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

Day 1
Valid 16Z Tue May 21 2024 - 12Z Wed May 22 2024


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

...16Z Update...
Focus for excessive rainfall remains fixed on the Upper
Midwest/Upper Mississippi Valley. A well-defined coupled jet aloft
takes on a negative tilt during the day with abundant moisture
already in place and more moisture being advected in from the
south (as seen by the greater than 95th percentile of advected PW
field from CIRA). The broader-scale global models and the higher-
resolution guidance both point to one region of heavier rainfall
with the deformation zone located to the north/west of the surface
low track with 2 to 4 inch amounts. While that may may be enough to
result in some instances of excessive rainfall...expanded the
Slight Risk areas a bit on the south and east side of the track.
The pre-convective should be nearly saturated and warm cloud depths
are rather deep...suggesting storms could be efficient rainfall
producers despite meager instability (especially with given the
upper level divergence associated with the coupled jet aloft taking
on a negative tilt). The 06Z and 12Z HREF both show a low-end risk
of 2 inch per hour rates forming in southwest Minnesota that
spreads north and east with time...roughly in tandem with the upper
jet. Only minor adjustments were made in the Ozarks based on
somewhat higher numbers shown by the 12Z HREF 3-hour LPM
amounts...apparently resulting from a more robust IVT than in
previous runs. It did not reflect a fundamental shift in forecast
reasoning.

Bann

...Previous Excessive Rainfall Discussion...

Models remain fairly consistent with the overall evolution of the
large scale flow across the mid section of the nation. A strong
vort ejecting northeastward on the eastern side of the amplifying
Rockies trof forecast to becomes increasingly negatively tilted,
pushing northeast across the Central Plains into the Upper
Mississippi Valley. This will help deepen surface low pressure
across the eastern portions of the Central Plains early day 1,
moving northeast into the Upper Mississippi Valley/Upper Lakes
region later day 1. While there has been some run to run changes
with qpf details, there continues to be pretty good consensus for a
widespread axis of heavy precip across large portions of the Upper
Mississippi Valley. Hi res models continue to show two well
defined areas of precip pushing northeastward across the Upper
Mississippi Valley into the Upper Lakes, comprised of a comma
head/deformation precip area on the northwest side and a more
progressive convective precip area on the southeast side. Heaviest
day 1 totals expected to be in the comma head/deformation precip
area, where precip will be somewhat slower moving and may pivot for
a time, compared to the expected more progressive convection on
the southeast side of the system. The latest HREF neighborhood
probabilities show the two precip mode areas well and was used to
model the new day 1 Excessive Rainfall Outlook. With respect to
changes, there were no significant changes from the previous
issuance. The risk gradient was tightened up on the western end and
minor changes made elsewhere based on the HREF neighborhood
probabilities.

Oravec

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: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt