Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
333 AM EDT Wed Apr 30 2025

Day 1
Valid 12Z Wed Apr 30 2025 - 12Z Thu May 01 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH-CENTRAL AND SOUTHEAST OKLAHOMA, NORTH TEXAS, AND WESTERN
ARKANSAS...

The overall synoptic pattern remains unchanged in terms of the
general longwave progression as an amplified mid-level trough
begins to open up overnight and kick eastward with a strong
vorticity maxima riding the base of the mean trough. Ample mid-
level ascent within a fairly moisture rich environment thanks to
coupled LLJ advection and strong 700-500mb moisture feed from the
above trough will lead to a period of prolonged convective
maintenance and initiation over the course of Wednesday into early
Thursday morning. A strong mid-level shortwave ejection is
currently migrating eastward out of NM with WV and IR satellite
already showing instances of cooling cloud tops marking the sign of
the final wave of convective initiation off the Caprock and
adjacent Permian Basin. This expanse of convective development will
usher east-northeastward within the mean flow, also anchoring to
the quasi-stationary boundary positioned back across the Permian
Basin through the Western Rolling Plains of TX. Heavy thunderstorms
prior have led to a swath of very low FFG's in their wake creating
an antecedent soil moisture condition incapable of taking on much
more rainfall before causing flash flooding, some significant in
nature given the ongoing issues near the Red River.

As the shortwave trough migrates through North TX and the Red River
Basin, cold pool convergence during nocturnal convective cycles
will lead to a conglomeration of heavy thunderstorms extending from
Central OK down through North TX, advancing eastward within the
confines of the stationary front. By 12z Wednesday, rainfall rates
between 1-2"/hr at peak intensity will encompass a pretty large
area with the western extent of the precip footprint likely
overlapping areas that were hit recently this evening. This area
will represent the western extent of the inherited MDT risk as
outlined as 00z CAMs have come into relative agreement on the
timing of the convective cluster moving through the region in the
initial stages of the forecast cycle. The primary energy will
continue to traverse eastward with the afternoon period likely to
see the shortwave axis bisecting Southeast OK between the 18-00z
time frame. This is where the next round of convective impacts will
transpire.

As of the latest ensemble bias corrected mean and HREF blended mean
output, the heaviest precip has shaded a bit further south compared
to run-to-run consistency, a tendency typically seen in these
setups due to the greater instability located further south, as
well as cold pool progression tending to move southeastward within
the theta_E gradient pattern(s). This is no different with a strong
signal for 2-4" areal averages located over Northeast TX with the
2" mean now even southeast of the DFW metro. 3-5" is the average
over the ArkLaTex bisecting the Red River Basin between OK/TX/LA,
something that has remained steady despite the overall shift
further south in the heavier convective forecast. This is the most
likely location for flash flood prospects, certifying the centroid
of the current MDT well-positioned and left untouched from previous
forecast.

With the scope of the heavier precip now aligned a bit further
south to include the DFW metro and areas along I-20, the MDT was
pulled a bit further south to encompass the 00z HREF >5" neighborhood
probs of at least 30% or higher, with the general maxima located
along I-30 towards Texarkana. This correlates well with the
forecasted position of favorable PWAT anomalies +2 or better when
assessing the most recent NAEFS and ECENS outputs. Hourly rates
between 1-2"/hr will be most common at peak intensity according to
the HREF hourly prob fields with 1"/hr running between 50-90% at
any given time between 18-06z in the forecast across Northeast TX
up into Southeast OK and Western AR. 2"/hr probs are not as
prolific in the signal, but considering the sheared profile and
deep boundary layer moisture advection pattern, would not be
surprised to see some cells percolate >2"/hr given the expected
environment.

The DFW metro is one of the areas of focus due to the risk of flash
flooding being higher with the urbanization factors at hand. There
are some CAMs hinting at significant totals within the metro
proper, but some are just missing the population center to the
north and east during the overall evolution. Considering the
environmental conditioning and the nature of the heavy precip being
within a short proxy, regardless of eventual outcome the MDT risk
was sufficient to cover for the threat. HREF EAS prob fields for at
least 2" running between 30-50%, west to east across the metroplex
is a pretty good signal for the threat and that signal only
improves as you move eastward through the I-20/30 zones.

There is a large SLGT risk that encompasses much of the Lower
Mississippi Valley and basically the rest of the northeastern
quadrant of TX up through Central OK. Scattered to bordering
widespread convection will be forecast from the Ozarks and points
southwest with some locally enhanced cores likely to spur some
flash flood potential with low to medium grade probabilities
suggest. There's some question on the exact location of these more
isolated heavy cells, however there's some indication a secondary
maxima could be within south-central TX near the eastern flank of
the terrain east of I-35, or across MO where diffluent upper flow
will be positioned well to enhance regional convective coverage
within a fairly moist environment as the anomalous PWATs funnel
poleward.

Kleebauer


Day 2
Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD
AREA FROM THE GREAT LAKES TO MID MISSISSIPPI VALLEY TO WESTERN GULF
COAST...

Surface cyclogenesis across the Southern Plains will lead to a
steady low progression to the northeast up through the Mississippi
Valley into the Great Lakes towards the end of the forecast cycle.
A cold front will trail the vacating low allowing for some frontal
convergence to occur across the above areas with the greatest
convective coverage likely to be within the warm sector of the Ohio
and Tennessee Valley into the southern Great Lakes. PWAT anomalies
between +2 and +3 deviations will be present for much of the CONUS
situated east of the Mississippi lending to a fairly rich
environment conducive for moist convective cores from the Gulf
coast up to the Great Lakes region. The reasoning for the heavier
convective threat north is the added sheared profiles within the
bounds of the warm sector creating better mesocyclone maturity and
stronger storms. All this to say....heavy rain prospects for longer
periods more likely within the zone encompassing the Tennessee and
Ohio Valleys. Current forecast totals region-wide over the
Southeast up into the Ohio Valley are pretty consistent with an
areal average between 0.5 to 1" with some locally enhanced totals
reaching upwards of 2-2.5" in the hardest hit locations. Given
rainfall rates peaking between 1-1.5"/hr when assessing the
probabilistic output of both the NBM and 00z HREF, there's a cap on
the higher end potential of any flash flood scenario leaving this
overall threat in the broad MRGL risk coverage, as inherited.

There is another area of interest down across the Rio Grande
extending from the Big Bend down to about Laredo for tomorrow
evening as a shortwave perturbation ejects east off of Coahuila
with some convection spawning over the Serranias del Burro. Some of
the CAMs are pretty significant in the overall convective
development, but much of the CAMs reach the end of their runs prior
to the activity making its way towards the Rio Grande Valley. Other
deterministic and ensemble bias corrected means tell the story in
full with some respectable QPF outputs within the above bounds
leading to potentially some localized flooding within some of the
urbanized zones between Del Rio down to Laredo. Considering the
environment to be favorable and this scenario having a long history
of potential, opted to keep the MRGL risk from previous forecast
issuance to outline the threat for a quick 1-3" of rainfall with
locally higher amounts plausible pending cold pool convergence
evolutions.

Kleebauer


Day 3
Valid 12Z Fri May 02 2025 - 12Z Sat May 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
CENTRAL TEXAS...

A cold front progression stemming from a trailing cold front and
pressing surface high over the Northern Plains will lead to
significant low-level convergence regime over Central TX by Friday
afternoon. Return flow around the western flank of a broad surface
ridge positioned over the Western Atlantic will reach back towards
the southern half of TX leading to a convergent pattern in
conjunction with slow-moving cold front moving north to south.
Models have been steadily increasing precip coverage within the
confines of the boundary once it makes headway into the Hill
Country of the Lone Star State by late-Friday morning, carrying
through the rest of the period as the front encounters higher
theta_E airmass within the western periphery of that surface ridge.
PWATs between +1 and +2 deviations will be present once the front
makes its presence across the region, a sufficient environment for
deep moist convective development along and ahead of the boundary
as it slowly migrates south. Ensemble means have upped precip
output in recent runs with a solid 1-2" areal average now
encompassing a large portion of the state from the Stockton
Plateau, east through Hill Country, the I-35 corridor, all the way
back towards the Piney Woods area of East TX. Favorable environment
with likely training echoes in proxy to the cold front will
generate locally enhanced QPF maxima, especially within the bounds
of the terrain where an added weak upslope component could yield
greatest results (3+" totals). The convergent pattern can be seen
as far east as the Southeastern CONUS, but the greatest surface
frontogenesis signal is situated back over the Southern Plains,
south of I-20.

A SLGT risk was added across the above area with a broad MRGL
encompassing the higher risk as scattered convection will still be
present up towards I-20 extending east through the Lower
Mississippi Valley up through Northern AL and Eastern TN. Will be
monitoring the setup closely as the setup will likely be most
prolific within one of the flashier areas of the country.

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