Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Tue Apr 1 2025

Day 1
Valid 16Z Tue Apr 01 2025 - 12Z Wed Apr 02 2025

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

16Z Update: Upon examination of the 12Z CAM guidance suite and in
consultation with the affected WFOs, a Slight Risk is now in effect
across extreme northeast Kansas, extreme southeast Nebraska, and
portions of northwest Missouri. A nocturnal MCS is likely to
develop to the northeast of the main surface low across central
Kansas, with the CAM guidance in agreement on indicating a band of
heavy convection with short term training potential, mainly during
the 3Z to 12Z time period. Three-hourly HREF neighborhood
probabilities of flash flood guidance exceedance are on the order
of 15-30 percent across portions of this region, with rainfall
totals up to 3 inches possible by 12Z and 1-2 inch per hour rates
possible. The MCS should weaken and become more progressive after
12Z Wednesday. Otherwise, the forecast remains on track and the
previous discussion is appended below for reference. /Hamrick
--------------------

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 2 2025 - 12Z Thu Apr 3 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN
ARKANSAS TO SOUTHERN INDIANA...

19Z Update: The overall forecast remains in good shape compared to
the overnight issuance with only some minor adjustments needed. The
12Z CAM guidance generally has the heaviest QPF signal on the
southern edge of the MCS events that will develop along the stalled
out frontal boundary, and combining this fact along with historical
trends for max QPF to occur slightly south of the multi-model
consensus in many cases, the updated Marginal Risk area was
adjusted to the southeast by about a row of counties across western
Tennessee, western Kentucky, and eastern Arkansas. The HREF flash
flood guidance exceedance probabilities are generally highest
during the 00Z to 9Z time period Thursday, when multiple rounds of
training convection are expected to produce widespread 2 to 4 inch
rainfall totals, with isolated 4 to 6 inch totals possible. It
appears more likely the 3-hourly guidance values will be exceeded
compared to the 1-hourly guidance, so this will likely be more of
a prolonged duration event. This will also saturate the soils and
elevate stream and river flows ahead of the additional heavy
rainfall expected during the Day 3 period 12Z Thursday to 12Z
Friday, with a major flooding event looking more likely. The
previous forecast discussion from overnight is appended below for
reference. /Hamrick
-------------------


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

Day 3
Valid 12Z Thu Apr 03 2025 - 12Z Fri Apr 04 2025

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

The D3 period is a general extension of the D2 time frame with the
synoptic scale pattern yielding very little difference in what is
driving the setup. The biggest change from day to day with the
positioning of the quasi-stationary front will be the northern
extent becoming a bit more elongated from west-southwest to east-
northeast as we move through KY leading to a better push of
moisture and surface based ascent focused across more of the
western 1/3rd of KY up through the Ohio River basin. This will
encompass areas like Louisville to Cincinnati that might be on the
edge of the heavier QPF footprint on D2, but will be within the
northern periphery of the heavier precip come D3. The key to the
extent of the significant flood prospects will be where the D2 and
D3 overlap in the overall QPF distribution. There is a considerable
probabilistic output within the 01z NBM for D3 totals to exceed 2"
with a wide swath of 60-80% probs for the threshold situated from
southwest AR all the way up through southern OH, an area that will
experience significant rainfall the period prior. The 72-hr
probability for >6" of total rainfall is between 30-60% across
northeast AR up through northwest TN and western KY with the max
prob field positioned between Paducah, KY down to Jonesboro, AR.
This corridor is located within a notorious flood plane thanks to a
litany of streams and river channels with the Mississippi River the
main stem that bisects the region.

The forecasted surface wave in question is well-defined when
assessing the global deterministic 500mb vorticity pattern. A
developing 250mb jet coupling is also being orchestrated by some
of the global deterministic with, at minimum a substantial right-
entrance region jet dynamic well- positioned over much of the
Mississippi Valley. This setup is textbook for a continuation of
the "meteorological highway" of moisture and mid-level vorticity
advection that historically has caused significant flash flood
issues over the Lower Mississippi and Ohio Valley's. This is only
day 2 of the forecasted 4 day event, so the priming of soils will
have been underway and only getting progressively worse as time
moves on.

The previous MDT risk inherited from the D4 was relatively
maintained, however there was some expansion of the MDT to the
northwest to account for a small trend within the forecasted
alignment of the quasi-stationary front and amplitude of expected
surface wave ejecting out of the southern plains. This period is
considered to be a higher-end Moderate, meaning the prospects for a
targeted high risk are within reason pending the previous period's
QPF outcome and the convective expectation as we get closer to the
D3 time frame. The area of vested interest for a potential upgrade
includes that corridor of northeast Arkansas up through western
Tennessee, far southeast Missouri, western Kentucky, and slivers of
both southern Illinois and Indiana.

This setup is shaping up to be a more extreme flooding scenario
considering the multi-day prospects and forecasted 72 to 96 hr
rainfall totals reaching 10-15+ inches in the hardest hit
locations. 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


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