038
FOUS30 KWBC 272026
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 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 EASTERN 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. Synoptically, the mid to upper-level trough
over the southern/central High Plains on Saturday will lift NNE
toward Minnesota through Monday morning. As this occurs, a lack of
height falls across the Gulf Coast region will coincide with a
retreating dryline in Texas, but locations from Arkansas down the
Sabine River Valley will remain beneath diffluent and divergent
flow aloft for much of the period (though jet-induced divergence
may be focused better across northern locations than those closer
to the Gulf Coast).

Current thinking is for new convective development along the
front/dryline or perhaps remnant outflow (from the early morning
convection) in the 18-00Z window over northeastern Texas into
southeastern Oklahoma and western Arkansas. The leading edge of the
cold pool from the afternoon/evening thunderstorms is expected to
be intercepted by 25-45 kt of southerly flow at 850 mb, forcing the
development of additional thunderstorms within the unstable
environment. Orientation of convection is likely to match mean
steering flow from the southwest at times, supporting training and
rainfall rates of 1 to 3 in/hr (although localized rates in excess
of 3 in/hr will also be possible).

Overall this event remains as a higher end Slight risk from
east and northeastern Texas into southeastern Oklahoma and western
Arkansas. A more focused and concentrated area of numerous and
significant flash flooding is still a possibility with higher
neighborhood probabilities of exceeding 5 inches in 24 hours
(ending 12Z Monday) showing 40-60 percent from near the Arklatex
down to a Huntsville to Jasper to Fort Polk line and 20-40 percent
for EAS probabilities for 3+ inches along the northern
Texas/Louisiana border. Along with the typical convective
uncertainties, the 12Z global and hires guidance (FV3, NAM_nest)
showed some southward displacement compared to their previous runs,
into drier antecedent conditions where flash flood guidance is
about 4+ inches in 3 hours. Due to these factors, confidence in
placement of a Moderate is not there yet, although Moderate Risk
coverage of flash flooding (locally significant possible) is
expected somewhere within the Slight Risk. 24 hour QPF from the 12Z
model suite ranged from 4 to 7+ inches ending Monday morning.

Across the Midwest into Wisconsin and Michigan, a broad Marginal
Risk remained in place with minor adjustments from continuity.
With northern extent, there remains uncertainty with the
availability of instability but a slow moving front with steering
flow parallel to the boundary will be in place across northern
Illinois/southern Wisconsin into Lower Michigan. While potential
for higher coverage of rates over 1 in/hr is not as high for these
northern locations, flash flood guidance is low in pockets across
the northern half of the outlook area, especially for portions of
Nebraska/Iowa due to recent heavy rain.

Otto/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
Mississippi Valley on Monday. Thunderstorms with heavy rain are
likely to be ongoing at the start of the period Monday morning and
the main flash flood threat looks to be focused in roughly the
first 12 hours of the period. As the closed low from over the
weekend moves north, an upper level trough/shortwave is forecast to
swing across the Lower Mississippi Valley from the west during the
day on Monday. Out ahead of this feature, the 12Z forecast
guidance continues to show 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 with perhaps
the best potential for stalling/training occurring across
Louisiana into portions of Mississippi. 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.

Other than a southward trend in the GFS/UKMET/CMC QPF, no
significant changes to mass fields were noted and the models remain
in pretty good agreement on the axis of heaviest rainfall. The
12Z ECMWF continues to be a bit of a northern outlier with QPF
maxima from southern Illinois, down into eastern AR, just west of
the Mississippi River. While a Marginal exists for many of these
northern locations, the Slight was kept where the better agreement
for higher rainfall existed, from near the Missouri Bootheel to far
southeastern Texas and southern Louisiana, overlapping eastern
Arkansas and western Mississippi.

Otto/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