FOUS30 KWBC 021533
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1133 AM 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
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