FOUS30 KWBC 090755
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Sat Aug 9 2025

Day 1  Valid 12Z Sat Aug 09 2025 - 12Z Sun Aug 10 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM FAR NORTHERN
MISSOURI THROUGH MUCH OF EASTERN IOWA AND INTO SOUTHWESTERN WISCONSIN...

...Upper Midwest...
The synoptic setup for heavy rainfall is becoming increasingly 
favorable as a near-textbook Maddox Frontal Pattern develops across
the Upper Midwest. This will likely result in numerous instances 
of flash flooding from northern Missouri through southern Wisconsin.

The period begins with a closed 500mb low moving across southern
Manitoba/Saskatchewan, which will drive an elongated trough 
southeast into the Central Plains and Upper Midwest with 
accompanying height falls. Embedded shortwaves shedding around the 
primary closed low will help keep the trough axis aligned SW to NE 
from the Four Corners into the Upper Midwest, and this is a 
critical piece to the evolving setup. As the trough lingers back to
the west, the parent upper jet streak rotating into central Canada
will maintain a tail back into the Central Plains, leaving 
favorable RRQ diffluence atop the region, and aligned with the 
greatest height falls. Since the trough will move little during 
this time, this pronounced synoptic support will be quasi- 
stationary, resulting in prolonged lift across the region.

At the surface, a cold front will try to push east, but become 
generally stationary from MN to KS as the upper trough lingers to 
the west. This will enable a plume of elevated moisture to 
continually surge northeast ahead of the front, as PWs of 2 to 2.2 
inches (above the 97th percentile according to NAEFS) get drawn 
northward on 850mb inflow of 20-30 kts, pushing an impressive theta-
e ridge axis northward into IA. This anomalous moisture will 
combine with MUCAPE of 2000 J/kg or more to supply extreme 
thermodynamics into the region, supporting heavy rainfall rates for
which both the HREF and REFS indicates has a 30-40% chance of 
exceeding 2"/hr, and the HRRR (as far out as it goes) suggesting as
much as 1"/15min (4"/hr rates) at times. 

The concern for Saturday, especially within the inherited MDT risk 
area, then, is that multiple rounds of convection are likely as weak 
impulses track aloft and 850-300mb winds remain parallel to the 
front. At the same time, Corfidi vectors become increasingly aligned
anti-parallel to this mean wind, indicating a high potential for 
backbuilding and training, which will support total rainfall above 
5" (40-60% chance), with locally as much as 8" possible (10-15% 
chance). However, there is still considerable uncertainty into the 
exact placement as rounds of convection, especially through any 
clusters/MCS, drive the effective boundary (merged outflow with 
front) north and south at times. This could result in a slow 
southern push of the heaviest rainfall through the day, although 
the northward advance of the theta-e ridge at times should again 
push heavy rain northward as well. With a lot of spread in place, 
the HREF EAS probabilities (which are extremely significant for 
2"/24hrs at above 35%) were used to help tailor the MDT risk area, 
which aligns quite well with the latest ECMWF EFI above 0.7.

Despite the uncertainty in exact placement, the MDT risk remains 
needed for the likelihood of training/repeating intense rainfall 
rates of 1-3+"/hr leading to FFG exceedance probabilities that 
peak above 40%. The MDT risk was expanded slightly to the SW to 
account for the latest high res CAMs as well, and numerous 
instances of flash flooding are likely.

...Southeast... 
Another active day of showers and thunderstorms is expected across
Florida and the coastal Southeast. The driving factor for this 
widespread convection will be a trough of low pressure centered 
over the eastern Gulf, with a secondary surface stationary front 
draped along the Panhandle of FL to off the SC/GA coast. This
results in generally weak flow within a col in the vicinity of a 
mid-level ridge, with modest onshore flow from the Atlantic 
becoming subtly accelerated by the trough over the Gulf. 
Additionally, weak impulses rotating around the Gulf trough will 
push W/NW through the period, providing locally enhanced ascent 
across the area.

Although ascent will be generally modest outside of any impulses or 
sea breeze/frontal convergence boundaries, the environment will be 
favorable for periods of heavy rainfall. PWs above 2.25 inches, or 
above the 90th percentile according to NAEFS, will encompass the 
region, and overlap with MUCAPE of 1000-2000 J/kg to support robust
thermodynamics for heavy rainfall. Any ascent working into this 
environment will support convection with rainfall rates of 2-3"/hr.
Cells that develop will likely move very slowly W/NW on 0-6km mean
winds of just 5-10 kts, and as propagation vectors become 
increasingly aligned back into the higher moisture and anti-
parallel to the mean wind, backbuilding and repeating cells will 
become likely. This could be enhanced by regeneration along sea 
breeze boundaries or storm mergers, lengthening the duration of 
these heavy rain rates. 

Although identifying where the heaviest rainfall will occur today is
challenging due to the weakly forced environment and slow-moving 
storms, the HREF neighborhood probabilities support generally two 
areas of heavier rainfall. The first is along the Gulf Coast of the 
FL Peninsula where the W/NW inflow may allow for a more focused 
region of heavy rainfall as the sea breeze becomes pinned and cells 
moving westward merge with it. The other region is along the SC/GA 
coasts where a weak impulse moving onshore within modest WAA will 
help deepen warm cloud depths and focus heavier rainfall along the 
frictional land/sea convergence boundary. In both of these areas, 
HREF probabilities for 5"/24hrs peak above 50%, and this could occur 
atop primed soils from heavy rainfall on Friday. After coordination
with ILM/CHS/JAX, a targeted SLGT risk has been added for the 
coastal plain of GA/SC where the HREF signal is most impressive and
rounds of storms effectively converging along the coast could 
produce repeating rounds of heavy rainfall.

...Southwest... 
A mid-level ridge centered across southern NM will gradually 
weaken today in response to an impressive trough amplifying across 
the Northern Rockies and a weak mid-level impulse rotating westward
from Texas beneath it. This feature is progged to track along the
international border before gradually rotating northwest late in 
the period, and locally enhanced ascent in the vicinity of this 
impulse will drive renewed and focused convection. As this 
shortwave tracks W/NW, it will impinge into a narrow corridor of 
elevated PWs above 1.25 inches aligned with a similar narrow 
channel of MUCAPE above 1000 J/kg. Together, this will provide an 
environment that will support thunderstorms with heavy rainfall as 
reflected by simulated reflectivity from the available CAMs 
including the HRRR-forced and GFS-forced UA WRF. 

Storm motions beneath this weakening ridge will be quite weak and 
variable, as expected, with 0-6km mean wind vectors showing a 
variety of directions and speeds of just 5-10 kts. With bulk shear 
remaining minimal, this indicates that pulse-type storms with 
generally limited lifespans will be the primary storm mode, and in 
the weakly forced environment this will allow for outflows and storm 
mergers to result in additional, and at times more intense, 
redevelopment. Rainfall rates could exceed 1"/hr at times (20-40% 
chance) which through mergers and slow motions could produce 1-2" of 
rain with locally higher amounts. Briefly considered a small SLGT 
risk over the area of greatest instability near Tucson, but wide 
variation in coverage among the various CAMs combined with recent 
dryness across the area precluded the confidence for an upgrade at this time.

...Central High Plains... 
The tail end of a front stretching from the Upper Midwest will 
linger across the Central High Plains today. As a trough upstream 
deepens over the Northern Rockies and sheds spokes of vorticity 
eastward, these will interact with the lingering front to produce 
scattered showers and thunderstorms late this afternoon through 
tonight. Impressive bulk shear of 35-50 kts combining with moist 
E/NE 850mb flow will result in slow moving thunderstorms organizing
into clusters, with repeated development likely upstream. Coverage
of storms may be isolated to scattered according to high-res CAMs,
but the slow motion of any cell that develops combined with 
anticipated rain rates above 1"/hr may produce isolated flash flooding.

Weiss
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