Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
838 PM EDT Sun Apr 27 2025

Day 1
Valid 01Z Mon Apr 28 2025 - 12Z Mon Apr 28 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH-CENTRAL AND SOUTHEASTERN MONTANA...

...01Z Update...
Few changes needed to the previously issued Excessive Rainfall
Discussion. Convection began increasing in coverage during the
afternoon across portions of south central/northeast Wyoming in
proximity to a quasi-stationary north-south oriented deformation
zone. Concern for excessive rainfall is driven by modest amounts of
rainfall falling on a warming snowpack at mid-levels across
portions of south-central/southeast Montana into northeast Wyoming.
Farther to the east in portions of southeast Wyoming into western
South Dakota...flash flooding from convection firing along a well-
defined dry line with easterly flow north of a warm front providing
amply shear. The main concern from this convection looks to be hail
but there is ample moisture flux to support increasing rainfall
production and risk of flash flooding from slow moving cells given
Flash Flood Guidance values ranging from 1 to 1.5 inches per hours
or near 2 inches in 3 hours across far southeast Montana into the
Black Hills of South Dakota. The activity farther west is expected
to take on a broader coverage of stratiform character later,

The Marginal Risk area extending across the remainder of the
Dakotas into Minnesota remained in place without modification. The
expectation remains for thunderstorms to develop later tonight with
an increasing chance for heavy rainfall as the low level jet taps
deeper moisture and transports the moisture/instability into the
region.

Bann

...16Z Update...

Very few changes were needed to the outlook areas with this update.
Guidance remains largely on track, with the Slight Risk across
south-central and southeastern Montana remaining in place.
Moderate rainfall rates up to an inch per hour (heavy in upslope
areas) will combine with snowmelt and burn scars to locally
increase the coverage of instances of flash flooding to widely
scattered. Most of the event will occur tonight, continuing in some
areas into Monday morning.

Southeastern portions of the Marginal were trimmed over Minnesota,
Iowa, and South Dakota with the latest guidance shifting the
potential for training thunderstorms north and west.


Wegman

...Previous Discussion...

...Summary...
Minimal modifications were made to the Slight and Marginal Risk
areas from yesterday's Day 2 ERO.

...Northern High Plains to the Mid Mississippi Valley...
Mid-Upper low migrating across the central Great Basin-central
Rockies during the period will set the stage for robust deep-layer
forcing over the outlook areas as the upper ridge axis moves east.
Helping matters will be coupled upper level jet streaks, i.e. with
the ribbon of enhanced lift within the right-entrance region of the
upper jet streak traversing south-central Canada, while also
within the left-exit region of the upper jet streak lifting into
the Front Range and Central Plains. Robust low-mid level moisture
transport turning westward into the maturing Cold Conveyor Belt
(CCB) and maturing TROWAL will reach 2-3 standard deviations above
normal across the Northern Plains per the 00Z GEFS, while
anomalies peak between +3 and +5 across the Upper MS Valley Sunday
night. This as the south-southwesterly LLJ peaks by Sunday night
(~50 kts at 850 mb), while strengthening ENE-NE low-level flow
peaking between 30-40 kts across the backside of the CCB will lead
to a considerable upslope component to the lift across the
foothills and High Plains of south-central and southeast MT into
northeast WY.

The Slight Risk area was largely unchanged. Peak rainfall totals of
2-4+" west and southwest of Billings will combine with high snow
levels and burn scars to locally enhance the flash flooding
potential in this area of Montana. Farther east across the northern
Plains and Upper MS Valley, deep-layer instability will become more
plentiful (MUCAPEs peaking aoa 1,000 J/Kg), while the low-level
moisture transport becomes more robust Sunday night given the
aforementioned uptick in the LLJ. Despite the more favorable deep-
layer thermodynamical profile, the spread in the model QPFs is much
higher over this area, particularly with the heavier rainfall. This
is likely to the more transient (less persistent), mainly WAA-
driven upper level forcing, along with the low-level fronts
remaining upstream of the Upper MS Valley through Sunday night. In
addition, the forward-propagating Corfidi vectors would support a
rather progressive motion of any organized convection that is able
to grow upscale. As a result, the Marginal Risk area was
maintained across the lower (Northern) Plains and Upper MS Valley.

Hurley


Day 2
Valid 12Z Mon Apr 28 2025 - 12Z Tue Apr 29 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PLAINS AND INTO THE UPPER MIDWEST...

...20Z Update...

In coordination with UNR/Rapid City, SD; ABR/Aberdeen, SD;
BIS/Bismarck, ND; FGF/Fargo, ND; MPX/Twin Cities, MN; ARX/LaCrosse,
WI; and DLH/Duluth, MN forecast offices, the inherited Slight Risk
was removed with this forecast update.

A mid-level circulation will open into a shortwave trough across
the northern Plains and Upper Midwest on Monday-Monday night. Ample
pre-frontal low-mid layer moisture transport and PW values
climbing to 1.5-1.75" will make for a favorable thermodynamic
profile, along with deep-layer instability as MUCAPEs peak between
2,000-3,000 J/Kg. There remains some concern during this period
with cell training, especially late Monday-Monday night across the
Upper Midwest and Central Plains with the strengthening LLJ (50-55
kt at 850 mb) and weakening downwind Corfidi Vectors. The 12Z
guidance remains highly variable with where any convection along
the cold front will set up, regardless of training potential, with
some of the guidance into northern Wisconsin and the UP of
Michigan, with more limited potential further west from southeast
ND into northern MN. This more limited potential is due to a
preponderance of the guidance suggesting the dry slot will move
over that area, greatly reducing the time of any rainfall through
the period.

For much of North Dakota southwest into southeastern Montana and
far northern Wyoming, there is a bit higher potential for flash
flooding stemming from rainfall and snowmelt from the northeast
flow against the Bighorns, but for most of that area, soils remain
very dry with an ongoing drought, and it's unlikely with limited
instability on the cold side of the low that rates will overcome
the otherwise sandy soils of the area to result in anything other
than isolated instances of flash flooding.

Hurley/Wegman


Day 3
Valid 12Z Tue Apr 29 2025 - 12Z Wed Apr 30 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OKLAHOMA AND NORTH TEXAS...

...20Z Update...

In coordination with OUN/Norman, OK forecast office, a Moderate
Risk upgrade was introduced with this update.

Another potential significant rainfall event will be taking shape
across western portions of the TX-OK Red River Valley northeast
into the Ozarks and Lower MO River Valley. This as the upper trough
west/ridge east amplify and in-turn exhibit slower W-E
progression. This will allow for a more prolonged period of
favorable upper level forcing (coupled southern and northern stream
jet streaks) along a surface front that will become quasi-
stationary across OK- southeast KS into central MO. Robust
synoptic-scale forcing, favorable thermodynamic profiles (Mixed
Layer CAPEs 1000-2000 J/Kg and TPWs 1.5-1.75") and the the enhanced
risk of repetitive convection along the slow moving/stalled
frontal boundary will likely lead to impressive 24 hr rainfall
totals within the Slight Risk area (3-5+ inches, with higher
localized totals per the higher-resolution GEM-Region and RRFS
runs). Tuesday night in particular may bring a more enhanced flash
flood threat, as the LLJ veers and becomes nearly parallel (and of
similar magnitude of) the mean 850-300 mb flow, thereby leading to
weaker forward-propagating Corfidi Vectors and allowing for a
greater risk of cell training.

Much of the Moderate Risk area picked up several inches of heavy
rainfall from a slow-moving and persistent MCS that tracked across
southwestern Oklahoma and north Texas yesterday. This has greatly
diminished FFGs and swelled local rivers, streams, and creeks.
While both today and most of tomorrow will be dry, (with severe
thunderstorm potential tomorrow), it appears unlikely that there
will be enough recovery to return soil conditions to normal by the
time Tuesday's heavy rainfall event gets going. There are therefore
heightened concerns for flash flooding. The Moderate Risk
highlights the hardest hit areas expected to pick up heavy rain
from both yesterday and expected again on Tuesday. Additional
expansions may be needed if forecast rainfall increases further
south and west in areas that were also hard-hit Wednesday, as well
as towards the Northeast from Tuesday's rainfall alone, as the
highest rainfall amounts for the day may be closer to the
Oklahoma/Kansas/Missouri/Arkansas corners region. As is common for
forecast areas of thunderstorms, shifts in the axis of heaviest
rainfall are common and therefore adjustments to the outlook areas
may be needed in the coming days.

Hurley/Wegman

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