FOUS30 KWBC 071142
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
742 AM EDT Thu Aug 7 2025

Day 1
Valid 1125Z Thu Aug 07 2025 - 12Z Fri Aug 08 2025

...THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS
PORTIONS OF THE NORTHERN PLAINS...UPPER MIDWEST/GREAT LAKES...AS
WELL AS THE SOUTHEASTERN COAST INTO NORTHERN FLORIDA...

...Northern Plains...

The 00Z model consensus shows a potent upper-level shortwave
tracking through the northern Rockies today/tonight with a lead
mid-level impulse out head helping to ignite thunderstorms over
eastern Montana with movement into North Dakota this
afternoon/evening. 00Z GFS forecast standardized anomalies of PW
are +2 to +3 over the northern Plains with moderate to strong
CAPE and increasing upper level divergence/diffluence ahead of the
approaching upper trough. 850 mb winds are forecast to increase
into the 30-40 kt range across SD, ahead of a related surface low,
helping to fuel the potential for multiple rounds of thunderstorms
through the period and some 2 to 4 inch rainfall totals (locally
higher possible). While movement of any organized convective
clusters should be generally be progressive toward the
east/southeast, potential for training will exist on the
southwestern flank and the environment will likely support 1-2
in/hr rainfall rates.

...Upper Midwest...

1124Z Update: Expanded the Marginal Risk area a bit west-southwest
based on the latest observational trends. Veering late-stage LLJ
(southwesterly ~25kts) orthogonal to the elevated frontal boundary
along the northeast periphery of the upper ridge has resulted in
some upscale growth with the backbuilding/training convection across
south-central and southeast IA this morning. A bit later than
normal given the nocturnal-diurnal cycle transition, however in
this case the elevated convergence became maximized late given the
orientation of the veering LLJ and the frontal zone. For further
details, please refer to the latest Mesoscale Precipitation
Discussion or MPD #880.

Hurley

Previous discussion...
Thunderstorms are likely to develop during the afternoon hours in 
the vicinity of Iowa/Wisconsin/Illinois with weak warm advection 
across a subtle theta-e gradient that strengthens with daytime 
heating. Moderate to strong instability is forecast to develop from
southern Minnesota into Iowa and points eastward at the nose of a 
20 to 30+ kt 850 mb jet axis oriented southwest to northeast into 
the Upper Midwest and continue into the first half of the 
overnight. While uncertainty remains, indications are for a small 
cluster of thunderstorms to form with potential for upstream 
development and slow net movement of heavy rain cores. 12Z and 
previous 00Z members of the HREF suite showed a relatively large 
spread in location and magnitude of heavy rainfall but decent 
agreement for at least 3 to 5 inches. Low confidence in location 
resulted in a broadening of the previous Marginal RIsk area over
Wisconsin/Illinois to include locations to the west, south and east.

...Northeastern Gulf Coast to Coastal Mid- Atlantic...

High moisture will remain across the Coastal Plain of the Carolinas
into the Florida Panhandle today into Friday morning, ahead of a
mid-level trough axis in place over the Southeast and an embedded
vorticity max forecast to move eastward from the southern
Appalachians. Precipitable water values are expected to remain near
2 inches along the coast and into northern Florida.

Thunderstorms will increase with daytime heating and low level flow
from the east to northeast will originate from the higher moisture
airmass. Scattered coverage of thunderstorms is expected across
portions of the Mid-Atlanitc and Southeast with cells exhibiting occasional
slow movement and training. The high moisture and sufficient
instability (at least 500 to 1500 J/kg MLCAPE) will be capable of
producing 2 to 3+ in/hr rainfall rates at times, some of which
could fall on urban centers or locations with above average
rainfall over the past week.

Otto


Day 2
Valid 12Z Fri Aug 08 2025 - 12Z Sat Aug 09 2025

...THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS
PORTIONS OF THE NORTH-CENTRAL U.S. AND SOUTHEASTERN COAST INTO
NORTHERN FLORIDA...

...Northern Plains into Upper and Middle Mississippi Valley...

There is potential for a lingering MCS to be in place over
Minnesota at the start of the period (12Z Friday). The potent
closed low/upper trough from Thursday night will continue to
advance across the north-central U.S. on Friday/Friday night.
Thunderstorms, some possibly slow moving, are expected to impact
portions of northeastern Montana during the day on Friday while
thunderstorms expand in coverage during the afternoon over North
Dakota into portions of South Dakota. By 00Z Saturday, a cold front
is expected to become better defined over the Dakotas and move into
a fairly unstable and moist airmass from the eastern Dakotas into
Minnesota. Numerous thunderstorms are expected ahead of this
boundary for Friday night, with fairly progressive movement but
potential for training within a strengthening nocturnal low level
jet, out ahead of the front.

...Northeastern Gulf Coast to Coastal Mid-Atlantic...

Eastward movement of the mid-level trough over the Mid-Atlantic
states will allow the higher moisture along the coast to push
offshore but anomalous moisture will remain over Florida (1 to 2
standardized precipitable water anomalies). Sea breeze convergence
along the coast in the Southeast will allow for at least some
potential for flash flooding due to high rainfall rates from
thunderstorms with 2 to 3+ in/hr rates possible. Farther south, low
level easterly flow and a lingering surface boundary are expected
to focus convection from northern Florida into the western
Peninsula during peak heating hours. There will be spotty potential
for 3 to 5+ inch rainfall totals across the coast of the Carolinas
down into portions of northern Florida.

Otto


Day 3
Valid 12Z Sat Aug 09 2025 - 12Z Sun Aug 10 2025

...THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS
PORTIONS OF THE CENTRAL PLAINS INTO THE UPPER MIDWEST AND OVER THE
SOUTHWESTERN U.S...

...Upper Midwest...

A cold front will advance eastward into the Upper Mississippi
Valley on Saturday ahead of a strong mid-level trough moving across
the north-central U.S. Standardized precipitable water anomalies of
+2 to +3 are forecast by the 00Z GFS to be in place ahead of the
front over the Upper Mississippi Valley, supporting growing
instability with daytime heating. Unidirectional southwesterly flow
ahead of the front will allow for periods of training with
thunderstorms that form in the pre-frontal environment with
potential for multiple rounds of storms.

...Central Plains...

The cold front will move through the central Plains with low level
upslope flow in its wake over the High Plains region. Precipitable
water values are forecast to be near to slightly above average and
ascent will be aided by the region's placement within the
divergent and diffluent right- entrance region of the upper jet
located on the eastern side of the advancing upper trough axis.
Thunderstorms will likely shift east from eastern Colorado into
Kansas, meeting with an increasing low level jet of 40 to 50 kt
beyond 00Z Sunday. The 00Z model consensus is for an axis of
locally heavy rain stretching from eastern Colorado into Kansas and
southern Nebraska. Current thinking is for some 2 to 4 inch
rainfall totals, most of which could fall over a 2-3 hour window.

...Southwest...

Flow around a mid-level ridge centered over New Mexico should
prompt increasing moisture into the Southwest with precipitable
water values reaching near climatology for early August over
Arizona and New Mexico. Assuming a lack of widespread cloud cover
beneath the ridge should allow for sufficient CAPE generation by
mid-afternoon and typical monsoonal thunderstorm coverage by
evening. The concern for flash flooding will be enhanced by slow
storm motions given 00Z model forecasts of 850-300 mb mean wind
speeds of 10 kt or less.

Otto
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