FOUS30 KWBC 250751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Sat Apr 25 2026

Day 1
Valid 12Z Sat Apr 25 2026 - 12Z Sun Apr 26 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX
REGION INTO EASTERN OKLAHOMA AND SOUTHEAST KANSAS...

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. The most notable change to
the existing outlook was to reduce the size of the Marginal Risk
and focus it more directly in the Central Plains. That is where the
forcing is expected to be strongest: the left exit region of an
upper level jet and strong positive vorticity advection. Further
south, in Oklahoma and Arkansas, the trend in QPF guidance has been
drier, with expectations for weaker forcing and less favorable
low-level lapse rates associated with a warm nose. Some models keep
areas that far south almost completely dry now; therefore the
Marginal Risk has been removed and focused further north. Where the
risk area still 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 was constrained by the
presence of less favorable soil conditions.

Lamers


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 was introduced 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 with this update.

Lamers
$$
                        
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