FOUS30 KWBC 100835
QPFERD

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

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

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

...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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