FOUS30 KWBC 010745
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Tue Apr 1 2025

Day 1
Valid 12Z Tue Apr 01 2025 - 12Z Wed Apr 02 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL PLAINS & MIDWEST...

Consensus continues to be strong for the overall evolution of a
broad surface cyclone developing in-of the High Plains to Midwest
over the next 24-36 hrs. A strong mid- level trough has ejected
into central CA and will maneuver eastward out of the western 
CONUS, aiding in the initiation of a deep surface cyclone across 
the Colorado Front Range by later this morning, intensifying 
during its life cycle through Wednesday morning and beyond. The 
process is manifested through a well- defined axis of difluence 
downstream of a robust mean trough/closed-low reflection over the 
Rockies, kicking eastward into the High Plains of the Dakotas. 
Current deterministic output indicates sub-990mb pressures by later
this afternoon with significant low-level moisture advection 
within the lead side of the circulation when assessing the forecast
850-700mb RH field. A textbook warm conveyor belt signature will 
mature with a connection based from the Gulf driving warm, moist 
air poleward and enhancing a blossoming QPF shield by the time we 
reach this evening. The primary focus for heavy rainfall will ensue
after sunset as the initiation of a powerful LLJ across the 
Central Plains on the order of 65-75kts will create an axis of 
growing low to mid-level shear juxtaposed within a budding warm 
sector downstream of the primary circulation. As the surface 
cyclone matures, a more defined cold front will develop along the 
tail end of the low as it moves into the Plains creating a focus 
for convective development as it maneuvers towards the warm sector.

All major deterministic, including the latest 00Z CAMs output indicate
a rapid development of convection across portions of northern KS 
into southeast NE advancing east- northeast within the bounds of 
the cold front and warm sector of the cyclone. There is also some 
growing consensus of a further south and southwest extension of 
the convective field within eastern KS down into OK due to the
ejection of a weak mid-level perturbation caught up in the mean
flow that intersects the prolific LLJ bisecting the areas above.
This correlates well with the current ML output that has been
steady with a small sector of convection displaced from the main
line that will ultimately develop over KS into NE and IA.  

HREF neighborhood and EAS probs for the primary axis of convection
over northeast KS and points northeast are very much elevated with
a core of 50-70%+ for at least 2" total via neighborhood
probabilities and 40-60% for at least 1" total via a the more
conservative EAS, a testament to a solid overlapping signal for
heavy rainfall within the above zone. A wide swath of 30-50% probs
for at least 2"/3-hrs is also situated within the primary zone of
interest, alluding to modest FFG exceedance probabilities for the
3-hr time frames between 00-12z Wednesday. Further south, the
probabilities are lower, but still within the lower bounds to at
least consider the potential. Latest First Guess Fields have been
steady to include that area across OK and eastern KS and a few CAMs
are worthy of the consideration. These types of synoptic and
mesoscale setups being depicted could offer some surprises due to 
the increasingly favorable thermodynamic and kinematic evolutions 
forecast within a rapidly improving environment, so to refrain 
from missing out on potential, was sure to maintain the previous 
MRGL risk with only minor adjustments within the southern and 
northern bounds of the risk area attributing to current 
probabilities and CAMs signals.

Kleebauer


Day 2
Valid 12Z Wed Apr 02 2025 - 12Z Thu Apr 03 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI AND OHIO VALLEYS...

The D2 period will represent the initial stages of a prolonged 
heavy rain event that will reside over the Lower Mississippi and 
Ohio Valleys with a multitude of frontal waves riding along a 
persistent quasi-stationary front that will undulate at times when 
interacting with the aforementioned surface waves. The general 
longwave pattern will become increasingly favorable for a "highway"
of moisture and mid-level impulses to eject northeastward out of 
the southern Rockies/Plains and up through the Mississippi and Ohio
Valleys beginning Wednesday afternoon, onward. The consensus among
all ensembles and much of the deterministic fields are the first 
wave of the pattern to maneuver over the mid-Mississippi Valley 
around the base of a broad trough located over the Plains/Midwest 
creating ample surface-based ascent to couple with the powerful 
upper jet component situated over the Central CONUS in response to 
the evolving pattern the day prior.

At the surface, a cold front will run into a stout western ridge
extension from a surface high located over the Western Atlantic
into the Canadian Maritimes. The pattern essentially comes to a
stalemate thanks to an expected +2 to +3 standard deviation height
anomaly centered over the Western Atlantic. This ridge will counter
the deepening trough over the Central U.S. creating a funneling
affect for deep moisture advection through the Mississippi and Ohio
Valley's, along with ample mid-level impulses that will be preside
over the same areas. The gridlocked setup will create a formidable
moderate to heavy rain event across the above areas with
depictions from guidance indicating a widespread 2-4" with some
areas exceeding 5" increasingly likely within the corridor impacted
by the multitude of convective elements spawning southwest to
northeast in proxy to the quasi- stationary front.

Latest prob fields based within the NBM blend and relevant EC/GEFS
ensembles signal a high likelihood of totals >3" over the span of
18z Wed to 12z Thu with more rain continuing beyond the D2 period.
The maxima is focused within a narrow corridor from central and
northeast AR up through the mid-Mississippi Valley encompassing far
southeastern MO, western KY, and southern IN with the probability
for >2" running between 60-80% and >3" settling between 40-60%
without a full implementation of the CAMs into the blend. Rainfall
rates will be generally between 1-2"/hr at peak intensity when
assessing the current HREF/NBM outputs, however, there's a chance
for higher rates located within the southwestern flank of the
stationary boundary (Much of Arkansas up into western KY) as 
assessment of the PWAT advection regime signals potential for
localized PWATs exceeding 2" at times during organized convection.
Historically, this tends to lead to locally higher rates with
prospects of 2-3"/hr possible, especially when accounting for
intra-hour rates during maturing convection. In all, the setup is
very much conducive for substantial rainfall totals in a span of
12-hrs that will allow for high flash flood concerns within urban
zones and an increasing potential for significant flash and river
flooding outside those areas just due to the overall QPF footprint
anticipated.

This is an increasingly significant setup approaching with
potential for high impacts and life-threatening flash flooding
spanning the course of several days. Be sure to prepare if you live
in a flood zone anywhere from Arkansas, northeast through the
Ohio/Tennessee Valley's. Anyone surrounding will want to monitor
this setup closely as small changes could have heightened impacts
given the forecasted setup.

Kleebauer


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