FOUS30 KWBC 110748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
348 AM EDT Fri Sep 11 2026

Day 1  Valid 12Z Fri Sep 11 2026 - 12Z Sat Sep 12 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN FAR SOUTHERN
ARIZONA & NORTHERN MINNESOTA...

...Southern Arizona...

The upper level ridge over southern AZ will direct embedded 500mb
vorticity maxima westward along the US/Mexico border on Friday.
Much of AZ, southern UT, and western CO will continue to contend
with anomalous moisture content with PWs >1.5" throughout much of
southern AZ. Latest 00Z REFS/HREF probabilistic guidance does show
a section of far southern Arizona particularly at-risk for flash
flooding. The latest 00Z HREF shows moderate-to-high chances
(50-70%) for 3-hr QPF surpassing 3-hr FFGs around Santa Cruz
county Friday afternoon. For these reasons, the Slight Risk remains
on track for far south-central AZ.

...Upper Midwest...

A 500mb shortwave trough and an accompanying frontal boundary will
track through the Upper Midwest at the same time a plume of >90th
climatological percentile PWs manifests over the region. Storm
motions should be progressive, although some training thunderstorm
activity closer to the warm front is becoming inc as low pressure
heads for the MN Arrowhead. Rainfall totals could range between
1-2" (locally higher amounts up to 3" in northern MN, while
rainfall rates could approach 1.5"/hr as storms track across the
region Friday afternoon and evening.

...Lower MS Valley, TN Valley, Central Gulf Coast, and the
Southern Appalachians...

A 500mb shortwave trough tracking towards the Mid-MS Valley and a
sharp 200mb trough axis aloft will generate healthy divergence
over the Mid-South, while copious amounts of moisture reside over
much of the Southeast. A nearby frontal boundary will trigger
widespread thunderstorms that are likely to contain >2"/hr rainfall
rates within the more intense storms. Model guidance remains in
fairly good agreement on the widely scattered t-storm activity
before storms dissipate for most areas by late evening. PWs >2.0"
will be common from the central Gulf Coast to the mid-MS Valley,
while PWs in the OH/TN Valleys range between 1.75-2.0", which are
still above the 90th climatological percentile. Storms are likely
to form as far east as the Southern Appalachians and Carolinas,
although the flash flood threat is less likely along the Southeast
coast. Localized flash flooding is possible, especially in poor
drainage areas and within more urbanized communities.

Mullinax
 

Day 2  Valid 12Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026

...THERE ARE SLIGHT RISKS IN PORTIONS OF THE OHIO VALLEY AND MID-ATLANTIC...

...Ohio Valley & Mid-Atlantic...

The broad 500mb troughing setup in the Mid-South will induce
healthy upper-level ascent over the Ohio Valley and Mid-Atlantic
on Saturday. A warm front is forecast to lift north through the
Mid-Atlantic on Saturday that will act as a trigger for storms,
while daytime heating and height falls support widespread showers
and thunderstorms from the Gulf Coast to the northern Mid-Atlantic.
PWs will rise above 2.0" as far north as the I-95 corridor from
Richmond and D.C. to Baltimore and Philly by Saturday afternoon
with increasing instability as the warm sector drifts north. Warm
cloud layers are likely to be at least 12,000ft, favoring highly
efficient warm rain processes within developing thunderstorms.
Where the heaviest thunderstorm activity unfolds remains unclear.
However, the preference was to favor a Slight Risk along the I-95
corridor largely due to the greater concentration of metropolitan communities.

West of the Appalachians, soils throughout the OH Valley are
growing increasingly more sensitive with 1-hr FFGs hovering as low
as 1" over southeast OH. It will not take much from this favorable
atmospheric and hydrologic environment to support a concern for
scattered instance of flash flooding. For these reasons, a Slight
Risk was issued for portions of the Upper OH Valley for Saturday.

...Central Plains...

An approaching 500mb disturbance and a strengthening southerly
low-level jet will interact with a frontal boundary that results in
scattered thunderstorms from the central High Plains to the Mid-MS
Valley. PWs ranging between 1.5-1.75" are generally around the 90th
climatological percentile and ample instability will be supportive
of strong thunderstorms producing >2"/hr rainfall rates.
Guidance has trended lower in QPF in recent forecast cycles,
although the reduction in QPF is largely because model spread in
where the axis of heavier storms occur remains unclear. A Marginal
Risk remains in place given the favorable atmospheric setup with
localized flash flooding possible.

...Florida...

Much of the Sunshine State will remain mired in an environment
containing copious amounts of moisture that will continue to
support scattered thunderstorms containing torrential downpours.
PWs between 2.0-2.25" will be common and storms will be capable of
causing localized flash flooding, especially in poor drainage areas
and in the more urbanized sections of Florida.

...Rockies & Southwest...

A strong 500mb closed-low with heights below the 10th
climatological percentile will work with an increasingly anomalous
PW field to produce scattered thunderstorms with efficient rainfall
over the Northern Rockies. Farther south, the pool of anomalous PWs
that supported daily rounds of excessive rainfall in the Southwest
through the week will stick around and cause additional
thunderstorms that produce locally excessive rainfall rates.
Instability will be lacking over the northern Rockies, but soils
are more saturated following recent rainfall and may provide a
slightly more favorable environment for localized flash flooding.

Mullinax
 

Day 3  Valid 12Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026

...THERE IS A SLIGHT RISK OVER PORTIONS OF THE MIDWEST...

...Midwest...

To start Day 3 (Sunday) a frontal passage that is tied to a fast
moving 500mb shortwave trough heading for the Great Lakes will
trigger rounds of showers and thunderstorms from South Dakota and
the Nebraska Sand Hills to as far south as northern KS/MO and as
far east as the MS River. As the first shortwave trough exits
east, a strong 500mb closed low will head through the Northern
Rockies, prompting the development of low pressure in the northern
High Plains. This area of low pressure will work in tandem with
high pressure over the Great Lakes Sunday night to generate a 30-40kt
LLJ over the Central Plains. The LLJ will act to advect rich
moisture and warmer theta-e air into the Midwest with the frontal
boundary that passed through early Sunday morning lifting as a warm
front Sunday night and into early Monday morning.

Latest ECMWF guidance shows a pool of anomalous PWs (above the
97.5 climatological percentile) sitting over the Central Plains on
Sunday that lifts north into the Midwest Sunday night. With the LLJ
intersecting the frontal boundary, and elevated instability
available, additional strong thunderstorms are likely to form late
Sunday night and into early Monday morning. While exact placement
of the heaviest rainfall still differs, most guidance is in
agreement that the Midwest will be the focus for potential flash
flooding on Sunday and into Sunday night. The latest WPC QPF calls
for 1-2" of rainfall in central IA with locally higher amounts possible.

...Southern Rockies & Four Corners...

The troughing pattern in the Northwest and the mid-upper level
ridge axis over the Southern Plains will direct additional
monsoonal moisture into the Southwest and Southern Rockies. ECMWF
percentiles suggest PWs will rise to above the 97.5 climatological
percentile Sunday evening as IVT values >300 kg/m/s direct moisture
into the regions. Some locations continue to sport saturated soils,
thus making these areas more prone to potential flash flooding.
While confidence in higher rainfall rates is lower at the moment,
the rgion's sensitive soils and potential for excessive rainfall
rates >1/0"/hr support a Marginal Risk as of this forecast cycle.

Mullinax
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