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FOUS30 KWBC 031909
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
309 PM EDT Thu Apr 3 2025

Day 1
Valid 12Z Thu Apr 03 2025 - 12Z Fri Apr 04 2025

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER TO MID-MISSISSIPPI VALLEYS WITH MAJOR FLOODING EXPECTED...

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

...16Z Outlook Update...
The overall forecast philosophy is on track and only minor spatial
adjustments have been made to categorical areas. The High Risk
areas remains in place across portions of Arkansas, Tennessee, and
Kentucky, where a nearly stationary frontal boundary should linger
throughout the day and foster multile rounds of deep convection.
12Z model QPFs suggest another 2-4 inches of rainfall generally
from Little Rock northeastward through south-central Kentucky, with
much of that rain falling on sensitive ground conditions (low to
near-zero FFGs) from prior rainfall and flood impacts last night
and this morning. Significant impacts remain possible, with the
only lingering uncertainty for convective coverage tied to
gradually rising geopotential heights from this afternoon onward.
Little Rock metro was added to the High Risk area after
consultation with WFO LZK, with ongoing high-water impacts
continuing as of this discussion issuance.

The Slight Risk area was expanded eastward into more of West
Virginia, southeastern Kentucky, and Middle Tennessee for this
outlook. Portions of West Virginia have experienced impacts from
heavy rainfall already this morning, and an extensive fetch of
moderate to heavy rainfall is expected to persist much of the day
given renewed convective development across the Mid-South per
latest radar mosaic imagery.

Lastly, Marginal risk areas were maintained across 1) west Texas
for overnight redevelopment of convection in areas that experienced
heavy rainfall this morning and 2) expanded into the
Philadelphia/New York City and adjacent areas for banded/training
convective potential in the 08-12Z timeframe. A quick 0.5-1.25 inch
of rainfall is possible across urban areas of the I-95 corridor,
which could pose a few runoff issues.

See the previous discussion below for more information.

Cook

...Previous Discussion...

Significant rainfall over the past 12-24 hrs will only add to the
favor of widespread flash flooding later this afternoon and evening
as the next round of heavy rainfall occurs as the next surface wave
rides up the quasi-stationary boundary in place across the Lower to
Mid-Mississippi and Ohio Valleys. Widespread 2-4" of rain with
maxima between 5-6" has fallen this evening across a zone spanning
from southern Arkansas up through western Tennessee and Kentucky
leading to several flash flood warnings as the first surface wave
lifts north. Some overturning of the atmosphere is anticipated by
morning with areal theta_e`s decreasing across areas north of the
Ohio River and a tightening instability gradient positioned along
and south of the quasi-stationary front nestled from southwest to
northeast over the Mississippi Valley. Despite some limiting
factors early on, small mid-level perturbations embedded within the
mean flow will still cause some scattered light to moderate
convective development through the morning and afternoon leading to
a continuation of priming from the prior evening.

The nocturnal period will once again become the main period of
interest as the nocturnal LLJ kicks back in across the Deep South
and banks into the stationary front bisecting the Mississippi and
southern Ohio Valley. The same areas that saw the significant rain
and flash flooding this evening will be hit once again with another
wave of convection that will initiate across the ArklaTex and
southern Arkansas and eventually move northeast as the flow aligns
parallel to the stationary front. Additional precip totals of 2-4"
with local to 6" will once again be in the cards for much of
Arkansas, western Tennessee, and southwestern Kentucky which will
ultimately put 48-hr totals between 4-8" with locally up to 10"
during the time frame. Local FFG`s are already very low after
today`s precip with much of the corridor spanning eastern AR up
through north-central KY exhibiting 1 and 3-hr FFG intervals
running below 1/1.5" respectively. 00z HREF is very much indicating
that rainfall rates during the peak of diurnal convection will
reside within that 1-2"/hr marker with FFG exceedance probabilities
above 50% for the 1/3/6-hr intervals in place. Neighborhood
probabilities for >3" total are between 40-60% across eastern AR
with as high as 60-80% over northwest TN and southwestern KY. >5"
neighborhood probs are running between 25-40% within the same
corridor, backed up by EAS probs >2" sufficiently above 50% along
the Mississippi north of Memphis to points northeast.

Despite the overall output being slightly lower than what occurred
this evening, the prescience has been set for significant flood
potential given the already saturated soils, expected rates, and
regional rivers running high from the previous period of rainfall.
The previous High Risk inherited was generally maintained with
small adjustments accounting for recent rainfall trends, updated
FFG`s, and encompassing elevated probabilities from both the hi-res
and national ensemble blends. A Moderate Risk still in effect for
a large area surrounding the High Risk in place across the Lower to
Mid-Mississippi Valley with a span that covers areas of southwest
Arkansas up to as far northeast as Cincinnati.

This is quickly becoming a high-end flash flood scenario with
another day or two of rainfall expected on top of what has occurred
and what will occur. If you lie within a flood plane or any area
that is prone to flash flooding across Arkansas, western Tennessee,
and western Kentucky, you will want to pay close attention and
have a plan to seek higher ground, if possible.

Kleebauer


Day 2
Valid 12Z Fri Apr 04 2025 - 12Z Sat Apr 05 2025

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OZARKS AND MID-SOUTH...

...LIFE-THREATENING FLASH FLOODING EXPECTED FRIDAY AFTERNOON
THROUGH EARLY SATURDAY...

...21Z Outlook Update...
Both global models and CAMs have come into agreement with
development of at least one north-northeast to south-southwest
oriented band of convection located generally from the ArkLaTex
into southeastern Missouri. Convection should be oriented generally
parallel to fast, meridionally oriented flow aloft, with abundant
moisture, low- level shear, and instability profiles all favoring
abundant rainfall (potentially exceeding 10 inches on a localized
basis). These totals will fall on areas that will have experienced
2-6 inches of rainfall leading up to the forecast period, with wet
soils/terrain variations all suggestive of a potentially widespread
and significant flash flood event.

Expansion of Moderate Risk (and attendant probabilities) was
considered across more of Illinois and central Indiana, but pre-
existing dry conditions and uncertainty about duration of
precipitation were mitigating factors for a higher flash flood
threat. This region will be evaluated for a possible upgrade in
later outlooks.

Outside of these changes, the overall outlook was on track with an
eventual broad axis of heavy precipitation expected from
north/central Texas northeastward to Ohio/Pennsylvania.

See the previous forecast discussion below for more details.

Cook

...Previous Discussion...

The previous synoptic scale forecast was relatively unchanged
within the run to run assessment over the course of Friday into
Saturday morning. 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 48 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 +3 to +4 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 east 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. The heaviest
precipitation axis based off the latest NBM probabilities still has
a significant footprint of at least 3" located near the Dallas
metro and points just east (40-60%) to as high as 60-80% in a large
area spanning the Red River of northeast TX up through much of
western and northern AR, through the Ozarks of MO, finally reaching
the Mississippi River south of St.Louis. Ensemble mean QPF output
signals a massive area of at least 2" with urban centers like
Dallas, Little Rock, St. Louis, and Springfield, MO all within a
core of the heavy precip footprint.

Considering the probabilistic output of the NBM, the consistency
within guidance for a significant corridor of very heavy rainfall,
and a well-defined axis of training precip potential, a broad
Moderate Risk was maintained and even expanded to account for the
expanse of likely flash flooding. A high-end Moderate is a likely
tag for places within southeast OK up through western and northern
AR through the Ozarks of Missouri as these areas will have the
greatest threat of enhanced rates over top of either complex
terrain or primed soils. An upgrade is plausible in future updates
pending additional convective probability details.

Kleebauer


Day 3
Valid 12Z Sat Apr 05 2025 - 12Z Sun Apr 06 2025

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
OZARKS AND LOWER TO MID-MISSISSIPPI VALLEYS WITH MAJOR FLOODING
EXPECTED...

..LIFE-THREATENING FLASH FLOODING IS EXPECTED ON SATURDAY AFTERNOON
THROUGH SUNDAY MORNING..

...21Z Outlook Update...
Substantial changes were made to the ongoing outlook with the
expectation that deep, slow-moving and intense convection would
still be ongoing across Arkansas at 12Z Saturday. The High Risk
expansion in Arkansas is essentially a continuation of the
significant flash flood threat from the D2/Fri forecast period.
South-soutwesterly flow aloft at the start of the period is
expected to continue fostering a very slow eastward progression of
convection, while 1.75+ PW values,, 1500+ J/kg MLCAPE, and
convective training/mergers should continue to foster high to
locally extreme rain rates and FFG exceedence on a widespread
basis. At least 3-5 inch rainfall totals are expected, with locally
higher amounts possible.

Eventually, veering flow aloft will result in a faster forward
speed of any convective complex(es) toward the Ohio and Tennessee
River Valleys during the latter half of the forecast period. As
this occurs, low FFGs are expected to remain from abundant
antecedent rainfall, and it is very likely that ground conditions
will not have completely recovered from abundant antecedent
rainfall. In this scenario, widespread significant impacts are
expected with both ongoing and newer instances of flash flooding.

Additional details are available in the previous discussion below.

Cook

...Previous Discussion...

The combination of intense right-entrance region jet dynamics,
highly anomalous atmospheric moisture, and very saturated
antecedent soil moisture will lead to another high impact flash
flood forecast for Saturday afternoon through Sunday morning across
the Lower to Mid-Mississippi Valley. This threat will extend
further east as a cold front helps finally sweep the stagnant
pattern out of the region, but not before another round of heavy
rainfall inundates areas that will be sensitive to any appreciable
rainfall.

A stout 190-200kt upper jet located over the Great Lakes will lend
to a pronounced upper ascent pattern situated over the Mississippi
Valley during the period. At the surface, our quasi-stationary
front that will linger through the course of several days will
still be confined to similar areas that have been impacted by its
presence in the days prior. Upper low located over the Southwest
U.S. will begin to open up and eject eastward, becoming sheared on
the northern edge due to an amplifying trough digging out of
Canada. To the east, our Western Atlantic ridge that has provided a
stalemate to any frontal progression moving east of the Mississippi
will provide one last period of favorable low to mid-level
convergence as the pattern evolves with the large trough to the
west countering the ridge across the Southeast. In the middle of
the atmospheric squeeze-play, elevated moisture presence and
favorable instability will remain present to be activated with the
addition of diurnal destabilization and increasingly favorable
ascent to match. The combination will allow for a rapid development
of widespread convection across the Missouri Valley with slow
advancement eastward towards the Low to Mid-Mississippi Valley
convergence point.

This setup has trended prolific in the QPF realm of the forecast
with very little deviation in all major deterministic at this
juncture. An expanse of 3-6" with locally up to 8" of rainfall is
forecast across a large portion of Arkansas extending east-
northeast through far southeastern Missouri, all of western and
central Tennessee and Kentucky, to as far north as the
Ohio/Kentucky border along the Ohio River. Northwest Mississippi
will also lie within the axis of heaviest precipitation leading to
quite a large area of not only heavy precip, but significant
overlap of areas that will have super saturated grounds that will
struggle to maintain the ability to absorb more rainfall. This
setup will yield a very dangerous scenario where widespread flash
flooding with considerable and potentially catastrophic impacts
would be favored. The agreement among all major NWP for substantial
rainfall totals and projected hourly rates >1-2"/hr is a testament
to the overall gravity of what is to come.

In coordination with the local WFO`s across Arkansas, Tennessee,
and Kentucky...a High Risk area for excessive rainfall has been
added over central and eastern AR, all of western TN and KY, far
southeastern MO, and the southern edge of IN. Additional storm
totals will bring multi-day amounts between 8-12" with some areas
potentially reaching as high as 14" over the span of 5 days. A
broad Moderate Risk spans the outer perimeter of the High Risk
leading to several more areas prone to significant flash flood
concerns due to the setup. This is becoming an increasingly
life-threatening situation for areas of the Lower and Mid-
Mississippi Valley region.

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