Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Wed Apr 30 2025

Day 1
Valid 16Z Wed Apr 30 2025 - 12Z Thu May 01 2025

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

The threat of significant flash flooding continues today, with the
focus increasingly shifting to portions of north Texas based on
observational trends and the latest 12Z hi-res model guidance.

A bowing convective line continues to advance eastward through
S OK, with a trailing line of convection extending into N TX on
the southern periphery of strong cold pool and mesohigh situated
over SC OK. Over the past couple hours, the convective line in N
TX has become increasingly oriented NE-SW, or even ENE-WSW; in
other words, more parallel to the mean flow vector. This has
increased the potential for training and backbuilding, especially
into the afternoon hours as the LLJ begins to weaken and low-level
winds start to veer to the southwest. For a more detailed mesoscale
analysis, please refer to MPD #202 issued at 14Z for this region.

The Moderate Risk area was maintained in SE OK and W AR, even
though confidence in flash flooding has decreased somewhat due to
the fairly steady forward progress of the bowing line, and how thin
the convective line has become. However, it was maintained out of
deference to continuity and the potential to reinvigorate the
convection in the next few hours with some clearing occurring ahead
of the line, which could increase rain rates again.

Greater concern now exists in N TX. The forward progression of the
line and associated cold pool in S OK and overall convective
evolution has led to far more west-east orientation to the
convective line in N TX than models had originally been
advertising. This certainly increases flash flood potential and
over the next few hours this places the Dallas-Ft. Worth metro area
squarely in the greatest risk. Hourly rainfall could exceed 2
inches per hour, with unobstructed inflow from a very moist and
unstable air mass (MLCAPE 1500-2000 j/kg with minimal CINH; PWs
1.6 to 1.8 inches per RAP analysis.) In an urban environment, this
could lead to significant and life-threatening flash flooding,
particularly if the rain rates are sustained for a couple hours,
which is a distinct possibility in this case.

As the afternoon and evening progress, the hi-res models do
indicate the potential for the southwestern periphery of the
convective line to continue to slow or stall. While they don't
necessarily agree on the specifics, many HREF members, as well as
the RRFS, indicate backbuilding and training convection could
become more likely in the corridor from Waco to Longview. This
seems to be well supported by observational trends, and in fact
many hi-res members are: (1) already not extending the convective
line far enough southwest based on current radar trends, and (2)
have the convective line oriented far too much in a N-S fashion.
Therefore, they may be underestimating rainfall potential a bit,
and also underselling the potential for a significant rainfall and
flash flood event further to the southwest. Probabilities and
associated risk categories were increased a bit down the I-35
corridor between DFW and AUS, and this will be monitored for
further updates based on observational trends. Rain rates in much
of C/N TX could reach or exceed 2 inches per hour given the
available instability and moisture.

The other biggest change for this outlook was to extend the Slight
Risk a bit further northeast through MO and into W IL based on more
elevated HREF probabilities of exceeding 1hr and 3hr flash flood
guidance and a fairly consistent signal in the guidance for a
corridor of enhanced rainfall associated with an MCV and
corresponding surface low.

Lamers


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