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FOUS30 KWBC 101556
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sun Aug 10 2025

Day 1
Valid 16Z Sun Aug 10 2025 - 12Z Mon Aug 11 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
KANSAS, MISSOURI, AND NORTHERN OKLAHOMA...

16Z Update...

The meteorological synopsis provided by the overnight shift remains
in sound shape with updates this cycle based off changes in the
12Z guidance suite. Most guidance has shown the heaviest rainfall
axis is shifting farther south and west with HREF/REFS neighborhood
probabilities for >3" of rainfall around 40% in southwest KS and
far northern OK. The expectation remains for a blossoming area of
thunderstorms activity over the central Plains as the LLJ
strengthens tonight and favorably orients itself along a frontal
boundary draped over the region. 12Z HREF and 00Z REFS guidance
continue to highlight southeast KS as featuring the highest odds
for rainfall totals >5" (30-45% 40km neighborhood probabilities on
new 12Z HREF) through tonight. Farther west, the Slight Risk was
expanded as far west as the Palmer Divide with new 12Z HREF PMM
guidance showing some isolated max amounts approaching 2" in some
cases. Only minor tweaks to the risk areas were made in the
Southeast and Southwest based on a blend of new incoming 12Z
guidance, which largely shows the risk areas remain on track.

Mullinax

---Previous Discussion---

...Central Plains through the Upper Midwest...
The synoptic pattern today continues to be indicative of a Maddox
Frontal rainfall event, although confidence in the exact evolution
has decreased and significant changes are needed to the inherited
ERO. Some of these changes are due to exceptional model uncertainty
at this time range, but also accounting for what occurred Saturday
when the event did not really pan out as expected.

Synoptically, the setup still remains extremely favorable for an
axis of very heavy rainfall. An anomalous mid-level trough (500mb
and 200mb heights locally below the 10th percentile according to
NAEFS) will remain aligned from ND/MN southwest through the Four
Corners. Although the leading impulse embedded in the core of this
trough will advect east across southern Canada, spokes of vorticity
and associated shortwaves rotating through the flow to the south
will force the trough axis to remain to the west. Downstream of this
trough axis, a persistent upper jet streak will arc poleward into
Ontario, placing at least modest but persistent upper level
diffluence within its RRQ. Beneath this evolving setup, a surface
front will oscillate at least subtly NW to SE as a stationary front,
providing additional focus for ascent beneath the weak height falls
and periods of PVA. Additionally, any stronger impulse interacting
with the surface front could result in waves of low pressure
rippling along to the northeast.

East of this front, low-level return flow will continue to advect
from an open Gulf, drawing elevated thermodynamics and theta-e
northward. PWs are progged to climb to above 2 inches as the 850mb
LLJ surges to 20-30 kts, overlapping intense MUCAPE of more than
3000 J/kg to provide an extremely favorable environment for heavy
rain, which is additionally supported by specific humidity values
between 925-850mb above the 97th percentile (NAEFS). At the same
time, the theta-e ridge will blossom and align into the front, with
an intense gradient suggesting heavy rainfall on its NW axis as is
typical with the Maddox Frontal conceptual model. In this setup,
with persistent ascent acting upon impressive thermodynamics, and
mean 0-6km winds aligned to the front, it is likely streaks of heavy
rainfall will occur, and most deterministic models showing 1-4
inches of rainfall through 12Z Monday, with high-res CAMs indicating
locally 5-7 inches, so heavy rainfall is almost a certainty.

However, uncertainty is exceptional (especially for a 24-hr
forecast) with the placement of this heavy rain, and the spread in
placement ranges from southwest WI to southern KS, and almost
anywhere in between, leading to a challenging ERO forecast.

While the front will be the primary focus for convection, along with
a wave of low pressure expected to develop along it, and the LLJ is
likely to be parallel to it, there is increasing concern that
convective clusters (both overnight Saturday night into Sunday, and
additionally on Sunday) will leave outflow boundaries aligned more
west-to-east outrunning the front. This could drastically change the
position of the heaviest rain axes, and would support heavy rainfall
south of the global members in a typical bias. If these clusters do
not develop, convection and heavy rainfall would likely remain more
aligned SW to NE, but have noted a trend in the Corfidi vectors to
be more pointed S/SE after SW early, which would additionally
support more W-E heavy rainfall. With all the uncertainty in
placement, the ERO was modified heavily and tailored primarily to
HREF/REFS neighborhood probabilities, but with additional changes
influenced by the EAS probabilities, the CSU first guess fields,
and coordination with the impacted WFOs.


...Southeast...
Another active day of convection is likely from the coastal
Carolinas through Florida.

Broad ridging extending from a Bermuda-type but elongated high
pressure over the Atlantic will be modified by an inverted trough
aligned from the eastern Gulf to the NC coast. Onshore and confluent
flow into this inverted trough will maintain impressive moist
advection into the area, manifesting as PWs above 2.25 inches, well
above the 90th percentile according to the SPC sounding climatology.
This will overlap with MUCAPE of 500-1000 J/kg, modest overall
instability indicative of a tropical airmass, to produce impressive
thermodynamics to support heavy rainfall. Additionally, this robust
thermodynamic advection onshore will result from 850mb winds as high
as 15 kts, which, while modest, will exceed the mean 0-6km winds
which are progged to be just around 5 kts. This suggests widespread
convergent ascent across the Southeast, with the accompanying WAA
surging warm cloud depths to around 15,000 ft, supporting efficient
warm rain processes within a deeply saturated column noted in
regional forecast soundings.

The CAMs are in generally good agreement in widespread convection
today. Scattered slow-moving thunderstorms with rainfall rates of 1-
3"/hr are expected across much of FL and into GA/AL. However, the
focus for the heaviest rainfall is likely along the coastal plain of
GA through southeast NC. Here, propagation vectors are aligned
against the weak mean wind, and with subtle mid-level impulses
lifting northwest within the weakly forced environment, waves of
repeating rounds of convection with heavy rainfall are likely. In
this region, the HREF indicates a 20-30% chance of 2"/hr rain rates,
with locally more than 3"/hr possible at times, at least briefly.
With backbuilding offshore leading to regenerating and training
rounds of heavy rain, total rainfall could exceed 5" (30-50% chance
from the REFS/HREF), with the greatest potential along the SC coast.
The inherited SLGT risk was cosmetically adjusted, but scattered
instances of flash flooding appear likely today, even across these
areas that are generally more difficult to flood.


...Southwest...
A MRGL risk of excessive rainfall continues for portions of the
Southwest today as monsoonal moisture returns to the area leading
to a more active convective pattern. A mid-level trough extending
down from the Northern High Plains will deepen today, producing
lowered heights across the Four Corners and Desert Southwest,
through which weak spokes of vorticity will rotate. These lowered
heights and locally enhanced ascent will impinge into an
increasingly favorable thermodynamic environment to support
convection with heavy rainfall as low-to-mid level flow backs
gradually to the SW, allowing for a better draw northward of PWs
exceeding 1", which may spread as far north as the Mogollon Rim and
combine with MUCAPE of 500-1000 J/kg across the southern half of
AZ and NM. This will support scattered to widespread showers and
thunderstorms which will move slowly eastward through the day. With
rainfall rates as much as 0.75"/hr to 1"/hr at times as shown by
the UA WRF and HREF neighborhood probabilities , this could result
in at least isolated instances of flash flooding, especially across
more vulnerable soils or terrain. The MRGL risk was curtailed a
bit on the eastern side across NM where CAPE is progged to be much
weaker, but in general changes to the inherited risk were minor.


...Central High Plains...
The inherited SLGT risk was maintained across eastern CO,
primarily for renewed thunderstorm activity on Sunday evening
falling atop soils that will be primed from ongoing (Saturday
night) convection. The elongated stationary front across the
Central Plains early today will drop steadily southward, leaving NE
upslope flow in its wake. This will be most pronounced into
eastern and southeastern CO, where PWs will likely surge to above 1
inch. As convection develops across the higher terrain of CO
during the aftn, it should steadily progress east and then grow
upscale as it moves off the terrain in response to bulk shear of 40
kts. While these clusters should move steadily eastward on mean
0-6km winds of 15-20 kts, rainfall rates of 1-2"/hr are likely
which could be sufficient to overwhelm soils that will be saturated
from antecedent rainfall. Additionally, some local backbuilding is
possible as reflected by CAMs across southeast CO, lengthening the
potential duration of heavy rainfall in a few areas.


Weiss


Day 2
Valid 12Z Mon Aug 11 2025 - 12Z Tue Aug 12 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN PLAINS THROUGH THE UPPER MIDWEST, AND THE COASTS OF THE
CAROLINAS...

...Central Plains through the Great Lakes...
The anomalous trough drifting slowly eastward from the High Plains
will weaken slightly as heights rise across the Central Plains
Monday. This will have limited overall affect on the downstream
rainfall, however, as spokes of vorticity embedded within the flow
and the accompanying surface front remain in place to drive ascent,
with only slow eastward advance expected through D2.

Despite this eastward translation, pinched 850-500mb flow between
this trough and a ridge to the east will maintain efficient moist
advection out of the Gulf, albeit with PW anomalies lower than
forecast for the weekend (PWs around 1.75 inches or 0.5 to 1.0
standard deviations above the climo mean). This suggests that
another day of training convection is likely along the front from
Oklahoma through Michigan, but with somewhat reduced intensity from
prior days. Another challenges for D2 is the uncertainty in frontal
position and convective timing, as lingering boundaries from
Sunday`s convection (with additional uncertainty in placement) will
play an important role into the training potential and heavy rain
axes on Monday. This suggests that despite a still favorable
synoptic pattern and elevated thermodynamics, the SLGT risk remains
the highest needed category for the D2 ERO.

On Monday, rainfall rates will likely still peak above 1"/hr at
times, which through training could produce 1-3 of rainfall
(ECENS/GEFS probabilities for 3+ inches below 10%), and where this
rain overlaps with heavy rain from the weekend (most likely on the
W/SW edge of the SLGT risk area), scattered instances of flash
flooding are possible. Otherwise, any cells training along the
front could produce isolated responses and a broad MRGL risk draped
along the length of the front into the southern High Plains
continues.


...Southeast...
Yet another active day across the Southeast is expected Monday as
an inverted low-to-mid level trough drifts northeast along the
coast of the Carolinas. This trough will move within a favorable
thermodynamic environment characterized by still highly anomalous
PWs of 2 to 2.25 inches, although the slow retreat of the mid-level
ridge to the east should help ease some of this moisture the latter
half of D2. Still, the overlap of these elevated PWs with MUCAPE of
1000-2000 J/kg will support convection with heavy rainfall.

In general, convection across the Southeast on Monday should be
scattered in coverage, but some organization, or at least better
clustering, is likely in the vicinity of any shortwaves and the
inverted trough, primarily from the Grand Strand of SC to the
Crystal Coast of NC. Here, repeating rounds of convection with
efficient rain rates exceeding 2"/hr are likely, driven by
impressive low-level confluence. Elsewhere across the Southeast,
heavy rainfall rates are also likely, but with less organization,
but still slow storm motions. The rainfall probabilities for more
than 3" peak along the SC/NC coasts and the SLGT risk was adjusted
cosmetically for the new guidance. Otherwise, a broad MRGL risk
continues for slow moving thunderstorms across areas that have had
recent heavy rainfall, and an expansion was made into the Upstate
of SC where some of the high res CAMs suggest enough upslope to
enhance rainfall locally in that area.


Weiss


Day 3
Valid 12Z Tue Aug 12 2025 - 12Z Wed Aug 13 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER MIDWEST/GREAT LAKES, THE SOUTHWEST, AND GULF
COAST/SOUTHEAST...


...Upper Midwest...
The anomalous mid-level trough that will produce the heavy rainfall
D1 and D2 across the Upper Midwest/Central Plains will weaken
Tuesday as shortwaves embedded within the trough become more
rapidly absorbed into westerlies across the northern tier of the
CONUS. A lingering shortwave trough, likely convectively enhanced,
pivoting across KS/MO will drive sufficient ascent for additional
rainfall along the weakening front, but with lesser moisture and
instability than was available earlier in the week. Still, at least
short term training of intense rainfall rates is likely as mean
layer winds align to the weakening front, which could result in
isolated instances of flash flooding.


...Southeast...
Elongated mid-level ridge extending westward from the Atlantic to
along much of the Gulf Coast will subtly retreat Tuesday as a
shortwave emerging from the Gulf lifts northward. The modest height
falls and PVA accompanying this feature will drive ascent into an
otherwise weakly forced environment, producing a favorable setup
for scattered to numerous thunderstorms from the central Gulf Coast
northeast along the periphery of the ridge into Georgia.
Thunderstorms that develop will contain heavy rainfall rates of
1-2"/hr in response to favorable thermodynamics characterized by
PWs of 2-2.25 inches overlapping MUCAPE as high as 2000 J/kg. The
general SW return flow will push storms along steadily to the N/NE,
but with repeating rounds possible and some organization beneath
the shortwave, a few instances of flash flooding could result as
rainfall reaches 2-3" in some areas.


...Southwest...
Mid-level ridging will expand but shift subtly westward on Tuesday
while weak impulses rotate around its periphery. This will help to
intensify instability across the region, and progs indicate a
larger plume of 1000 J/kg MUCAPE spreading across the southern
portions of AZ and NM, especially during peak aftn heating. This
will combine with PWs that will be above 1" in many areas south of
the Mogollon Rim, leading to a favorable environment for
convection. Where any of these impulses rotate aloft, enhanced
ascent will yield scattered to widespread thunderstorms with rain
rates approaching 1"/hr at times, and these storms will move
slowly/chaotically beneath the ridge. This could result in
isolated instances of flash flooding, most likely where any storms
track across more vulnerable terrain features or burn scars.


Weiss


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