501
FOUS30 KWBC 022027
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Wed Apr 2 2025

Day 1
Valid 16Z Wed Apr 02 2025 - 12Z Thu Apr 03 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN
ARKANSAS TO CENTRAL KENTUCKY...

..A PROLONGED LIFE-THREATENING FLASH FLOOD EVENT WILL BEGIN TODAY
WITH SEVERAL DAYS OF HEAVY RAINFALL IMPACTING A LARGE PORTION OF
THE LOWER MISSISSIPPI AND OHIO VALLEYS..

...16Z Update...

Generally minor updates were made to the ongoing ERO risk areas,
but they were numerous. Most of the changes were based on changes
among the latest CAMs regarding where the heaviest precipitation is
expected through tonight. In summary though, little has changed
with the prior forecast reasoning across the Moderate Risk area.

A High Risk upgrade was precluded for this update for a few
reasons: First, generally the least coverage of expected rainfall
of 2 inches or more compared with subsequent days, second, dry
antecedent soil conditions and relatively low river levels prior to
the start of this event, and third, a good portion of the event
occurs into the Day 2/Thursday period, so the time for heavy rain
to cause flooding will be limited as the storms associated with the
front are progressive/fast-moving for the first half of today as
they`re moving into place. There is little doubt that portions of
the moderate risk area will see training storms resulting in flash
flooding, but the coverage of flash flooding are not expected to rise
to High Risk levels today and tonight.

A squall line and attendant cold front moving across Missouri and
northwestern Arkansas will stall along the southern portion of the
line (where the Moderate Risk remains and been expanded), whereas
the northern portion of the line should continue moving eastward
with the upper level energy, which should limit the training
potential. It`s important to note that some of the guidance in
southern Indiana suggests a second round of storms develops and
follows closely behind the initial line. However, this second round
of storms is also rather fast moving. Thus, think there should only
be a few hours of heavy rain potential, which suggests the Slight
would be the more appropriate category for that area.

Elsewhere, the Moderate Risk was expanded east to include metro
Nashville due to the slowing of the line and much greater potential
for training than areas further north. For similar reasons, the
Moderate risk was also expanded east to include much of south-
central Kentucky.

The CAMs guidance is in very good agreement that the line will
stall over the northwest corner of Mississippi and is unlikely to
progress much further south and east in northern Mississippi
through 12Z (but likely will after 12Z and into the Day 2 period).
Thus, the risk areas were both trimmed back about a row of counties
and intentionally left with a really tight gradient between the
Moderate risk to the northwest and nothing. This reflects the high
likelihood that the forward progress of the storms stops, resulting
in towns getting little or no rainfall close to others that get
flooding rains only a short distance away.

The warm front of the developing Plains low causing all of this
rainfall appears more likely to get hung up across western New York
tonight. This could result in multiple hours of moderate rainfall
in the Buffalo area and along the Lake Ontario shoreline. Thus, the
Slight Risk area was expanded east. For northern New York into
Vermont, temperatures should remain cold enough that a significant
portion of the precipitation falls as snow or a wintry mix, which
should prevent flooding concerns at least until after 12Z, so the
Marginal Risk area was trimmed.

The broad synoptic setup described in the previous discussion below
remains valid and has been included for reference.

Wegman

...Previous Discussion...

The setup for the D1 period is already underway with a broad upper
trough pattern across the Central and Western CONUS taking shape
and interacting with a building Western Atlantic ridge that`s
already flexing its muscle in the overall longwave pattern. A
deepening surface low over the Plains will migrate in-of the Upper
Midwest with a cold front advancement towards the Mississippi
Valley as we move into the first half of the period. The issue
becomes two-fold by later this afternoon as the frontal boundary
slows its forward momentum due to the surface reflection over the
Midwest becoming vertically stacked and losing its vigor in
progressing the pattern. Meanwhile, the ridge over the Atlantic
will continue to strengthen leading to a stout western edge of the
surface ridge pattern acting as a "wall" to prevent much more
motion of the surface front migrating out of the Plains. By the
evening, the front will likely be deemed quasi-stationary, meaning
the overall advancement of the boundary will be limited and will
only move based on subtle surface wave propagations that ride along
the boundary, or from well-defined cold pools that originate from
organized convective clusters that form within the vicinity. In
this case, both of these outcomes are likely to play a significant
role in location of the frontal positioning, as well as an
inflection point for daily organized convection across the Lower
Mississippi Valley up through the western half of the Ohio Valley
for D1.

Assessment of the 500mb vorticity field based off the past several
deterministic outputs indicates the first in a set of 3 distinct
surface waves that will originate out of the Southern Rockies and
Plains that will eject northeastward out of the base of the mean
trough and migrate right up the stationary boundary in place. This
is a textbook signal for enhanced convective development with a
sharp low to mid-level moisture advection regime along and ahead of
the surface amplification, much of which is aided by a budding LLJ
presence within the eastern flank of the surface wave. The latest
CAMs and global deterministic are in agreement on a large areal
extent of southwest to northeast oriented convection that will
likely spawn between 18-00z this afternoon and evening, mainly once
the primary mid-level perturbation enters the picture and provides
sufficient ascent to trigger convection along the surface front.
00z HREF probabilities for >2" PWATs are generally modest between
25-50% across the Lower Mississippi Valley with the primary axis
located across southern and eastern AR. >1.5" PWATs is very high,
however across the same areas with a northern expansion all the
way beyond the Ohio River near IL/IN/KY/OH by later in the evening.
This is thanks to the strengthening nocturnal LLJ pattern that will
transpire ~00z, maintaining a nearly steady state as we move
through the overnight hours Wednesday into Thursday.

Considering the variables above, a significant precip core is
anticipated extending from the ArklaTex, north into the north-
central Ohio Valley with a sharp eastern edge defined by the
greater subsidence provided by the western fringes of the low to
mid-level ridge anchored over and off the Southeast U.S. coast.
This sharp delineation is typically an artifact of strong surface
based convergence present just upstream, especially within a primed
upper pattern fighting against the ridge itself. Across AR up
through western TN and KY, a well-defined axis of heavy precip is
progged across all major NWP, a testament to the agreement on the
placement of the primary convective cell motions, but also where
there is an expectation for the future cell mergers and forward
propagation to occur leading to repeated rainfall threats during
the course of Wednesday evening to Thursday morning. HREF
neighborhood probabilities for >3" of rainfall indicate a large
corridor of 70-90+% probs located across south-central and eastern
AR, extending northeast up through western TN and the far
southwestern KY area to the south of PAH. The >5" probs are also
elevated within that same zone running 25-50% across that corridor
leading to general continuity in what has been forecast the past
several cycles. EAS probabilities are also very impressive for >2"
over much of the above area with 60-80% located over almost all of
western TN, including the Memphis metro. These types of outputs
from the prob fields are historically aligned with higher-end risk
days, especially over regions that have already primed soils or
exhibit large scale urban footprints privy to runoff potential.
This setup is shifting towards the significant category in terms of
expectation for the first day of a long-lasting period of enhanced
rainfall concerns thanks to the pattern hitting a "longwave
roadblock".

Totals for the D1 period are likely to reach 2-4" across a large
domain with localized totals of 5-8" plausible in areas that see
greater training and focused cell mergers once the storm mode
shifts from supercellular to more of a multi-cell cluster after
cold pool mergers late in the evening. The previous Moderate Risk
was relatively maintained, but did extend the risk a bit further
east due to CAMs output signaling a greater risk of outflow
dominant regimes pushing the eastern extent of the heavier precip
further into northern MS and western TN. A high-end Moderate is
most likely from the Pine Bluff, AR up through the southwestern KY,
northwest TN border south of Paducah. The Memphis, TN metro and
the eastern side of Little Rock, AR is the most likely large metro
corridors under the greatest threat for significant flash flood
prospects when assessing the latest probabilities and general QPF
output. Further north into KY and IN, a Moderate Risk still exists
for most of western KY up into southern IN as guidance indicates a
secondary maxima of QPF due to cell initiation late Wednesday into
early Thursday along the leading edge of the surface wave as it
migrates northeast. Some guidance is very aggressive with precip
outputs >2" over a short time scale, enough to cause flash flood
concerns within any urban corridors and valleys located within the
river flood plane. Considering the signals for >1" at the very
least, the Moderate Risk was drawn north to account for the
potential with a general expectation this setup will evolve into
that area as we work into the D2.

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 2
Valid 12Z Thu Apr 03 2025 - 12Z Fri Apr 04 2025

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR EXTREME SOUTHEAST
MISSOURI TO WESTERN KENTUCKY WITH MAJOR FLOODING EXPECTED...

..A PROLONGED LIFE-THREATENING FLASH FLOOD EVENT WILL BEGIN TODAY
WITH SEVERAL DAYS OF HEAVY RAINFALL IMPACTING A LARGE PORTION OF
THE LOWER MISSISSIPPI AND OHIO VALLEY`S...

20Z Update: The updated ERO does not require many changes with the
arrival of the 12Z models and CAM guidance. There was a very slight
southward adjustment of about 10-20 miles with the inherited High
Risk area to include the greater Memphis area. There is strong
model agreement for additional 3-5 inch rainfall totals on top of
extremely heavy rainfall expected Wednesday night across much of
this same area. This will result in severely reduced flash flood
guidance values that will easily be exceeded with this next round
of intense rainfall. There may be a lull in the activity during
portions of the day Thursday, but the rainfall intensity should
rapidly increase once again Thursday night as more MCSs train over
the same areas. The one model that differs from the overall model
consensus is the NAM conest which depicts considerably lighter QPF
on the order of 1-2 inches over this region, but this is considered
a lower probability solution at this time.

Elsewhere across the nation, a Marginal Risk area has been added
across portions of far eastern Colorado, northwestern Kansas, and
southwestern Nebraska as a shortwave moves through the region with
moderate to locally heavy rainfall developing ahead of a surface
low forming in the lee of the central Rockies. Although no
widespread flooding is expected, some of the storms may briefly
train and possibly result in some nuisance level flooding on a
localized basis. The previous forecast discussion is appended below
for reference. /Hamrick

------------------

The D2 period will be an ongoing atmospheric evolution with the
first half of the time frame being a continuation of the
significant rainfall from the D1 as the succession of cold pool
mergers and convective clusters propagate across the Ohio and
Tennessee Valleys before finally fading as they lose steam moving
into the wall of subsidence to the east. Scattered showers and
storms will still be ongoing within the confines of the quasi-
stationary front just due to continued low-level convergence and
smaller mid-level perturbations ripping through the "atmospheric
speedway" setup from southwest to northeast across the Southern
Plains up through the Ohio Valley. Despite the expectation for
lower rates during the early to mid-afternoon period on Thursday,
any additional rainfall will act as a priming mechanism for what
will transpire later in the evening as the second of 3 major
surface waves takes shape across the Lower Mississippi to Ohio
Valleys.

The introduction of yet another stout LLJ pattern and attendant
low-level moisture flux will yield another round of widespread
convection within proxy to the surface front that will modestly
undulate north to south within the confines of the aforementioned
areas. The positioning of the front will make all the difference in
the inevitable convective cell motions and subsequent QPF
distribution, something that will be important when assessing where
the worst flooding will occur as the grounds around much of the Low
to Mid-Mississippi Valley and western Ohio Valley will be flood
prone from the previous convective impacts. As of this time, there
is ample agreement from global deterministic, as well as the
relevant ensemble packages for an axis of heavy precipitation to
impact a zone spanning central and northeastern Arkansas up through
western Tennessee and Kentucky with the northern edge of the
heavier precip aligned with the KY/OH schemaborder near the Ohio
River basin. Additional amounts of 2-4" with locally up to 7" will
impact those above zones, overlapping several areas that will have
seen significant rainfall the previous 24 hrs. 01z NBM
probabilities for rainfall amounts >6" over the D1-2 period are now
upwards of 40-60% located from Pine Bluff, AR up through western
KY between Paducah and Bowling Green. The area with the highest
probabilities is across western TN, just north of the Memphis
metro, but very close by the major metro center. Considering the
elevated theta_e alignment and +2 to +3 deviation PWATs present,
rainfall rates within matured convective cores will likely breach
1"/hr with 2-3"/hr within reason, especially in the early evening
when instability is highest and the LLJ is at peak intensity. The
deep moist convergence regime will be well-established and cell
motions will remain generally parallel to the frontal boundary
thanks to the stagnant synoptic scale pattern in place. This will
lead to significant, life-threatening flash flooding to impact a
large area where FFG`s will likely lean critically low.

Considering all the above variables, there was no reason to remove
the High Risk. Instead, the risk area was expanded to account for
the latest probabilities and relevant run to run continuity from
the pertinent NWP. A Moderate Risk spans from southwestern Arkansas
to the northeast, just outside Cincinnati on the Kentucky side of
the Ohio River. Memphis, Little Rock, Pine Bluff, Bowling Green,
Louisville, and Evansville are all major urban corridors within the
Moderate Risk with areas like Paducah and Union City the larger
towns/metros embedded within the High Risk. The travel routes most
susceptible to flash flooding will be along the I-69 and I-55
interstates within western KY, western TN, and northeastern AR.

Kleebauer


Day 3
Valid 12Z Fri Apr 04 2025 - 12Z Sat Apr 05 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR A LARGE
PORTION OF THE LOWER AND MID-MISSISSIPPI VALLEYS...

20Z Update: The existing Moderate Risk area did not require many
adjustments for this time period. Given the axis of heaviest
rainfall is expected to be displaced about 100 miles northwest of
the Day 2 event, this currently avoids the need for any High Risk
areas at this time. However, if models trend farther southeast with
the main QPF axis in future runs, it is possible a High Risk could
eventually be needed for eastern portions of the current Moderate
Risk area. Rainfall totals on the order of 3-6 inches, locally
higher, are expected throughout most of the Moderate Risk area. The
outer Marginal Risk area was extended to include northeast Ohio
and also to near the Rio Grande in southwestern Texas. The previous
forecast discussion is appended below for reference. /Hamrick
---------------------


The final surface wave within the atmospheric parade across the
Southern Plains to Mississippi Valley will show itself by late-
Friday morning through the remainder of the period. This wave will
likely be the strongest of the 3 allowing a much more amplified
surface evolution that will shift the focus of heavier precip
further northwest while also providing some vigor in the overall
QPF distribution. The trend the past 24 hrs is for a zone of
significant (3-6+") totals to be recognized across portions of the
Southern Plains up through the Ozarks and Mid-Mississippi Valley
of Missouri, spanning as far north as south-central Illinois. When
assessing the theta_e environment, there is a noticeable push north
of more modestly unstable air with the most pertinent area of
available instability situated across southern MO down through AR,
southeastern OK, and even northeastern TX. A sharp uptick in
regional PWATs with deviations between +2.5-3.5 anomaly-wise are
forecast within that zone correlating with a substantial rise in
area convection blossoming late Friday afternoon, onward. This
setup is fortunately further west compared to the previous periods,
so the overlap of the heaviest rainfall is not anticipated to be
as problematic as the previous period.

Having said that, the pattern will yield high precip totals due to
widespread convective activity, falling over some areas that will
have seen appreciable rain, and over other areas that are more
prone to flooding due to complex terrain situated over the Ozarks.
The pattern is such that several areas from northeast of DFW up
through IL will see rainfall amounts of 3-6" with locally higher
beginning Friday afternoon with the heaviest rainfall likely to
occur by the evening with the implementation of yet another LLJ
structure thanks to the surface wave mentioned previously. Further
to the west, another closed upper reflection across the Southwest
U.S. will only entice greater upper forcing within a broad axis of
difluence situated over the Plains. This setup is more privy to
convection forming as far west as the Western Rolling Plains of TX
to points east and northeast. Even there, modest 1-2" with locally
higher totals are forecast in-of those zones. As a result, there
was a general expansion of the Slight Risk further southwest into
Central TX, an expansion of the Moderate Risk to include the
eastern Red River Basin up through the southeast half of Missouri
into southern Illinois, and a high-end Moderate impacting
southeastern OK up through western AR and much of southern and
southeast MO. The threat of widespread flash flooding is increasing
for areas within the Moderate risk and aforementioned higher-end
Moderate designation. Be sure to stay up to date for the latest
changes as we move through the remainder of the week as this
pattern remains very active and prone to significant life-
threatening flash flood concerns.

Kleebauer


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