Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Tue May 27 2025
Day 1
Valid 01Z Wed May 28 2025 - 12Z Wed May 28 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST U.S. AND TEXAS...
...Southeast and Mid-Atlantic...
A Slight risk was maintained from portions of the central Gulf
Coast into central TN. Ongoing convection will continue across the
corridor through the evening, and with lowered FFGs from previous
rainfall, isolated to scattered flash flooding will remain
possible. Recent HRRR and RRFS runs indicate some upscale growth
on the southern edge of this convection over the next few
hours...potentially allowing for some brief training across
portions of far southern MS, AL and the far western FL Panhandle.
Much higher FFGs over this area should limit the extent of flash
flooding...however localized rainfall over 3" in a short period
could result in a few flood related issues within any more
sensitive locations.
Portions of coastal NC will also need to be monitored into the
overnight hours. Convection along a coastal trough is resulting in
slow storm movements and locally heavy rainfall. Indications that
the MCV associated with ongoing GA convection may help enhance
this coastal convection overnight. The main question remains
whether the bulk of this potential training convection ends up
onshore or just offshore. Blending the 22z and 23z HRRR gives an
impressive 5-10" swath of rainfall in between Wilmington and
Newport. However recent radar trends indicate the convection slowly
propagating off to the east, getting close to moving offshore.
Thus quite possible the HRRR is too far inland with its axis of
more extreme rainfall amounts. So While the HRRR can not be ruled
out the more likely scenario seemingly keeps a bulk of any extreme
rainfall just offshore...either way will need to closely monitor
radar trends.
...Texas...
A Slight risk remains for portions of southwest to central TX.
Most signs point towards the ongoing convection organizing and
growing upscale into an MCS, or at least a cluster or two,
overnight. In the organization phase (near the western edge of the
Slight risk) cell mergers should result in isolated to scattered
flash flood issues. If/when the convection organizes it should
generally be more progressive off to the east...however FFG lowers
to the east and hourly rainfall could still exceed 2" even in a
quicker moving system...so a Slight risk still seems appropriate
for these areas overnight.
Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Tue May 27 2025
Day 2
Valid 12Z Wed May 28 2025 - 12Z Thu May 29 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE HIGH PLAINS, TEXAS, LOWER MISSISSIPPI VALLEY, CENTRAL GULF
COAST, AND THE COASTAL MID-ATLANTIC...
...20Z Update...
One change made to the previous ERO issuance was to add a Marginal
Risk to coastal North Carolina into the Mid-Atlantic. This is the
back end of the rainfall occurring on Day 1/Tuesday night that may
linger after 12Z Wednesday. Many CAMs in particular show some
lingering rain before pulling offshore later Wednesday, and some
localized flooding issues are possible.
With the existing Marginal Risk, expanded the Marginal Risk north
into Wyoming. Similar to the Day 1/Tuesday period, some moisture
and instability could lead to slow moving convection, and HREF
probabilities for 2 year ARI exceedances are localized at more than
50 percent, with low end potential for 10 year ARIs to be
exceeded. Also adjusted the Marginal to include more of the central
Plains per recent guidance. Highest QPF still looks to be in
south-central Kansas or so, from possible Wednesday morning
convection and the potentially heavier Wednesday night convection.
But continued to hold off on any Slight Risk upgrade given the
general antecedent dryness and the movement of the convection with
possibly only a brief period of training.
Farther south, certainly some moisture (probably above the 90th
percentile for PWs) and instability in southeast Texas for some
flooding risk. 12Z HREF probabilities of exceeding FFG showed some
relatively high probabilities for that region, but this was likely
influenced by the briefly lowered FFG from this (Tuesday) morning
storms. Given that FFG rebounds in these coastal and near coastal
areas relatively quickly, expect these probabilities may go down in
future HREF cycles. Still, will continue to monitor if this may
need a Slight Risk in the Day 1 period. Farther east, global and
CAM guidance has decreased with QPF toward the Florida Panhandle,
with just some scattered showers and storms. Though localized
storms may be slow moving, the scattered nature and higher FFG in
that region seems like the risk for flash flooding is less than 5
percent.
Tate
...Previous Discussion...
...Central High Plains...
Longwave pattern across the CONUS will become "blocky" with a large
closed ULL centered over the Midwest with vorticity maxima pivoting
around the general circulation. A shortwave currently analyzed over
the High Plains of Canada will get caught in the western periphery
of the ULL and rotate quickly to the south and southeast by the
middle and latter stages of D2 leading to convective enhancement
with a likely MCS definition as we move into the end of the period.
There's a growing consensus on the evolution beginning upstream
over the Front Range, eventually crossing through the Central High
Plains of KS with upscale growth of any convective segment sliding
down prevailing northwesterly steering flow. As the complex loses
latitude and reaches the southern periphery of the sprawling ULL,
the disturbance will slow and eventually pivot eastward allowing a
training axis to occur somewhere over Southern KS. Ensemble bias
corrected QPF is fairly aggressive in the signature with max QPF
orientation aligned from DDC down towards North-Central OK. This
has been pretty well-documented so far with relatively consistency
in guidance, so the threat is certainly increasing for a regional
bout of heavy rainfall. The area expected to see the most rain is
actually one of the "drier" areas of KS leading to modest FFG
indices in the 1/3/6-hr temporal scales. Areal average of 2-3" with
max potential of up to 4" is forecast across the axis referenced
above. 1-2" will surround the forecast maxima leading to some
isolated flash flood concerns in those areas if the orientation of
the complex shifts. Overall, a MRGL risk is in place for the
threat with a potential for a targeted upgrade if the current
assessment proves to be favored leading in.
...New Mexico and Texas...
Persistence in a narrow theta_E ridge from West TX up through the
Southern Rockies will lead to another shot at scattered
thunderstorms over the terrain with locally heavy rainfall causing
some flash flood concerns in complex topography. Remnant burn scars
up over Northern NM have been hit the past few days with high
runoff capabilities leading to flash flooding in those targeted
zones. This threat will remain for D2 with some isolated heavy
cores also possible as far south as the Sacramento Foothills.
West Texas will see an initiation of thunderstorms across the Big
Bend up to the Stockton Plateau and adjacent terrain of Southwest
TX. Threat is very localized, but considering the environment in
place, any cells will have the opportunity for a quick 1-2" of
rainfall that would ultimately cause issues over the area. A
relative min in QPF is anticipated over the Permian Basin and
Caprock area of Eastern NM and High TX Plains. This is well
documented within the QPF distribution allowing for a "donut hole"
for a nil ERO to be positioned in the area of the QPF minimum.
Across Central and East TX, thunderstorm complex will materialize
the back end of D1 and roll into D2 with heavy rainfall forecast
across the Piney Woods and Upper coastal areas of TX. This will be
back to back days this is forecast or will have occurred leading to
some threat of localized flooding pending soil saturation and
recovery response in the area. Totals between 2-3" will be possible
across portions of the TX coast up into the Lower Sabine, so
realistically, localized flooding will remain a threat for at least
another period. Any convective development across Central TX
Wednesday will lead to another isolated flash flood threat after
this evenings rainfall and any Tuesday additions will lead to
solidly primed soils within a region that is prone to flash flood
concerns due to higher runoff capabilities.
For each of the areas mentioned in the above sections, a MRGL risk
is forecast with the best opportunity for upgrades potentially
across the Upper TX coastal area into the Piney Woods.
...Lower Mississippi Valley and Gulf Coast...
Complex developing upstream over TX will eventually work its way
into the Lower Mississippi Valley with more heavy rainfall concerns
in-of the energy associated with the progressing complex. This area
has relatively high FFG's leading in, so the threat will remain
fairly isolated overall with max QPF outputs generally within the
2-4" range with the heaviest more likely to be closer to the Lower
Sabine than areas further east. Still, there's reasonable consensus
on a stripe of elevated QPF from the expected convective impacts
moving overhead leading to a lower-end MRGL risk for the Lower
Mississippi area, mainly LA. Across the Central Gulf Coast, heavier
cells within a moisture rich environment could spawn over areas a
little more sensitive (urban areas) between New Orleans to
Tallahassee. The risk is low-end, but some guidance has been trying
to put heavier QPF signatures within proxy of those areas. Steering
flow will be relatively weak so a shot for cells to retain some
longevity over the same areas. A MRGL risk remains in place over
each area.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Tue May 27 2025
Day 3
Valid 12Z Thu May 29 2025 - 12Z Fri May 30 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST...
...20Z Update...
As mentioned in the previous discussion, the pattern Thursday is
quite tricky, with a possible MCV tracking east across the
Southeast the start of the period while a shortwave rounding the
upper low will both provide upper level support for thunderstorms
but with uncertainty in the placement of both. Generally the signal
this cycle is for the MCV to push through faster and show the
heaviest QPF farther southeast on Thursday than in the previous
cycle, and perhaps lighter with max QPF amounts, but still with
considerable model spread. Adjusted the Slight Risk southeast given
the model trends. Hated to leave the Slight out of the higher FFG
in the central Gulf Coast because of models (like the UKMET) and
ensembles (like the EC ensemble) show the heaviest QPF more along
the coast. Thus show the Slight Risk covering from the central Gulf
Coast and inland into the Southeast, but future adjustments will
likely be needed. The Marginal Risk is similar to the previous
issuance though with some minor expansion northward into the Lower
Ohio Valley for a possibly separate QPF axis there closer to the
vort max.
Tate
...Previous Discussion...
A tricky setup is on tap for much of the CONUS situated east of the
Rockies with a broad ULL helping dictate the pattern encompassing
two-thirds of the nation. Across the Southeast, a corridor of
elevated PWATs and associated instability will be present south of
the primary low. A cold front will begin a southward advancement
with cooler, stable air making headway through the Ohio Valley the
beginning of the period. As the front sags south a broad axis of
pre-frontal convergence will transpire with scattered to widespread
thunderstorms likely situated across much of the South. The issue
becomes a discrepancy in timing of the front, precip location, and
any boundaries that could enhance convective posture. What is known
is the environment favors heavy rainfall for anything that does
develop and that trend will remain steady all the way through the
eventual setup. Ensemble depictions are more stable in the QPF
distribution compared to the deterministic where several
iterations of output are available with maxima situated anywhere
from Tennessee down to the Gulf Coast to as far west as TX.
Ensemble means are positioned over an axis from Jackson, MS towards
Western GA, a corridor that will be well-saturated by the D3 time
frame leading to a greater threat for flooding in those areas. With
a pretty high chance of somewhere in the South getting anywhere
from 2-4+" of rainfall, the forecast SLGT risk in place outlines
the region that would have the greatest threat considering the
ensemble QPF and relevant FFG's expected by D3. The risk area
could very well adjust, but went with the area of greatest
interest, correlating well with the current ensemble output of
heavier QPF.
More scattered to widely scattered thunderstorms will develop
across much of Texas leading to localized flash flood concerns for
yet another day with the best threat occurring Thursday afternoon
and evening. The threat will be most suitable within areas of
higher saturation from previous periods of rainfall, urban areas,
and complex topography.
Kleebauer
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
