Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sat Apr 27 2024

Day 1
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 CENTRAL AND EASTERN OKLAHOMA, SOUTHEAST 
KANSAS, SOUTHWEST MISSOURI, AND FAR NORTHWEST ARKANSAS...

A significant rainfall event still appears likely Saturday night
across portions of north central TX into central/eastern OK and
portions of southeast KS and southwest MO. Numerous instances of
flash flooding are expected, some of which will likely be 
significant in nature.

Overall not much has changed with the expected setup over the past
couple days. The environmental ingredients remain in place for 
training/backbuilding convection, especially during the overnight 
hours. Persistent and strong mid/upper level forcing, a 
significant ramp up in 850mb moisture transport into a stalled 
boundary, mean flow parallel to the boundary and weak Corfidi 
vectors (due to the strong low level jet aligned parallel to the 
mean flow)...all are favorable for training/backbuilding 
convection. The overlap of impressive upper level divergence and 
strong 850mb moisture convergence , both of which persist upwards
of 6-12 hours, combined with substantial instability and well 
above average PWs...all point to a numerous flash flood threat, 
ramping up by later in the day into the overnight hours.

Convection will likely get going pretty early in the day and then
persist into the overnight hours. The initial convection should be
more scattered in nature and moving at a decent clip...so would
expect just an isolated flash flood risk initially. However as the
low level jet and moisture transport really ramp up this evening 
into the overnight expect we will see upscale growth of convection 
into one or more training lines. Of course, the early convection 
could play a role in where this boundary and most favorable 
training corridor ends up, but overall models remain fairly well 
clustered with an axis from north central TX into central/eastern 
OK.

Probabilities from the 00z HREF are pretty impressive. EAS
probabilities of exceeding 3" are rather broad and as high as 
50-80%. Given EAS probabilities can be considered a smooth point 
probability...this indicates an event with a widespread 3"+ QPF 
footprint is probable. Embedded within this 3" area will likely be 
higher totals...with 5" neighborhood probabilities over 60%, and 8"
probabilities around 30%. This generally is in line with 
expectations from previous shifts...that this event has the 
potential to drop a swath of 4-8" of rain (isolated 10" max 
possible)...with the most likely location from just north of the 
Red River into central/eastern OK..including the Norman to Tulsa 
corridor. Overall think the 00z HREF QPF and probabilities are 
reasonable...although tend to think the southwest flank of 
convection could over perform relative to the HREF...with the 00z 
GEM Reg and FV3LAM potentially showing plausible outcomes of a QPF 
max a bit southwest of the 00z HREFbm max. The northeast extent of
the MDT risk over Southeast KS and southwest MO may see a bit 
lower QPF than areas further southwest, however with wetter 
antecedent conditions here, flash flood impacts are still likely.

One thing to continue to consider is that convection should become
intense and organized enough to develop a stronger cold pool 
resulting in some eastward cell progression with time. While we do
think there will be some eastward progression, 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 the organized and 
significant flash flood threat described above.

Further north, a Marginal risk extends from northeast CO, across 
NE and into IA and portions of WI/IL/IN. Gave some consideration to
a Slight risk upgrade over portions of NE and western IA given the
heavy rainfall over these areas on Friday. However the extent of 
convective training is a bit more uncertain here, and the higher 
probabilities of heavy rain end up a bit south of areas hardest hit
Friday. Thus tend to think flash flooding will be more isolated in
nature today, which fits more into the Marginal risk category. 
However will continue to monitor.


Chenard

Day 2
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
eastern OK and TX, into western AR/MO. Current expectations are 
that convection will have enough of a cold pool by this time to 
result in a decent eastward propagation, while also running into a
less unstable airmass with time. While some lingering flash flood
risk into Sunday morning is likely over these areas, the risk 
should be on a decreasing trend.

The bigger question becomes what happens by later in the day into
the overnight hours. Ingredients do appear to be in place for
upscale convective growth and some training/backbuilding 
potential. There is a slowing front over the area, with strong and 
persistent upper level divergence moving over top of this 
boundary. Moisture transport is not as strong as what we will see 
Saturday night over the Plains, but still note a pretty strong low 
level jet and 850mb moisture convergence signature. 

Overall this event remains as a higher end Slight risk from
northeast TX into much of AR, and would expect to see scattered
flash flooding across this corridor. A more focused and
concentrated area of more numerous and significant flash flooding
is still a possibility. Based on the ingredients in place, the most
likely location for this appears to be in the vicinity of the
Arklatex. This is also where the 00z GEM reg, UKMET, ECMWF and
FV3LAM have a relative max in QPF. However there remains some
spread with this scenario, and model QPFs are generally lower than
what we are seeing on Saturday night over the Plains. Thus this
event will probably have a bit less flash flood coverage. Still
think a MDT risk upgrade may eventually be needed, with the
Arklatex region the most likely location. However we will have 
much more high res guidance to evaluate over the the next 24 hours
than we do now, so would prefer to hold off on any upgrade and 
continue to evaluate trends. Either way some flash flooding is 
expected, some of which could be locally significant in nature.

A broad Marginal risk extends from MO northward into MN, WI and MI, 
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, some of these
areas will have seen heavy rain on previous days...so can not rule
out an eventual need for an embedded Slight risk pending soil and 
stream response from this earlier rain.

Chenard


Day 3
Valid 12Z Mon Apr 29 2024 - 12Z Tue Apr 30 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...

The convective and heavy rain threat shifts eastward into the lower
MS Valley on Monday. Still seeing an impressive combination of
mid/upper forcing, low level convergence, instability and moisture
to support a flash flood risk. At this time these ingredients look
a bit more progressive than previous days, which should result in 
quicker cell motions and an overall lowered flash flood threat. 
Nonetheless, rainfall rates should still be quite high given the 
instability and moisture in place, so 2"+/hr remains probable in 
spots. This should be enough to drive an isolated to scattered 
flash flood risk, especially over urban areas.

Models are in pretty good agreement on the axis of heaviest
rainfall, generally trending a bit further east compared to 
previous runs. The greater uncertainty lies with where to place 
the northern end of the Slight risk. The ECMWF remains further 
north than the GFS, GEM reg and UKMET. For now will keep the 
northern edge of this Slight risk over western TN, as this better 
aligns with the model consensus, with even the machine learning EC 
AIFS further south than the deterministic ECMWF.

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