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FOUS30 KWBC 271618
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1218 PM EDT Sat Apr 27 2024

Day 1
Valid 16Z Sat Apr 27 2024 - 12Z Sun Apr 28 2024

...A HIGH RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR A SMALL
PORTION OF EAST-CENTRAL OKLAHOMA WHERE NUMEROUS SIGNIFICANT FLASH
FLOOD IMPACTS ARE EXPECTED...

...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...

...16Z Update...

Confidence continues to be high in a major flash flood event
unfolding for portions of east-central Oklahoma with higher end
rainfall totals Saturday beginning late this evening into Saturday
night. Therefore, a small High Risk was introduced for locations
east of OKC along I-40, but higher-end rainfall totals (5 to 10
inch 24 hour totals) and flash flooding is also expected from the
Red River Valley into portions of northeastern Oklahoma as well.

12Z components of the HREF show unanimous agreement with 3 to 6+
inches for east-central OK where the High Risk was placed, and
while there remains some potential for cold pools to shift the axis
of higher rainfall to the south, it is difficult to ignore the
consistent (00Z to 12Z cycle) placement of HREF probabilities for
higher end rainfall. The probability values listed in the previous
discussion below still hold true for the 12Z HREF, with perhaps a
5 to 10 percent increase overall for the EAS (3 inch) and
neighborhood probabilities (5 and 8 inch) for the 24 hour period
ending 122 Sunday.

Elsewhere, changes to the outlook were minor but the Slight Risk
across the Midwest was extended east to include northern Illinois
into northwestern Indiana, due to the likelihood of cell training
from WSW to ENE and 1-2 in/hr rainfall rates late this evening and
overnight. Given flash flood guidance for this region of the
Midwest is only 1 to 2 inches in 3 hours and after collaboration
with LOT and IWX, an upgrade to Slight was introduced for this
update.

Otto

...previous discussion follows...


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