FOUS30 KWBC 102016
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 PM 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
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