the farthest south global
model, closer to the hi res and HREF mean axis.  WPC has followed
the southward trend in our latest qpf and we will also trend
farther to the south with the Excessive Rainfall probabilities.
Training of cells parallel to the boundary is a concern for heavy
to locally excessive rainfall totals day 1, although confidence is
not great with placement given the spread and significant changes
run to run. With respect to the previous outlook for this period,
there has been approximately a 60-80 mile southward adjustment to
the marginal and slight risk areas, extending them farther south
into northeast Texas and northern Louisiana and reducing the risk
from the OH/MS river confluence into northern Arkansas.

Oravec

Day 2
Valid 12Z Thu May 23 2024 - 12Z Fri May 24 2024


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
TEXAS/ FAR SOUTHEAST OKLAHOMA, ACROSS SOUTHERN ARKANSAS INTO
NORTHERN MISSISSIPPI...

...2030Z Update...

...Mid-South...

No significant changes were made to the Slight risk area a from
northeast Texas to northern Mississippi this afternoon. The second
day in a row of strong and moisture-laden showers and thunderstorms
expected to develop across the Slight Risk area will be capable of
producing widely-scattered flash flooding. Expect the FFGs in this
area to be lower by Thursday after today and tonight`s rain.

...Mid-Atlantic...

The surrounding Marginal Risk area was expanded several states east
to include much of the Mid-Atlantic. In addition to localized
decreases in FFG expected as a result of today`s storms, another
several lines of storms may track along the front from the Southern
Appalachians to the Mid-Atlantic. While each line will be rather
fast-moving, if 2 or 3 separate lines move across the region, then
isolated flash flooding is possible, especially in urban and
flood-prone areas.

...Northern Plains...

The Marginal in the upper Midwest was expanded west to include much
of North Dakota and southeastern Montana with this update. A
developing storm system will tap Gulf moisture in place to become a
typical comma-shaped storm as it moves into the Dakotas. The
northern and western side of the storm, while on the "cold" side,
will cause steady rain over a long period of time over a typically
dry area. Thus, isolated flash flooding is possible, particularly
where convection can develop on the interface with the dry slot,
where higher rainfall rates will be possible.

Wegman

...Previous Discussion...

Similar to the day 1 time period, a stationary frontal boundary
expected to remain stretched in a west southwest to east
northeasterly direction across northeast Texas/southeast Oklahoma
into Arkansas, although this front will be weakening with time. PW
values expected to remain 1.5 to 2 standard deviations above the
mean along this weakening front during day 2. Additional shortwave
energy moving west to east across the Southern Plains will support
additional convection in this high PW axis and potential for
additional heavy rains from northeast Texas/southeast Oklahoma,
east across the Lower Arkansas Valley into the Lower Mississippi
Valley. Given the low confidence with how convection will develop
day 1 and whether there will be an outflow boundary still in tact
for the day 2 convection, confidence does remain low on qpf
placement. Models do seem to support a farther northward axis day 2
compared to day 1 as the stationary front weakens and southerly low
level flow pushes the high PW values farther to the north.  There
still is potential for overlap of day 1 heavy rains with day 2, but
with low confidence in the day 1 axis, confidence is low as to
where this overlap will be. The new day 2 Excessive Rainfall
Outlook did trend southward from the previous issuance for this
period by approximately 50 miles to better match the latest WPC
and model qpf consensus.

...Eastern portions of the Northern to Central Plains into the
Upper Mississippi Valley...

No significant changes made to the marginal risk area across
eastern portions of the Northern to Central Plains into the Upper
Mississippi Valley. Additional convection likely ahead of the next
strong area of height falls rotating from the Northern Rockies
into the Northern and Central Plains day 2. This region has seen heavy
rainfall amounts over the past 24 hours, keeping stream flows high
prior to the potential day 2 mounts. There is a lot of spread with
qpf details, leading to low confidence in any one solution.
However, with the strong upper trof, and associated strengthening
low level southerly flow and increasing PW values, there will be
potential for localized heavy rainfall amounts that may fall across
areas that received heavy totals over the past 24 hours.


Oravec

Day 3
Valid 12Z Fri May 24 2024 - 12Z Sat May 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE UPPER MISSISSIPPI VALLEY INTO THE UPPER GREAT LAKES AND FROM
THE LOWER ARKANSAS/LOWER MISSISSIPPI VALLEYS INTO THE TENNESSEE
VALLEY AND SOUTHERN TO CENTRAL APPALACHIANS...

...2030Z Update...

No major changes to the southern Marginal across many of the same
areas as Day 2/Thursday...from northeast Texas to the southern
Appalachians. While this may be the 3rd straight day of storms for
some, convective coverage is expected to be lesser on Friday as
compared with previous days...such that despite the lower FFGs
likely to be in place, lesser coverage of storms should keep any
new flash flooding isolated.

The northern Marginal across much of the upper Midwest and Great
Lakes was expanded south to include Milwaukee and Chicago over to
the Quad Cities. The same low spreading rain further west Thursday
will move into this region on Friday. Much of Minnesota and
Wisconsin were hard hit yesterday with heavy rain, so some of these
areas may still be sensitive to heavy rain and resultant flash
flooding again on Friday, particularly if Thursday`s rain in Day 2
overperforms.

For both Marginal Risk areas, confidence is not there for any
upgrades at this time.

Wegman

...Previous Discussion...

...Upper Mississippi Valley into the Upper Great Lakes...

The next strong area of height falls moving into the Northern High
Plains at the end of day 2 will continue to push eastward through
the Northern Plains and into the Upper Mississippi Valley during
day 3. The overall favorable large scale conditions ahead of these
height falls day 2 will continue into day 3 along the associated
front where strong frontal convergence in an axis of PW values 1 to
1.5 standard deviations above the mean will support potential for
moderate to locally heavy rainfall totals. This front does look
very progressive day 3, which may limit very heavy amounts. The
marginal risk was drawn to where there may be overlap of locally
heavy rains day 3 with heavy rains that have fallen over the past
24 hours.

...Lower Arkansas/Lower Mississippi Valley, Tennessee Valley into
the Southern to Central Appalachians...

A broad marginal risk area was maintained from the Lower
Arkansas/Lower Mississippi Valley region, east northeast across the
Tennessee Valley and into the Southern to Central Appalachians. PW
values across these regions will remain above average for the day 3
period, with additional shortwave energy moving west to east
supporting scattered convection and locally heavy rainfall
amounts. Not a lot of confidence in where the scattered convection
will become more organized, with a fairly large spread in model qpf
details.

Oravec

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