Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Fri Apr 26 2024
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 EASTERN OKLAHOMA, SOUTHEAST KANSAS
SOUTHWEST MISSOURI, AND FAR NORTHWEST ARKANSAS...

...2000 UTC Update...
Per collaboration with WFOs SGF, ICT, and TSA, we have expanded
the Moderate Risk area slightly farther downstream (N-NE-E) across
those CWAs, based on (1) the recent heavy rainfall/increased RSM
per NASA SPoRT/lower FFGs per the RFC, and (2) the latest QPF
trends in the guidance. Within the Moderate Risk outlook area, the
ensemble PMM of 12Z models depicts a heavy QPF footprint of 3-6+
inches, with the 12Z HRRR, FV3, and both ARWs indicating embedded
7-9+ inch totals. The latest HREF and RRFS ensembles 50-70%
neighborhood probabilities of 12hr QPF exceeding 5" within the
Moderate Risk area between 00-12Z Sun. During the same time, both
ensembles show small areas of 15-25% probs of >8 inches across
northeast OK into southwest MO.

Hurley

...Previous Discussion...
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