FOUS30 KWBC 200738
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
338 AM EDT Sun Jul 20 2025

Day 1
Valid 12Z Sun Jul 20 2025 - 12Z Mon Jul 21 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR
CENTRAL ILLINOIS...

...Midwest, Ohio Valley, & Southern Appalachians...
High moisture persists into Sunday -- precipitable water values
eclipsing 2.25" -- with the core of the instability pool more
significant across portions of the Ohio Valley on Sunday as warming
at 700 hPa occurs across the Central Plains, slowly strengthening
a mid-level capping inversion. Effective bulk shear remains 
sufficient for convective organization, including mesocyclone
formation. Two rounds of convection are expected -- a morning round
dropping out of IA into the Midwest and Ohio Valley with a second
round forming in the vicinity of northeast KS during the afternoon
and evening hours. Of some concern is the backing of the low-level
winds during the 18-00z time frame, which could act to anchor any
existing convection in place/limit forward propagation. Any complex
that forms in northeast KS and northern MO is expected to spill
into portions of IL during the evening and overnight hours late
this day (into early Monday morning). Considering the degree of
moisture, instability, and effective bulk shear, there is the 
potential for 3-4" amounts in an hour and local 8" totals. The
guidance has increased its signal for higher amounts with a few
pieces of guidance advertising 8-11" of rain in the 00z model suite,
so in coordination with LSX/St Louis MO, ILX/Central IL, and 
IND/Indianapolis IN forecast offices, added a Moderate Risk area 
in and near central IL.  Localized High Risk impacts cannot be 
ruled out should any training bands or cell backbuilding persists for 3+ hours.

To the southeast of the Moderate Risk area, a higher end Slight 
Risk appears to exist for much of the rest of the Ohio Valley and 
western slopes of the Southern Appalachians, with some guidance 
showing a secondary maximum between eastern KY and the KY/OH/WV 
border junction. Lingering model uncertainty kept us from upgrading
to a Moderate Risk for the possible secondary maximum. With the 
same rainfall potential -- hourly amounts to 3-4" with local 8" 
totals possible -- localized Moderate Risk impacts cannot be ruled out.

...Northern Plains...
The Dakotas/MN will be at the nose of a modest low level jet. A 
shortwave moving by the area may be all it takes to initiate a 
line of storms across the region, though there may be two separate 
rounds: one in the morning and a second Sunday night. The guidance
has been slowly shifting east and southeast with time, and the 
area of potential Slight Risk continues to shrink. Believe this is 
a lower end Slight Risk.

Southwest/Southern Rockies...
Enough moisture is available for scattered monsoon convection on
Sunday, with the most noted across south-central AZ. Hourly 
amounts to 2" are possible due to diurnal heating, which would be 
most problematic in urban areas, dry washes/arroyos, or burn scars.

New England/Northern Mid-Atlantic states...
A cold front is expected to sweep through the region, which could
lead to locally heavy pockets of rainfall. Available moisture and
instability supports hourly rain amounts to 2" where cells can
manage to train or merge. Both the 00z HREF and 18z RRFS have a
heavy rain signal from 12z onward this morning from the eastern tip
of Long Island near Block Island, Martha's Vineyard, and Nantucket.
Recent radar imagery (as of this writing) shows a vorticity maximum
in this general neighborhood which could be the responsible party.
If so, it appears to be running ahead of the model guidance and 
shouldn't be the threat advertised. Radar estimates are quite low 
in its vicinity as is.

Roth


Day 2
Valid 12Z Mon Jul 21 2025 - 12Z Tue Jul 22 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
KENTUCKY, TENNESSEE, SOUTHWEST WEST VIRGINIA, & WESTERNMOST VIRGINIA...

In and near KY & TN...
An axis of moisture and instability pivots between a convective
frontal wave offshore the Southeast and a strengthening warm
advection pattern across the Plains. Declining low-level inflow
is noted with time, but the GFS forecasts enough effective bulk
shear for some level of convective organization during the period 
of daytime heating, which doesn't drop off significantly until 
around sunset. Consideration the degree of saturation area soils 
have now and the expectation for further saturation on Sunday and 
Sunday night, adding a Slight Risk area here makes sense. Hourly 
amounts to 3" with local totals to 5" are possible where cells 
train or merge which would be most problematic in urban areas and 
over saturated soils. This appears more likely within the defined
Slight Risk than the surrounding Marginal Risk area.  There was 
some consideration for a Slight Risk for coastal sections of NC, 
but with coastal marsh in some of those areas, decided against it.

Plains/Southern Rockies... 
Surface boundaries/low pressure areas/upper level disturbances are
on this move this period, which along with warming temperatures at
700 hPa/increased mid- level capping across portions of the 
Central Plains and IA should limit QPF and flash flood potential 
elsewhere. Hourly amounts up to 2" and local totals to 4" are
considered possible in this region. At the moment, it appears any 
flash flooding would be on an isolated to widely scattered basis. 
In the Southwest and Southern Rockies, burn scars and dry 
washes/arroyos would be most at risk. Anywhere within these 
regions, urban areas would be of most concern.

Roth


Day 3
Valid 12Z Tue Jul 22 2025 - 12Z Wed Jul 23 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
MINNESOTA, NORTHERN WISCONSIN, & NORTHWESTERN MICHIGAN...

In and near the Western Great Lakes...
A strengthening low-level jet out of the southwest is expected
downwind of an amplified Western trough and under the base of a
cold low moving across central Canada. Precipitable water values
rise to 2"+ and a broad reservoir of CAPE up to 5000 J/kg builds
near southeast SD due to significant warming at 700 hPa/a
strengthening mid-level capping inversion across portions of the
Plains to the east of the Western trough. Effective bulk shear
should be more than sufficient for organized thunderstorms,
including mesocyclone formation. Weekly rainfall anomalies show
that portions of the Arrowhead Peninsula and Upper Peninsula of
MI have had 300%+ of their average rainfall, leading to some degree
of saturation.  Hourly rain amounts to 2.5" with local amounts to 
5" could lead to flash flooding, with urban areas most prone to 
issues. The Slight Risk was shifted somewhat north of continuity
per the latest guidance.

Southeast...
A potentially convective low retrogrades through portions of 
northern FL this period. Precipitable water values rise to 2-2.25".
CAPE should rise to 3000 J/kg due to either daytime heating where
present or due to onshore flow from the warm western Atlantic,
Gulf, and FL Straits. There does appear to be enough effective bulk
shear present for at least some loose convective organization.
Hourly amounts to 3" and local totals to 6" are possible here.
Considered a Slight Risk as some portions of northern FL saw above
average precipitation with the previous retrograding convective low
on July 16-17, but the heavy rain signal at the moment shows
minimal overlap with the the previous convective low's rainfall footprint.

Southern Rockies...
Enough moisture and instability is expected to hourly amounts up 
to 2" and local totals to 4", with a distinct diurnal cycle
expected. At the moment, it appears any flash flooding would be on
an isolated to widely scattered basis. Burn scars and dry 
washes/arroyos, and urban areas would be most at risk.

Roth
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