125
FOUS30 KWBC 220053
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Tue May 21 2024

Day 1
Valid 01Z Wed May 22 2024 - 12Z Wed May 22 2024


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY...

...01Z Update...
For the 01Z Update, removed the western extent of the Marginal and
Slight Risk areas over the upper Mississippi Valley where
favorable forcing aloft will support additional periods of rain
through the evening hours, but a lack of instability will likely
keep rainfall rates in check and limit the threat for flash
flooding. Also, removed areas further to the south that are now or
will soon be behind the advancing cold front.

A small Slight Risk area was maintained over Wisconsin. Ample
moisture (PWs ~1.5 inches) and instability supported by strong low
level inflow is expected to continue to support rainfall rates of
1-2 in/hr as the ongoing convective complex moves across this area
over the next few hours. The threat for flash flooding will likely
be confined to the evening hours as the system moves progressively
through the region.

Further to the south, broadened the Marginal Risk area across
Arkansas. Training storms are expected to continue to develop this
evening ahead of the advancing cold front, with rainfall rates of
1-2 in/hr expected. The hi-res guidance, including the most recent
HRRR, show notable differences on the scope and placement of heavy
amounts, but most show a good signal for heavy amounts on the order
of 2-4 inches developing somewhere within this area during the
evening into the overnight hours.

Pereira

Day 2
Valid 12Z Wed May 22 2024 - 12Z Thu May 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
TEXAS/SOUTHEAST OKLAHOMA, ACROSS THE LOWER ARKANSAS VALLEY INTO
THE MID MISSISSIPPI VALLEY...

20Z Update...
Previous outlook still looks to be on-track based on the 12Z suite
of guidance. The focus for heavy rainfall will be a slow moving
frontal zone that cuts into a moist and unstable atmosphere with
divergent flow aloft.  See below for details.

Bann


Previous Excessive Rainfall Discussion...

The trailing frontal boundary from the low moving northeastward
from the Upper MS Valley into the Upper Lakes region day 1, will
slow significantly on the southern end from the Southern Plains
into the Lower Arkansas and Mid Mississippi Valley during day 2 as
it becomes aligned more parallel to the southwest flow aloft. PW
values expected to rise to 2+ standard deviations above the mean
along this slowing frontal zone, supporting potential for heavy
rainfall total from far northeast Texas, southeast Oklahoma, east
northeastward across much of Arkansas and toward the Mid
Mississippi and Lower Ohio Valley regions. Shortwave energy
embedded in the southwest flow aloft will enhance uvvs in this
anomalous PW axis Wednesday afternoon continuing into the early
hours of Thursday. Training of cells parallel to the boundary is a
concern for heavy to locally excessive rainfall totals day 2. There
is some spread with respect to the latitude of this potential
training/heavy rains axis. The NAM is a northern outlier across
from northeast OK, northern Arkansas, far southern MO, far western
KY and far southern IL. The EC and GFS are considerably farther to
the south, while the FV3LAM and multi model ensemble qpf mean are in
between. We have drawn the slight risk area to better fit our
preferred more southern qpf solution. This is about 60-80 miles
farther to the south with the marginal and slight risk areas from
the previous issuance along the northern portion of the risk areas
across southern MO into northern AR. With respect to stream flows,
there is overlap between heavy qpf of 1.5 to 2.5" and high stream
flows, as per the National Water Model, for areas of far northeast
Texas into Southeast Oklahoma, with this area possibly having a
greater risk of runoff issues compared to other areas in the slight
risk area.

Oravec

Day 3
Valid 12Z Thu May 23 2024 - 12Z Fri May 24 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEAST
TEXAS/SOUTHEAST OKLAHOMA, ACROSS THE LOWER ARKANSAS VALLEY...

20Z Excessive Rainfall Discussion...
Few changes needed based on 12Z global/coarser scale models.

Bann

Previous Excessive Rainfall Outlook...

Similar to the day 2 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 day 3. 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 Arkansas and into the Lower Mississippi Valley. Still
some latitude differences with the model qpf axes. Similar to the
day 2 period, WPC is favoring the more southern qpf axis of the
GFS, EC, UKMET and multi model ensemble qpf mean. There is
potential for overlap of day 2 heavy rains with day 3. This may
lead to a higher threat level in later updates, especially given
the high stream flows across northeast Texas/southeast Oklahoma as
per the National Water Model.

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

Introduced a marginal risk area to eastern portions of the
Northern to Central Plains into the Upper Mississippi Valley for
convective potential ahead of the next strong area of height falls
rotating from the Northern Rockies into the Northern and Central
Plains day 3. This region likely will see heavy rainfall amounts
during the upcoming day 1 period, keeping stream flows high prior
to the potential day 3 amounts. 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 receive heavy totals day 1.


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