Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Fri Apr 26 2024

Day 1
Valid 16Z Fri Apr 26 2024 - 12Z Sat Apr 27 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHERN PLAINS INTO THE LOWER TO MIDDLE MISSISSIPPI VALLEY...

...Summary...
Based on both current observational trends (radar/satellite/
mesoanalysis), along with the latest trends per the 12Z CAM
guidance, the Slight Risk area was expanded a bit farther southwest
to include more of east-central TX, including Fort Hood), while
also trimmed a bit across parts of southeast KS.

...Eastern Portions of the Plains into the Lower Missouri and Mid-
Upper Mississippi Valley...
Ongoing QLCS traversing MO and the MOKSAROK 4-state region is the
main driver for the more enhanced (high-end Slight Risk) across
these areas, though despite airmass destabilization farther west
across central KS, much (though not all) of the latest CAMs show
limited additional rainfall during the remainder of the D1 period
behind the QLCS given the shortwave DNVA.

With time today the better synoptic forcing lifts to the north,
resulting in a downward trend in forcing across the southern half
of the Slight risk, with weakening lower level convergence and also
less pronounced mid/upper level ascent. These larger scale factors
would generally favor weaker and less organized convection with
time today into tonight over these southern areas. Countering this
is a rather favorable thermodynamic environment, with plentiful
instability and moisture. Thus it seems most likely that if
convection this morning over OK and TX can grow upscale enough to
generate an organized outflow/cold pool, which the 12Z HRRR, ARWs,
and NAM CONUS-Nest show, then this mesoscale feature could be
enough to sustain an axis of more robust convection into the
afternoon. Any convection that is able to sustain and grow upscale
may exhibit some training and backbuilding characteristics into
the strong low level jet in place.

Farther north a Marginal risk is in place from northern MO into SD
and MN. Elevated showers and embedded thunderstorms will move
across this region this morning, but will be outrunning the better
instability...which should keep rainfall rates low enough to
prevent any more organized flood threat. As the surface low and
strong mid/upper forcing ejects into the Plains this afternoon
additional convective development is likely from eastern NE/SD into
portions of IA and MN. Some of this activity should be intense,
but generally should be moving at a quick enough pace to limit the
flash flood risk.

Hurley/Chenard


Day 2
Valid 12Z Sat Apr 27 2024 - 12Z Sun Apr 28 2024

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
FAR NORTHERN TEXAS INTO OKLAHOMA AND FAR SOUTHEAST KANSAS...

A significant rainfall event still appears likely Saturday night
across portions of north central TX into central/eastern OK.
Overall not much has changed over the last couple model cycles,
and thus much of last nights discussion still holds true. Large
scale forcing really ramps up Saturday night with the approach of
the mid level trough and 250mb upper jet streak. With strong
ridging to the east, these features lift more northward than
eastward during the overnight hours, resulting in a prolonged
period of impressive upper level divergence centered over Oklahoma.
The persistence of this large scale forcing is typically
indicative of a favorable training convective setup.

In the lower levels we have a very strong 850mb jet in excess of
50 kts resulting in impressive moisture transport and convergence
near a slow moving cold front. This low level flow is nearly equal
and parallel to the deep layer mean flow, which typically supports
a backbuilding convective risk. The expectation is that with deep
layer mean flow nearly parallel to the low level boundary,
convective cells will tend to train and also backbuild into the
strong low level jet.

Convection should become intense and organized enough to develop
a stronger cold pool resulting in some eastward cell progression
with time. This eastward progression is the main wild card in
determining the magnitude of the flash flood risk with this event.
We do think there will be some eastward progression, but the
persistent forcing and strong low level jet both will be opposing a
significant eastward push...and thus tend to think we will see
enough persistence of convection to result in an organized flash
flood threat. Convection will be capable of 2-3"/hr rainfall, and
thus it will not take much persistence to start getting some
significant rainfall totals. While widespread 5"+ totals may not
occur, this event does appear to have the potential to produce a
swath(s) of as much as 4-8" of rainfall. This is expected to
result in an increasing, and locally significant, flash flood
threat Saturday night

Model guidance remains in decent agreement, all generally focusing
the highest swath of QPF from north central TX into
central/eastern OK. Although, as would be expected with any
convective forecast, there is still some uncertainty on where
exactly the highest QPF ends up, as the swath of the most excessive
amounts will likely be rather narrow. The GEM REG has been pretty
consistent with a swath heavy rainfall, and while the magnitudes
and areal extent are probably overdone in that model, do think it
represents a reasonable evolution of how things may play out
Saturday into Saturday night. The 00z ECMWF remains a bit further
southeast than the consensus. The 00z GFS has a secondary max over
eastern KS into western MO, and while heavy rain is expected here,
tend to think the GFS is overdoing this northern swath, and under
doing the swath over OK (this is a typical GFS bias). The
experimental machine learning GFS Graphcast actually appears to be
correcting this bias, and has an axis more similar to the GEM reg
(albeit lower magnitudes). The current placement of the MDT risk is
where the best overlap in ingredients and the model QPF consensus
resides, and was extended a bit more southwest into north central
TX with this update.

Across eastern KS into northwest MO and southern IA locally
excessive rainfall is also expected along an axis of stronger low
level convergence. But in general the thermodynamic ingredients
here are less favorable for a prolonged period of heavy rain
compared to the swath over OK, and thus a Slight risk should
suffice. A Marginal risk extends westward across NE into northeast
CO along/near the low track where convection near the warm front
may result in a localized flood risk.

Chenard


Day 3
Valid 12Z Sun Apr 28 2024 - 12Z Mon Apr 29 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHERN PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY...

Convection will likely be ongoing at 12z Sunday across portions of
OK and TX, a continuation of the Moderate risk day described above
for Saturday night. Current expectations are that convection will
have enough of a cold pool by this time to result in a decent
eastward propagation. Thus the flash flood risk should be
decreasing into Sunday morning, although some continued flash
flood risk is probable.

The bigger question becomes what happens on Sunday night. There is
some model disagreement on this, although the majority of solutions
would suggest an additional round of upscale convective development
is likely. This is also generally supported when looking at things
from an ingredients based perspective. With the main low lifting
off to the northeast over the Upper MS Valley, the portion of the
front over TX and AR should become nearly stationary. Meanwhile we
see a strong and persistent upper level divergence signature
centered near the TX/AR border, and a trailing mid level shortwave
ejecting into the southern Plains should only help to enhance lift
near the stalled low level convergence axis. Overall, ingredients
are similar to what we will see Saturday night over OK, thus
training and backbuilding convection appears probable.

Still think this event may very well eventually need a Moderate
risk given the setup and ingredients in place. However there
remains a bit of uncertainty with regards to where the best
convective training will be...with anywhere from northeast TX into
AR within the range of possibilities. Given this uncertainty, and
the fact that the overall synoptic setup, while similar to
Saturday night, is probably not quite as favorable (also noting
slightly lower model QPFs compared to day 2)...think sticking with
a higher end Slight risk is the best course of action for now.
Plus, rainfall Friday will likely play a role in if/where soil and
stream conditions will be more sensitive for flash flooding come
Sunday...so knowing how convection plays out Friday may help with
any eventual MDT risk placement Sunday. Thus the Slight risk
remains for now, and will continue to evaluate on future shifts.

A broad Marginal risk extends from MO northward into MN and WI,
generally along and east of the strong low and mid/upper forcing.
Convection here, while locally intense, should be quick moving,
limiting the extent of any flash flood risk. However this will be
the second strong low pressure and convective threat within a 3
day period...so can not rule out an eventual need for an embedded
Slight risk pending soil and stream response from round 1.

Chenard


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