Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
819 PM EDT Thu Apr 3 2025
Day 1
Valid 01Z Fri Apr 04 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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Apr 4 2025
Day 1
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...
Both global models and CAMs have come into better agreement over
the past 24 hours and have largely maintained that agreement in
the 04/00Z model production cycle. The development of at least one
north- northeast to south- southwest oriented band of convection
located generally from the ArkLaTex into southeastern Missouri was
still depicted...with convection being 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.
The overall outlook was on track with an eventual broad axis of
heavy precipitation expected from north/central Texas northeastward
to Ohio/Pennsylvania.
The previous synoptic scale forecast was relatively unchanged.
What should be the final surface wave had already started to
enhance convection across over southwestern Texas in the predawn
hours...and it should continue to make its way across the Southern
Plains to Mississippi Valley by later this 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.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 neighborhood probabilities from the 04/00Z
HREF...maintained the Moderate Risk area with only minor
adjustments based on the model and WPC QPF. Also made a slight
eastward expansion of the Marginal and Slight Risk areas in the
Ohio Valley and the Mid-South where Flash Flood Guidance was
largely reduced to less than half an inch per 3 hours.
Bann
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Apr 4 2025
Day 2
Valid 12Z Sat Apr 05 2025 - 12Z Sun Apr 06 2025
..LIFE-THREATENING FLASH FLOODING IS EXPECTED ON SATURDAY AFTERNOON
THROUGH SUNDAY MORNING..
Deep, slow-moving and intense convection is expected to be on-going
across parts of the Mid-South as the Day 2 period begins at 12Z
Saturday. Many of the adjustments made previously were still
supported by the 04/00Z model runs...with south to southwesterly
flow aloft at the start of the period expected to continue
fostering a very slow eastward progression of convection, while
1.75+ PW values,, 1500+ J/kg MLCAPE, and convective
training/mergers continue to foster high to locally extreme rain
rates and FFG exceedance on a widespread basis. Additional 3-5
inch rainfall totals...with locally higher amounts...are expected
across parts of the region which will be increasingly water-
logged.
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.
The synoptic forcing for enhanced deep layer lift will be strong
given the magnitude of an intense right-entrance region jet
dynamics in an area of highly anomalous atmospheric moisture and
very saturated soils. This spells another high impact flash flood
forecast for Saturday afternoon through Sunday morning across the
Lower to Mid- Mississippi Valley. This threat will extend farther
east as a cold front begins to 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.
All major deterministic models depict an expansive footprint of 3
to 6 inch rainfall...with locally up to 8" of rainfall 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.
As a result...the previously issued High Risk remained largely in
place with only modest adjustments based on the latest model and
WPC QPF with a broad Moderate Risk area that spans the outer
perimeter of the High Risk leading to several more areas prone to
significant flash flood concerns due to the setup. This remains a life-
threatening situation for areas of the Lower and Mid- Mississippi
Valley region.
Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Apr 4 2025
Day 3
Valid 12Z Sun Apr 06 2025 - 12Z Mon Apr 07 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST US ON SUNDAY|...
There should finally be enough of a shift in the upper flow pattern
such that moisture gets drawn northward across portions of Alabama
an Georgia by southerly low level flow ahead of an approaching
cold front and an upper level trough. Given the timing. There looks
to be enough instability and a shear profile that thunderstorms
should be able to produce locally heavy rainfall rates and maximum
rainfall amounts from 2 to 3+ inches over from southeast Alabama
into central/northern Georgia. As a result...rainfall rates may be
high enough to challenge current FFG in the area on the order of
2.5 inches per hour (although the 3+ inches per 3 hour FFG may be
be less likely to be met/exceeded).
Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Fri Apr 4 2025
The heavy rainfall potential decreases quite a bit to begin next
week in comparison to what is expected this weekend. However, there
will still be an axis of deep moisture over the coastal plain of
the Southeast states ahead of the cold front, with areal 1-2 inch
totals possible from southern Georgia to the Hampton Roads area of
southeast Virginia, mainly between 12Z Monday and 00Z Tuesday,
where a Marginal Risk of excessive rainfall is in effect for the
new Day 4 period. With the exception of the central and southern
Florida Peninsula, the threat of heavy rainfall should be over by
Tuesday across the remainder of the country with no risk areas
needed. If future models trend higher with QPF over South Florida
during this time, it is possible a Marginal Risk area may
eventually be needed there.
Elsewhere across the country, rain and mountain snow makes a return
to western Washington and Oregon courtesy of onshore flow and a
couple of shortwave trough passages, but this does not look like a
major event at this time. Lighter precipitation crosses the
Northern Plains and Midwest midweek, and the next good chance of
rainfall arrives across the Eastern U.S. by Friday as a surface low
approaches the region.
In terms of temperatures, much cooler conditions are coming for the
Eastern U.S. after the strong cold front exits the East Coast
Sunday night/Monday morning. Afternoon highs are likely to be on
the order of 15 to perhaps 25 degrees below early April averages on
Tuesday from the Ohio Valley to the eastern Great Lakes,
compliments of the upper low overhead. Readings return closer to
average by late in the week for the East Coast states. Meanwhile, a
warm pattern is expected for the Western U.S. through the week with
both daytime highs and overnight lows about 10-20 degrees above
average, with the greatest anomalies over the Great Basin.
Hamrick
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Fri Apr 4 2025
The heavy rainfall potential decreases quite a bit to begin next
week in comparison to what is expected this weekend. However, there
will still be an axis of deep moisture over the coastal plain of
the Southeast states ahead of the cold front, with areal 1-2 inch
totals possible from southern Georgia to the Hampton Roads area of
southeast Virginia, mainly between 12Z Monday and 00Z Tuesday,
where a Marginal Risk of excessive rainfall is in effect for the
new Day 4 period. With the exception of the central and southern
Florida Peninsula, the threat of heavy rainfall should be over by
Tuesday across the remainder of the country with no risk areas
needed. If future models trend higher with QPF over South Florida
during this time, it is possible a Marginal Risk area may
eventually be needed there.
Elsewhere across the country, rain and mountain snow makes a return
to western Washington and Oregon courtesy of onshore flow and a
couple of shortwave trough passages, but this does not look like a
major event at this time. Lighter precipitation crosses the
Northern Plains and Midwest midweek, and the next good chance of
rainfall arrives across the Eastern U.S. by Friday as a surface low
approaches the region.
In terms of temperatures, much cooler conditions are coming for the
Eastern U.S. after the strong cold front exits the East Coast
Sunday night/Monday morning. Afternoon highs are likely to be on
the order of 15 to perhaps 25 degrees below early April averages on
Tuesday from the Ohio Valley to the eastern Great Lakes,
compliments of the upper low overhead. Readings return closer to
average by late in the week for the East Coast states. Meanwhile, a
warm pattern is expected for the Western U.S. through the week with
both daytime highs and overnight lows about 10-20 degrees above
average, with the greatest anomalies over the Great Basin.
Hamrick
