FOUS30 KWBC 260802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Sun Apr 26 2026

Day 1
Valid 12Z Sun Apr 26 2026 - 12Z Mon Apr 27 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE CENTRAL
PLAINS...

The latest hi-res model guidance has trended substantially toward
a corridor of heavy rainfall in the Central Plains this evening and
tonight. Therefore, confidence has increased in a heavy rain event,
but considerable uncertainty still remains on the location. Some
guidance places the heaviest rain almost wholly into Iowa, while
other guidance is further south in the vicinity of the I-70
corridor in Kansas and Missouri -- almost 200 miles of variability
with approximately 18 hours of lead time. Given the consistent
signal for significant rainfall, we have upgraded to a Slight Risk,
and drawn it broadly to account for positional uncertainty. The
contours were largely based on EAS probabilities from the 00Z HREF;
and the major axis of the Slight Risk area intersects the QPF
maximum on the AI versions of the GFS and GEFS.

Convection is expected to develop this afternoon on the nose of a
strengthening low-level jet (LLJ) and plume of instability, likely
in northern Kansas or southeast Nebraska initially. The convective
evolution in the approx. 6 hours after initiation will be
important. In general, all models show a west-to-east progression
of thunderstorm clusters, consistent with the bulk shear vectors,
and along a focused area of low-level convergence near the nose of
the LLJ. Some models show scattered clusters progressing over
similar areas, but with intermittent breaks and not precise
training; others show a more continuous band of convection training
in a specific corridor (sustaining 1-2 inch per hour rain rates for
several hours in a given location). Comparing QPFs from those
scenarios suggests the latter scenario could double rainfall, with
amounts potentially high enough to lead to significant flash
flooding. Therefore, this will need to be monitored closely.

A Marginal Risk was considered in the Arklatex region as ongoing
convection will likely persist in some form beyond 12Z. However, a
squall line in southern Arkansas appears to be forward propagating
and accelerating, if anything. And other thunderstorm activity is
more isolated and generally less intense. Therefore, a non-zero
excessive rainfall risk does exist in the region, but no Marginal
Risk was outlined at this time.

Lamers

Day 2
Valid 12Z Mon Apr 27 2026 - 12Z Tue Apr 28 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL PRIMARILY OVER
SOUTHEAST IOWA AND NORTHERN ILLINOIS...

The training band of convection from Sunday Night is likely to
progress eastward into the Midwest on Monday; placement
uncertainties grow on Day 2, but there has been a general southward
shift in the location of the heaviest rainfall in model guidance
over the last couple cycles. Some guidance now shows the corridor
of heaviest rainfall on Monday and Monday Night as far south as the
I-70 corridor from St. Louis to Indianapolis, while other guidance
remains closer to the Illinois-Wisconsin border, where the existing
Slight Risk area was concentrated. This is about a 200 mile spread.
So while heavy rainfall is expected, confidence remains somewhat
low on the location. Given the overall trends, the main change was
to shift the axis of the Slight Risk southward, albeit only about
70 miles out of deference to continuity.

The overall environment will be very supportive of heavy rain
rates. A plume of 1.2 to 1.7 inch PWs (above the 95th percentile
for late April) and moderate to strong instability will support 1
to 2 inch rain rates in organized convective clusters and lines.
Although training convection may continue into Monday morning,
convective lines should become oriented more in a north-south
fashion later in the day as forcing along an accelerating cold
front increases. The best opportunity for training convection, and
therefore a longer duration of heavy rain rates, should be in or
near the Slight Risk area. Areas further south should not only see
steady progression to any convective lines, but have also been much
drier lately, so the risk of flash flooding is lower despite some
similar QPF from the latest model guidance.

Lamers

Day 3
Valid 12Z Tue Apr 28 2026 - 12Z Wed Apr 29 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
INTERIOR SOUTH...

Fairly widespread convection is expected on Tuesday and Tuesday
Night over much of the interior South, in a broad plume of
anomalously high PWs between 1.5 and 1.8 inches with strong
instability. Organized convective clusters and lines should be
capable of producing rain rates up to 2 inches per hour, and any
training could lead to localized flash flooding. Much of the region
has been very dry lately; over the past 60 days most of Kentucky,
Tennessee, and eastern Arkansas have seen less than half of their
average precipitation. Therefore, the region is generally not
primed for impacts from heavy rainfall, even though flash flooding
can occur given sufficiently high rain rates sustained for a long
enough duration. Given low confidence in mesoscale details, the
risk level has been held at Marginal over the entire region for
now. It's possible an upgrade to Slight Risk may be needed
eventually, and may be more likely in Arkansas, eastern Oklahoma,
or northeast Texas where there has been some rainfall in the past
couple weeks, including earlier today.

Lamers


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt