FOUS30 KWBC 250828
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Thu Apr 25 2024
Day 1
Valid 12Z Thu Apr 25 2024 - 12Z Fri Apr 26 2024
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
PLAINS AND MISSISSIPPI VALLEY...
Convection will be ongoing at 12z this morning across portions of
the central and southern Plains. There remains some spread within
the 00z HREF guidance whether to favor a northern training axis
across portions of KS and MO, or a southern axis from northeast OK
into northwest AR. At this point it remains a bit unclear which
axis will become the dominant one, but both will likely see some
upscale convective development this morning within an area of
increasing moisture convergence in the 850-700mb layer. Storm
motions will favor some training and backbuilding, and thus some
flash flood risk will probably exist this morning from KS into
western MO, and from northeast OK into northwest AR.
Stronger mid and upper level forcing will eject east out of the
Rockies tonight, which will trigger additional convective
development over the Plains. Over KS and OK this convection should
be quite progressive off to the east. However high rainfall rates
are likely, and so any areas that see heavy rain from the first
round of convection this morning/afternoon could potentially see
areas of flash flooding tonight. Otherwise the activity should be
quick enough to pose only an isolated flash flood risk. There is a
better chance of a convective training tonight closer to the warm
front near the KS/NE border, which could drive an isolated to
scattered flash flood threat.
Overall will continue with a broad Slight risk that stretches from
near the NE/KS border southeast into northwest AR. This covers
both the expected convection this morning/afternoon, and the
development expected tonight with the arrival of stronger forcing
from the west.
Chenard
Day 2
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 MID MISSISSIPPI VALLEY...
In what is shaping up to be an active stretch of excessive
rainfall potential, Friday may end up being a relative minimum of
flash flood impacts. The first system will be rapidly ejecting
northeast across the Plains and upper MS Valley. Given the quick
forward motion and the fact that the system will be moving east of
the stronger instability with time, tend to think any flash flood
risk will stay localized in nature from the central/northern
Plains into the upper Midwest.
The southern extent of the system may pose a slightly greater flash
flood risk, but even this remains uncertain. Convection should be
ongoing Friday morning across portions of TX/OK/AR/MO, although the
intensity and organization of this activity remains unclear. The
convection will be outrunning the surface front by this time
resulting in a gradual weakening of lower level convergence. In
the upper levels the forcing is also on a decline, so quite
possible activity is weakening Friday. However with plenty of
moisture and instability in place, and little reason for much
eastward progression of convection...any activity that is able to
persist and/or intensify during the day could end up pretty slow
moving and thus capable of producing flash flooding. Thus opted to
maintain a Slight risk from northeast TX into southwest MO to
account for this threat...while also accounting for the fact that
some of these areas may be more sensitive depending on the rainfall
that occurs Thursday and Thursday night. Also worth noting our
machine learning ERO also has elevated probabilities over
approximately this same area...likely indicative of the lingering
favorable ingredients in place.
Chenard
Day 3
Valid 12Z Sat Apr 27 2024 - 12Z Sun Apr 28 2024
...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
OKLAHOMA...
A significant rainfall event appears likely Saturday night across
portions of north central TX into central/eastern OK. 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, swaths of totals exceeding 5" are expected, which should
result in an increasing, and locally significant, flash flood
threat.
Model guidance is in decent agreement for a day 3 forecast, all
generally focusing the highest swath of QPF from north central TX
into central/eastern OK. Although, as would be expected with a day
3 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 current placement of the MDT risk
is where the best overlap in ingredients and the model QPF
consensus resides. To the north into southeast KS, some expansion
may eventually be needed depending on antecedent rainfall over the
next couple days leading up to Saturday night. Meanwhile some
expansion to the south southwest of the current risk area is a possibility...as
the south and southwest flank of convection is
often a favored region for more prolonged training/backbuilding in
these type of events.
Across eastern KS into northwest MO and southern IA locally
excessive rainfall is also possible. But in general the
environmental ingredients are not as favorable for a prolonged
period of heavy rain, 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
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