FOUS30 KWBC 100751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Thu Sep 10 2026

Day 1
Valid 12Z Thu Sep 10 2026 - 12Z Fri Sep 11 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE UPPER OHIO
VALLEY INTO THE CENTRAL APPALACHIANS...

...Ohio Valley & Central Appalachians...

A 300mb jet streak placing its diffluent right-entrance region over
the Ohio Valley and Central Appalachians paired with an
approaching cold front will foster a favorable environment for
developing thunderstorms. PWs within the Slight Risk will contain
anomalous PWs of at least 2.0", which are approaching the 99th
climatological percentile. In addition, MUCAPE >1,000 J/kg will
also be available within a highly saturated environment to support
between 2-3"/hr rainfall rates. Area-averaged soundings show mean
850-300mb winds are generally out of the WSW, and would parallel
the frontal boundary as it moves east, indicating there could be
potential for training. Soils have been drier of late, evident by
NASA SPoRT-LIS 0-10 and 0-40cm soil moisture percentiles. Still,
1-hr FFGs are generally around 2" within the region and given the
potential for training storms, scattered instances of flash
flooding are possible.

...Mid-Atlantic...

Some of these storms in the Central Appalachians and Ohio Valley are
likely to spill over the central Appalachians and track across
northern VA, northern MD, and the Mid-Atlantic I-95 corridor
Thursday evening. Storms should be progressive, but the same
anomalous moisture content will move into the region and some
training could occur. There is also westerly low-level wind field,
which could limit areas flash flood potential given downsloping
effects. Localized flash flooding cannot be ruled out, given the
potential for training storms and storms tracking into a higher
concentration of urban/hydrophobic surfaces.

...South-Central Plains & Mid-Mississippi Valley...

A passing cold front and a pool of anomalous PWs for mid-September
(1.8-2.0" PWs) will be available for thunderstorms as the develop
during the afternoon and linger into Thursday night. Plenty of
instability will be available (1,000-2,000 MUCAPE per 00Z HREF) to
support thunderstorms producing 2-3"/hr rainfall rates. FFGs are
higher in these areas, and given storms will also be generally
progressive, the flash flood threat will remain isolated. A
Marginal Risk for flash flooding remains in place in these regions.

...Southwest...

A Marginal Risk remains in place over portions of AZ/NM and as far
west as the Peninsular Range of southern CA. For southeast AZ and
much of southern NM (along and west of the Sacramentos), the 500mb
ridge axis remains located over central AZ will contain spokes of
500mb vorticity maxima that revolve around its southeast flank,
providing some weak upper-level support aloft. PWs remain elevated
for mid-September with >90th climatological percentile values
along the AZ/NM borders with Sonora and Chihuahua. Farther west,
PWs are above the 97.5 climatological percentile in southern CA
with strong daytime heating causing diurnally-driven thunderstorms
along the higher terrain of the Peninsular Range. Low-chance
probabilities (10-30%) remain present on HREF/REFS guidance there,
so maintained the Marginal Risk for Thursday where areas of complex
terrain have localized flash flood potential.

Mullinax
 

Day 2
Valid 12Z Fri Sep 11 2026 - 12Z Sat Sep 12 2026

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

...Southern Arizona...

The ridge over southern AZ will be remain in place, although small
lobes of 500mb vorticity will move east to west 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"
likely throughout much of southern AZ that supports a localized
flash flood threat. REFS/HREF probabilistic guidance does show a
section of far southern Arizona particularly at-risk for flash
flooding. The HREF most notably shows moderate chances (40-50%) for
3-hr QPF surpassing 3-hr FFGs around Santa Cruz county Friday
afternoon. These signals in guidance support an upgrade to a Slight
Risk in far southern 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 manifest over the region. Storm
motions should be progressive, although some training thunderstorm
activity closer to the warm front is possible as low pressure
heads for the MN Arrowhead. Rainfall totals could range between
1-2" (locally higher) 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 into the Southern Appalachians...

A 500mb shortwave trough tracking towards the Mid-MS Valley and a
sharp 200mb trough axis aloft is set to generate healthy divergence
aloft over the Mid-South, while copious amounts of moisture
continue to 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. 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 3
Valid 12Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026

...THERE IS A SLIGHT RISK IN PORTIONS OF THE CENTRAL PLAINS...

...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.
Uncertainty exists in the placement of the heaviest rainfall, but
the signal supports a scattered flash flood threat Saturday
afternoon and into Saturday night.

...Southeast & Mid-Atlantic...

The broad 500mb troughing setup in the Mid-South will induce
healthy upper-level ascent over the Southeast 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 to above 2.0" as far north as the DC/Baltimore area by
Saturday evening with no shortage of instability as the warm sector
drifts north. QPF has been increasing across most guidance,
although the location of the heaviest rainfall is still unclear.
This setup is ideal for a localized flash flood threat,
particularly in the more urbanized sections of the Mid-South and Mid-Atlantic.

...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
$$
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