FOUS30 KWBC 260823
QPFERD

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

Day 1
Valid 12Z 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...

A Slight risk was maintained with this update from portions of
northeast TX into MO, although there remains some uncertainty 
regarding the intensity and organization of this activity. 
Convection will likely be ongoing at 12z this morning across 
portions of TX and OK, with this activity gradually pushing 
eastward across these two states and into portions of MO and AR 
through the afternoon hours. 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, then this mesoscale feature could be 
enough to sustain an axis of more robust convection through today. 
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. More recent observational and HRRR trends 
suggest portions of MO have the best chance of seeing multiple 
convective rounds today with stronger upper forcing and WAA...with
one round this morning and another through the afternoon hours. 
Thus we did expand the Slight risk into more of MO to account for 
this risk.

Further 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.

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: 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