FOUS30 KWBC 260055 CCA
QPFERD
Excessive Rainfall Discussion...CORRECTED
NWS Weather Prediction Center College Park MD
855 PM EDT Sat Apr 25 2026
Day 1
Valid 01Z Sun Apr 26 2026 - 12Z Sun Apr 26 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
OKLAHOMA, NORTHEAST TEXAS, AND FAR SOUTHWEST ARKANSAS...
01Z Update...
Expanded the southern extent of the Slight Risk area a bit more
into Texas given where convection initiated and the trends seen
in short term radar imagery. There are some members of the WoFS
which show a corridor immediately either side of the Red River
where 5 minute rainfall amounts exceeding 0.25 inches in proximity
to where the SPC Mesoanalysis page focused MUCAPE values of 3000=20
to 5500 J per kg. Also nudged the Slight Risk region northward a=20
but into east-central Oklahoma where the WoFS signal was for a=20
cluster of training cells during the mid- to late-evening which=20
would support heavy to excessive rainfall. Some mid-level dry air=20
likely hold precipitable water values around 1.5 inches=20
which...when combined with forward speeds 10 kts or greater...may=20
be factors which ultimately limit coverage or amounts somewhat.
Refer to MPD 0131 and MPD 0132 for more specific details through=20
26/0530Z and 26/0641Z respectively.
Bann
16z Update...
Only adjustment to the previous outlook was to introduce a Slight
Risk for an area encompassing southeast OK, northeast TX, and far
southwest AR. This region has been highlighted by several CAMs,
including many recent HRRR and RRFS runs, with greatest potential
for training/backbuilding supercells as well as later tonight
redeveloping showers and thunderstorms along a quasi- stationary
boundary. This boundary may also be reinforced by outflow from
initial developing supercells and is why the Slight Risk was
hedged southward compared to some northern solutions. Timing of
this event appears to begin after 21Z as storms initiate across
central/eastern OK and even possibly north TX before spreading
eastward and gradually weakening by 03Z-06Z. However, continued
rainfall is possible after 06Z as the LLJ increases above 40 kts in
north TX and central OK overrunning the lingering outflow
boundary/stationary front.
Rainfall rates are likely to exceed 2"/hr within the most intense
updrafts and where hail doesn't become the primary hydrometeor.
Should these cells train or backbuild over the same area, rainfall
totals within a 3 to 6-hr period could exceed 3" and produce
scattered instances of flash flooding. The 12Z HREF highlighted
these probabilities (3" per 6-hr) at 25-30% across the Slight Risk
area through 06Z Sunday. There remains the potential for
thunderstorm coverage to remain isolated and/or include fast
forward propagation, which would limit the flash flood threat.
Snell
Previous Discussion...
Very strong instability is likely to develop later today across
the southern Plains as a consequence of considerable northward
moisture advection. Convective initiation may be delayed until
around or after 00Z as it will take time to erode an inversion.
Eventually this is expected to give way to scattered supercells,
which should then grow into convective clusters and progress to the
southeast during the overnight hours. The environment will be
supportive of heavy rain rates to around 2 inches per hour with PW
values around 1.5 inches (90th percentile for late April in the
region). Flash flooding would be possible where these rain rates
can be sustained for a couple hours; that would be most likely to
occur via cell mergers leading to brief periods of training.
A targeted upgrade to a Slight Risk may be needed later today, as
there is a chance for more focused training and backbuilding along
a composite outflow boundary and warm front that will gradually be
pushing north into southeast Oklahoma and southwest Arkansas. While
models have come into better agreement on the location of this
boundary, and the relatively strong low-level convergence
associated with it, many models also show somewhat limited
convective coverage, which would reduce opportunities for training
and a focused corridor of heavy rainfall. If greater development
along and near the boundary becomes apparent, the alignment of
shear vectors with the boundary orientation would increase the risk
of flash flooding and may warrant an upgrade.
Another area of higher QPF values (1-2 inches) is forecast from
the Poconos into the greater New York City area today and tonight.
This area was not outlooked in the ERO as the rain is expected via
steady light-moderate rain for much of the day, with a lack of
instability and chances of even 0.5 inch per hour rain rates
negligible to nil. Therefore, any impacts from the rain would not
be expected to be of the flash flood variety.
Lamers
Day 2
Valid 12Z Sun Apr 26 2026 - 12Z Mon Apr 27 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN THE CENTRAL
PLAINS...
Strong cyclogenesis in the central Plains on Sunday will facilitate
significant northward moisture transport and a more expansive area
of thunderstorm activity in the region. This is also where the
forcing is expected to be strongest: the left exit region of an
upper level jet and strong positive vorticity advection. Only minor
changes were made to the previous outlook, including trimming more
of the southern edge and expanding eastward a bit in northern MO.
Where the risk area exists, the primary question at this point will
be the amount of available instability. More unstable scenarios
could support convective clusters with higher rain rates (in excess
of 1 inch per hour) that could lead to isolated flash flooding.
Scenarios with less instability could limit rain rates. Given these uncertainties, expect further refinement of the outlook in
subsequent updates. Although some potential for heavy rain exists
from central Nebraska into central South Dakota in some guidance,
flash flood guidance is much higher in those locations, so the
northwest extent of the Marginal Risk remains constrained by the
presence of less favorable soil conditions. The 12Z HREF highlights
the greatest potential for storm total rainfall amounts exceeding
3 inches throughout southwest IA and far northwest MO, where 40km
neighborhood probabilities are 40-60%.
Lamers/Snell
Day 3
Valid 12Z Mon Apr 27 2026 - 12Z Tue Apr 28 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER NORTHERN
ILLINOIS AND FAR SOUTHERN WISCONSIN...
A deep cyclone will continue to make steady progress northeastward
on Monday from the central Plains into the western Great Lakes. A
broad 50-60 knot low-level jet to the east of the cyclone and
accompanying cold front will continue significant northward
moisture transport with 1 inch PW likely to extend to the Canadian
border and 1.5 inch PW potentially reaching to southern Wisconsin.
This would place most of the Mississippi Valley firmly in the
90-95th percentile for PW in late April. This will support
relatively high rain rates in the abundant convection expected
along and ahead of the aforementioned cold front. This is reflected
in a broad Marginal Risk over much of the Middle and Upper
Mississippi Valley. The risk is held at that level for most of the
region due to the expectation for relatively narrow squall lines
forced by the cold front as the primary contributer to rainfall
over the 24-hour period. Those squall lines should steadily (or
even rapidly) advance eastward and that would limit the duration
of heavy rainfall at any one location. Rain rates will be high
enough that isolated flash flooding is still possible.
A Slight Risk remains in a small area of northern Illinois and far
southern Wisconsin where there is an overlap of higher
probabilities of 2+ inch rainfall (25-40 percent per NBM v5.0) and
anomalously high streamflow and soil moisture due to significant
rainfall over the past couple weeks. While the risk of flash
flooding is slightly higher in these areas, the same limitation of
relatively fast progression of thunderstorms described above still
applies. Additionally, there is some uncertainty about the degree
of instability this far north; an easterly component to the low-
level flow may advect more stable air from the cool Lake Michigan
into adjacent land areas. Models that embrace that scenario tend to
show a rapid decay of convection as it progresses east into
southern Wisconsin and northern Illinois, so this will need to be
monitored as details come into focus. Nevertheless, enough signal
for heavy rainfall exists in an area of increased vulnerability to
justify higher ERO probabilities at the moment.
Lamers/Snell
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5mJ5JotKFux3QxZaXJSoxWSgJxKZvzNPWiyErZXDfjEd= pcdw7ZcNN_13t4y0W4IqMgXu8mMUsmY6ArHivgbEW9l63uo$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5mJ5JotKFux3QxZaXJSoxWSgJxKZvzNPWiyErZXDfjEd= pcdw7ZcNN_13t4y0W4IqMgXu8mMUsmY6ArHivgbEt3_EqpA$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5mJ5JotKFux3QxZaXJSoxWSgJxKZvzNPWiyErZXDfjEd= pcdw7ZcNN_13t4y0W4IqMgXu8mMUsmY6ArHivgbEWFSmnRg$=20
=3D =3D =3D
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