Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
340 PM EDT Mon May 25 2026
Day 1
Valid 16Z Mon May 25 2026 - 12Z Tue May 26 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL
GULF COAST TOWARD THE SOUTHERN APPALACHIANS...

16Z Update...

The meteorological rationale provided from night shift remains in
good shape. Within the large Slight Risk area in the Southeast,
the biggest concern is along the central Gulf Coast and farther
inland into south-central AL. Latest 12Z HREF showed moderate
chance probabilities (40-60%) for rainfall totals >5" from the NOLA
on north and east into southern MS. Southern AL and the western-
most portion of the FL Panhandle are also ideally placed within the
core of the anomalous >500 kg/m/s IVT that most CAMs show keeping
numerous thunderstorms producing 2-3"/hr rainfall rates in the
forecast. NASA SPoRT-LIS 0-10cm soil moisture percentiles are all
over 85%, indicating topsoil in the region will struggle to keep up
with these Excessive Rainfall rates. Scattered flash flooding is
of greatest concern along the central Gulf Coast this afternoon and
into this evening, with locally significant flash flooding
possible should pockets of >5" rainfall totals occur.

In west TX, 12Z HREF and REFS probabilities show that eastern NM
and central TX are most favored for localized flash flooding (as
eluded to in the previous discussion), but there are some CAMs
members sporting low chances (10-30%) for >2" of rainfall in West
TX between the two inherited Marginals. Given the messy convective
setup between the approaching trough to the west and the
stationary cut-off disturbance over central TX, opted to merge the
two Marginals together. In addition, 12Z HREF probabilities showed
low-to-moderate chances (30-50%) for additional rainfall totals
along I-35 south of Dallas and low chances (10-20%) for >3" close
to the Austin metro area. The atmosphere may be worked over enough
to keep most thunderstorm activity isolated, but given the
urbanized corridor along I-35 potentially at risk, expanded the
Marginal into these areas for this afternoon and evening.

Mullinax

---Previous Discussion---

...Gulf Coast to Southern Appalachians...

Stagnant upper level pattern across much of the CONUS is allowing
for a continued confluent axis situated from the Central Gulf Coast
up through the Mid Atlantic states with ample moisture comprised of
PWAT anomalies running between +1 to +2.5 deviations above normal
within the aforementioned zone. Prevailing southerly flow in-of the
Central Gulf Coast will allow for the advection of yet another
mid-level pulse out of the Gulf by the beginning of the period with
sights on southeast LA through the Southeast U.S. Increasing instances
of heavy rainfall from convective development spurred by the
disturbance and deep moisture presence will invoke yet another
round of flash flood potential, mainly within those urbanized zones
littered across southeast LA up through southern MS, much of AL,
and eventually into GA by the afternoon time frame. Instability
parameters are well-within the mode to drive heavy rain prospects
with many of the forecast point soundings indicating appreciable
tall-skinny CAPE outputs historically prevalent for heavy rainfall
potential and enhanced rates.

00z HREF neighborhood probs for >3" are running between 40-80% in
the corridor expanding from New Orleans to the I-20 corridor in AL
with the bullseye focused over southwestern AL towards the MS
border. This zone has been consistently signaled as the primary
focus of heavier precipitation in-part to multiple convective
episodes in the period that could very well put down 3-5+" of
rainfall prior to the end of the D1. Despite higher FFG's locally,
rainfall rates between 2-3"/hr with higher intra-hour rates are
likely given the PWAT's between 1.8-2.1" over the course of the D1,
a good proxy for those higher end totals locally, especially in
any repeatedly impacted areas. This environment is respectable for
the time of year and regional instability parameters are more than
sufficient for even scattered heavy cell cores focused away from
the shortwave migration anticipated. All CAMs have some instances
of heavy rain across the Southeast U.S. extending to as far north
as the Tennessee Valley and as far east as southeast GA into
portions of the SC Low Country. A SLGT risk remains forecast for
the area of southeast LA up through the Southern Appalachians and
neighboring Piedmont in SC and all zones in-between. MRGL risk
expands away on either side encompassing a broad area outside the
SLGT.

...Mid Atlantic...

Remnant quasi-stationary front across the Central to Southern Mid
Atlantic will slowly lift north through the day Monday with
increasing moisture profiles being advected from southerly low to
mid-level flow around the northern periphery of a ridge in the
Western Atlantic. PWAT anomalies will shove closer to +2 to +3
deviations across a good portion of VA, especially areas south of
I-66 down to the VA Tidewater where PWATs are forecast to jump
above 2" as expressed via multiple global ensemble outputs. This
would set up a pretty deep warm cloud layer sufficient for higher
rainfall rates for any convective pulses that arise during the
forecast. The proximity of the front will be a focal point for
enhanced low-level convergence which can be seen very well within
the theta_E and omega cross-sections off both the 00z NAM Nest and
HRRR runs this evening. Pretty much all guidance has a narrow, yet
prolific corridor of heavy rain of convective origin aligned near
and just south of the front by later this afternoon and evening
with a steady stream of rainfall likely to cause some localized
areas of 2-4+" in the time frame of impact. Pending the model of
preference, some hit the corridor from Richmond to Hampton Roads
pretty hard while others are just a bit north, closer to the
northern neck of the Tidewater. In any case, either pathway would
be sufficient for flash flood prospects, even with the ongoing
drought in the area. Rainfall of 1-2"/hr will be likely in the
heavier cells with even some stronger cores capable of brief
2-3"/hr periods when you factor the anomalous deep moisture layer
in place.

If it was not for such elevated FFG's, this would likely be a
higher risk, but for now, maintained continuity with a MRGL, but
will have to monitor for exact placement of the repeating
convection as that could be enough to tip the scales to a focused
higher risk, especially if the immediate Tidewater is impacted as
that area has historically been flash flood prone due to more
sprawling urbanization and poor drainage capabilities.

...Texas into New Mexico...

Couple of areas of interest across the Southwestern U.S. into the
Southern Plains for Monday. The first is within the bounds of a
remnant MCV that continues to cause issues across Central and
Northern TX as it drifts over the northern Hill Country, west of
I-35 between San Angelo and Fort Worth. Spells of heavy rain
continue to plague this zone with cells popping up over the course
of the evening as focused low-level convergence and available
moisture aids in these scattered, slow-moving thunderstorms capable
of multiple inches of rainfall as they drift over the area. CAMs
are split on the handling of this feature heading through the
morning and afternoon today, but the models that do maintain
organization of the MCV tend to have another scattered to
widespread convective enhancement during the diurnal instability
max leading to pockets of enhanced rainfall between 2-4" in wake of
the cells. Modest neighborhood probs from the 00z HREF for >2"
(20-40%) exist in this relative corridor of the Concho Valley into
north-central Hill Country between San Angelo and Fort Worth. With
the MCV likely to exhibit little forward propagation, areas that
were hit this evening could see more in the way of convection this
afternoon, so long as the MCV holds together and doesn't dissipate
as indicated via some guidance. Considering the variables at play
and the potential to be a sneakier setup overall, wanted to gauge a
conservative approach and add a MRGL risk to the area referenced
above to cover for the threat.

Further west, we have a different setup that will induced by a
favorably developing upper trough/closed-low scenario coming out of
a negatively tilted shortwave trough ejecting out of the Desert
Southwest during the period. Height-falls are all but certain
within all model output with the shortwave progression likely
develop something more formidable as it encroaches on NM by later
this afternoon. Enhanced upper forcing and well-defined surface
trough bisecting far western TX up through eastern NM and the
divide will provide the necessary focused ascent features capable
for scattered to widespread convective development during peak
diurnal instability and beyond as the upper forcing pattern becomes
most favorable by the evening. Probabilities for rainfall rates
>1"/hr remain highest within the confines of the surface trough as
indicated in the latest 00z HREF prob fields. The Sacramento
Mountains, adjacent High Plains of eastern NM down into the Upper
Trans Pecos are the most favored areas of heavy rainfall with
secondary maxima plausible in far west TX across the terrain in the
Guadalupe Mountains down to portions of the Davis and Delaware
Mountains in Southwest TX. The previous MRGL risk was expanded to
include more of the area comprised of at least modest probabilities
for >1" of precip, as well the more favored areas for rates >1"/hr,
at times during the period of interest.

Kleebauer


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt