FOUS30 KWBC 281957
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Tue Apr 28 2026
Day 1
Valid 16Z Tue Apr 28 2026 - 12Z Wed Apr 29 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX
REGION THROUGH PORTIONS OF THE DEEP SOUTH...
...16Z Update...
A plume of deep Gulf moisture will advect northeastward out of the
western Gulf across the lower Mississippi Valley today. A stalled
out front along the Red River and through central Arkansas is
providing the focus for the thunderstorms in southeast Oklahoma.
These cells are expected to multiply and expand across much of
southern Arkansas, leading to training of cells. Once the cells
merge together, they'll become a line that should push
southeastward across northern Louisiana and central Mississippi.
By then the "training" part should come to an end as the storms
move parallel to each other and in a line. A secondary cold
front/dry line will approach the stalled front from the north,
causing the storms on the northeast side to bow out across the
Tennessee Valley. There is some potential for storms to backbuild
on the north side of the line, roughly near the IL/MO/KY/TN area of
the Mississippi and Ohio Rivers, but significantly less instability
there should temper storm intensity a bit, despite soils in some
areas having been moistened by recent rains in the area the past
few days.
=46rom the inherited forecast, the Slight Risk area has been trimmed
on the north side to account for the aforementioned bowing segment
resulting in significantly less rainfall as compared with any
backbuilding storms to the north and especially to the south. The
Marginal was trimmed a bit on the north side as well, but largely
kept from the Ohio River confluence south. Meanwhile, the Slight
was expanded a bit to the west along the Red River dividing Texas
and Oklahoma, for expected backbuilding due to an evident vort max
in far southwest Oklahoma that is likely to result in renewed
thunderstorm development behind the current line of showers and
storms moving across southeast Oklahoma and northeast Texas. The
HRRR and RRFS 12Z runs were the models most heavily used for the
changes made to the outlook. The Marginal was kept very thin on the
south side due to aforementioned very dry conditions, high FFGs,
and higher likelihood that the training will be done by the time
the storms reach central Louisiana and southern
Mississippi/Alabama.
Wegman
...Previous Discussion...
A favorable environment will exist for numerous organized
thunderstorms over the southern U.S. today and tonight. Outflow
from steadily advancing squall lines over the Tennessee Valley
should provide a more distinct focus for renewed convective
development over the forecast period. The boundary is likely to be
oriented west-east, essentially parallel to 40-50 knot bulk shear
vectors. Although the air mass north of the boundary may modify
with time, it should still provide low-level convergence and a
contrast in instability. South of the boundary, the warm sector
will have very strong instability (CAPE over 3000 j/kg) and deep
moisture (PW approaching 2 inches; above the 99th percentile for
late April). This parameter space is very supportive of high rain
rates, potentially in excess of 2 inches per hour in the most
intense and organized thunderstorms.
If anything, recent hi-res model runs (06Z HRRR, 00Z RRFS in
particular) have focused the heavy rainfall signal even more into a
corridor approximately 100 miles wide, with 2-4 inch totals and
localized maxima even higher. This is consistent with the expected
pattern described above, with repeated rounds of thunderstorms
along a distinct mesoscale boundary. The main change with this
outlook was to narrow the Slight Risk a bit, especially by trimming
from the stable (northern) periphery well into the stable side of
the mesoscale boundary. It could potentially be focused a bit more,
and that will be a consideration later today. A mitigating factor
for flash flood impacts could be the severe to extreme drought
conditions in place across much of the Slight Risk area. With rain
rates potentially in excess of 2 inches per hour, and heavy rain
sustained for several hours in some cases, drought conditions may
not necessarily prevent impacts but could lead to more isolated
instances of flash flooding. This is still consistent with the
Slight Risk definition. Flash flood guidance factors in current
ground and streamflow conditions, and HREF and REFS probabilities
still show about a 15-30 percent chance of exceeding FFG despite
the overall dry antecedent setup.
Lamers
Day 2
Valid 12Z Wed Apr 29 2026 - 12Z Thu Apr 30 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
SOUTHEAST TEXAS TO THE CENTRAL GULF COAST...
...2030Z Update...
The inherited Marginal Risk was largely maintained with this
update. While high resolution guidance now fully encompasses the
entirety of the Day 2 period, a lack of agreement on the placement
and behavior of the convection does not lend itself to a Slight
Risk upgrade at this time. Were one to need to be issued, it
appears that portions of northern Louisiana and into southern
Mississippi would be the area to highlight. This would be largely
dependent on how much convection the area gets tonight with the
current convection, as saturated soils across Louisiana would make
flash flooding more likely again on Wednesday. As mentioned in the
previous discussion below, PWATs will be well above normal for=20
this time of year, supporting storms capable of very heavy rains.=20
Advection into the region will be weak...noting that the stalled=20
front along which any storms from Texas through Alabama would=20
likely form, will also be lacking in forcing. The upper levels will
be of minimal help for the storms as well, as a weak fast moving=20
shortwave traverses the region along the jet, likely initiating=20
some convection across the Lower Mississippi Valley, but then=20
quickly departing that convection.=20
Wegman
...Previous Discussion...
The outlook for Day 2 has remained mostly unchanged, with a
Marginal Risk in a corridor from Texas through Alabama along the
western and central Gulf Coast. A threat of excessive rainfall and
flash flooding is expected, due to a continuation of strong
instability, and the arrival of a plume of unusual amounts of mid-
level moisture. Backward trajectory analysis indicates moisture
around 3km above the surface can be traced along an anomalously
strong subtropical jet extending back into the tropical central
Pacific Ocean. This should push PW values over the risk area above
1.7 inches, and above 2 inches in portions of Texas, which is quite
unusual for April. Given this, it's possible an upgrade to Slight
Risk may be needed in future outlook updates. However, hi-res
guidance is just coming into view. While the extended HRRR and RRFS
do both show rain rates in this environment reaching (and even
exceeding, in some cases) 2 inches per hour, confidence is not high
on placement of greater concentrations of convection. Additionally,
there doesn't seem to be much signal for backbuilding and training,
except perhaps via cell mergers, which could lead to more limited
duration of heavy rain. Therefore, the risk level was held at
Marginal for now for the entire region.
Lamers
Day 3
Valid 12Z Thu Apr 30 2026 - 12Z Fri May 01 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN CENTRAL TEXAS...
...2030Z Update...
Few changes were needed for the risk outlook for Thursday. The=20
lion's share of the convection will occur late Thursday night,
generally after midnight, as a weakening but still potent
positively-tilted upper level shortwave dives southeastward from
the Intermountain West and into the southern Plains. Prior to that,
much of Thursday will look like Wednesday, just further west.
Plentiful Gulf moisture will run into a stalled out frontal
boundary across central Texas, but on rather weak (10-20 kt)
southerly flow at 850 mb. Thus, forcing will be the primary
limiting factor, since instability and especially moisture will not
be. However, on Thursday night as that shortwave ejects towards the
Plains, much more plentiful convection will blossom all along the
front. There is uncertainty as to where that convection will form,
with the Slight Risk highlighting the most likely locations across
central Texas.=20
The Slight Risk was expanded east into northern Louisiana as prior
days' rains will likely saturate soils such that lesser amounts of
rain, likely occurring Thursday night, will still cause widely
scattered flash flooding, especially in urban areas such as
Shreveport.
The Marginal Risk was expanded across much of eastern New Mexico
with a small Marginal into southeastern Arizona and far
southwestern New Mexico. This new Marginal highlights the potential
for showers and a few thunderstorms in an area generally
susceptible to flash flooding in an area far enough west to be
impacted by the incoming shortwave trough during peak heating, when
storms will have more instability to potentially produce localized
heavy rain.
Wegman
...Previous Discussion...
Increasing mid-upper level height falls over New Mexico and Texas,
due to a strong shortwave pushing through northern Mexico, should
lead to a low-level mass field response with increasing south to
southeast inflow across southern and central Texas. That will also
lead to a gradual uptick in low-level convergence between the
developing low-level jet, and an easterly cool conveyor belt to
the north, in a more stable region from Oklahoma into northwest
Texas. The result should be more numerous thunderstorms over the
course of the Day 3 period, particularly by Thursday Night. Deep
moisture will remain firmly in place, although the instability
should shift to a narrower CAPE profile with lower equilibrium
levels. That trend, in combination with PWs of 1.5 to 1.8 inches,
should support increasingly efficient, warm rain processes in
convective bands.
Furthermore, some training of the lower-topped convective bands may
occur. With stronger instability over South Texas, the Rio Grande
Valley, and far northeast Mexico, and steadily increasing forcing
from the approaching wave upstream, persistent new convective
growth upstream of convective bands is plausible.
Putting all these factors together, a Slight Risk was introduced
over Central Texas. This was placed where drought conditions either
do not exist or are much lower on the scale; that also happens to
be where some ensemble QPF signal is strongest. It's possible the
Slight Risk may trend southeast with time as AI guidance is
positioned on the southeast flank of the guidance envelope, and
would seem to be a logical corrective to the usual bias of QPF
maxima placed too far in the direction of a stable air mass,
especially with 2-3 days of lead time.
Lamers
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5zNvW0mLhCyeWs1oG1VsOF8DacbtmL6SFx_oXmULpzeE= eJNwRUqmLHyTrknsgK587m62mOgetJs3oe8pA3tCQmXo5e4$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5zNvW0mLhCyeWs1oG1VsOF8DacbtmL6SFx_oXmULpzeE= eJNwRUqmLHyTrknsgK587m62mOgetJs3oe8pA3tCbxuVAmc$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5zNvW0mLhCyeWs1oG1VsOF8DacbtmL6SFx_oXmULpzeE= eJNwRUqmLHyTrknsgK587m62mOgetJs3oe8pA3tCEIMruRw$=20
=3D =3D =3D
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