FOUS30 KWBC 270830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Sun Apr 27 2025

Day 1
Valid 12Z Sun Apr 27 2025 - 12Z Mon Apr 28 2025

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

...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 SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN PLAINS INTO THE UPPER MIDWEST...

...Summary...
Based on the favorable deep-layer synoptic forcing and
thermodynamic profile and the uptick in areal-average model QPFs
(especially the upper-bound totals), have hoisted a fairly large
Slight Risk area across parts of the Northern Plains and Upper 
Midwest. Also expanded the Marginal Risk area across SD, southwest
MN, and Iowa from yesterday's Day 2 ERO.

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 (even ~1,000 J/Kg wrapping westward into the
TROWAL zone). The biggest concern during this period will be with
cell training, especially late Monday-Monday night across the Upper
Midwest with the strengthening LLJ (50-55 kt at 850 mb) and 
weakening downwind Corfidi Vectors. The 00Z guidance has become a
bit more clustered with the heaviest QPF within the Slight Risk
area, with pockets of 3-5+ inches per the higher-resolution models. 

Hurley


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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS INTO THE OZARKS AND LOWER MISSOURI VALLEY...

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.

Hurley
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