FOUS30 KWBC 110851
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
451 AM EDT Sat Jul 11 2026

Day 1  Valid 12Z Sat Jul 11 2026 - 12Z Sun Jul 12 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
ARIZONA AND THE SOUTHERN PLAINS TO THE CENTRAL APPALACHIANS...

...OK into the OH and TN valleys...
An active convective regime is anticipated ahead of a
southeastward moving shortwave trough axis extending from MO to OH.
At 12z this morning, an initial round of convection will be
ongoing from southern IL into portions of KY and TN. Model
guidance supports a transition toward a primarily forward
propagating convective mode this morning, which should place the
overall flash flood risk on a declining trend, though localized
upwind propagation may keep an isolated threat going.

By this afternoon, the primary MCV/shortwave will shift over IL
and IN, interacting with a southward moving frontal boundary to
trigger scattered redevelopment across portions of IL, IN and WV.
Within this zone, PW values near or just under 2" will overlap with
CAPE of 1000-2000 J/kg. While widespread convective organization
is not anticipated, slow cell motions and localized mergers near
the frontal zone will support heavy rainfall rates capable of
producing localized pockets of 2-3" of rain. This is supported by
the 00z HREF and REFS, which both show FFG exceedance
probabilities of 15-40%.

A more organized, and potentially higher end, flash flood threat
is expected to evolve later this afternoon into tonight across
portions of KY and TN. Convection should develop to the south-
southeast of the main shortwave/MCV, likely focusing along remnant
outflow boundaries and instability gradients left behind by the
morning activity. Global deterministic and AI models favor a heavy
rainfall axis focused primarily from KY into northeast TN,
whereas high res models (including the RRFS/REFS) suggest the core
threat may track further south into TN. Given the strong westerly
low level moisture transport south of the MCV, the environment
will be favorable for backbuilding and training convective clusters
tonight. This environment, combined with PWs hovering around 2",
suggests that many HREF members may be underdoing the
afternoon/nocturnal threat. Do expect to see a swath of heavy
rainfall materialize somewhere across the KY/TN corridor, where an
axis of locally higher end flash flood impacts is possible. A
higher end Slight risk is maintained, with a MDT risk upgrade not
out of the question if we gain spatial model convergence and more
confidence on exact convective mode.

Further west, isolated to scattered flash flooding is also expected
across portions of central OK into AR. Low level flow will become
convergent on the southwestern flank of the broader convective
regime. Slow cell motions near a boundary will pose a threat over
OK, while enhanced northerly flow on the western periphery of the
MCV/shortwave will converge with the broad south-southwesterly
inflow to promote some backbuilding/training threat over parts AR.

...Mid-Atlantic...
A localized flash flood threat is forecast across portions of the
Mid-Atlantic. While low level flow will be northerly, one or two
low level convergence axes are projected to setup across the
region, potentially acting as a focus for convection. Despite the
northerly flow, ample moisture and instability will remain in
place. Convective cells will generally track from north to south,
but localized repeating cells will be possible along the trough
axis. The strongest signal for localized 3" totals with the HREF
resides along the coastal plain from NJ southward into southeast
VA where convergence is enhanced. Because FFG values are generally
higher here compared to inland locations, think the overall flash
flood risk should stay isolated, though localized urban and low
lying flood impacts are expected.

...AZ and NM...
The persistent terrain driven pattern continues across the
Southwest, with convection developing over the higher elevations
before propagating southwestward into a pooling instability axis
over the lower deserts of southern AZ. Instability and moisture
profiles should be a bit higher today relative to the past few
days, leading to a corresponding increase in the overall flash
flood risk. Both the 00z HREF and REFS depict greater convective
coverage and higher localized rainfall totals. While fast
southwestward storm propagation and cell speeds may act as a
limiting factor, the moisture/instability footprint should be
sufficient to promote some cell mergers and localized 1-2"/hr
rainfall, justifying the maintenance of the Slight risk.

A Marginal risk has been added to portions of eastern NM. This area
received locally heavy rain yesterday, and the model QPF signal is
a bit stronger today within an environment characterized by
slightly higher moisture and instability. Convection should remain
isolated to scattered, with the primary flash flood threat focused
over recent burn scars, particularly across the Sacramento Mountains.

...TX...
A Marginal risk was added to portions of the TX Hill Country, where
PWs are forecast to climb above 2". Isolated to scattered
convection is expected within a broad southerly low level flow
regime. While most cells will remain transient and produce only
brief downpours, any cells that establish mesoscale organization
could briefly train or backbuild into the low level inflow.
Guidance suggests a weak MCV currently south of the Rio Grande
that may track northward towards the region later today, which
could provide an additional focus for ascent. The risk area aligns
closely with the HREF and REFS 1" EAS probabilities of 10-20%
(signaling organization potential), and 3" neighborhood
probabilities exceeding 40%. Global deterministic and AI suites
also show a distinct convective focus over this corridor.

Chenard
 

Day 2  Valid 12Z Sun Jul 12 2026 - 12Z Mon Jul 13 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
EASTERN KENTUCKY AND TENNESSEE INTO THE CAROLINAS..

...Southeast Quarter of the Country...
A Slight risk has been introduced stretching from KY and TN
eastward into southern VA and the Carolinas. A well defined
shortwave/low over the KY/TN vicinity will interact with a slow
moving backdoor front to focus organized convection. High res
guidance, including the RRFS, FV3 and 3km NAM, indicates a
concentrated axis of convection capable of producing localized
totals exceeding 3". While the global GFS and ECMWF are less
pronounced with their QPF swaths the GEM regional and UKMET are
more in line with the high res guidance. The AIFS and AIGFS also
depict a corridor of enhanced QPF that generally matches the Slight
risk placement. Beneath the closed low over KY and TN, convection
may be less organized, but slow moving. Given PWs near 2" and the
possibility of saturated soils from day 1 rainfall, a flash flood
risk should continue. Across the central and eastern Carolinas PWs
over 2" combined with low level southerly moisture transport and
backdoor frontal convergence should create an environment for high
rainfall rate driven flash flood potential.

Surrounding the Slight risk, a broad Marginal encompasses eastern
NM, TX and much of the Southeast. A southward advancing cold front,
the aforementioned mid level shortwave/low, and upper jet support
will provide the necessary synoptic forcing for scattered
convection. PWs over 2" and CAPE of 2000-3000 J/kg where daytime
heating is maximized, will support heavy rainfall rates. Some
mesoscale uncertainty exists with the track and strength of the
shortwave and frontal positions. An embedded Slight risk or two
may be needed on future updates as confidence increases. Increasing
low level westerly flow along the central Gulf Coast provides a
signature for backbuilding and training, but model divergence on
the exact axis remains. Given the higher regional FFG, did not
want to go with an overly large Slight risk at this time.

...Southwest...
Monsoonal moisture and instability will expand northwestward on
Sunday, necessitating a broader Marginal risk across AZ. While an
embedded Slight risk may eventually be needed, lingering questions
regarding storm coverage and organization keep the risk at
Marginal for now. Current indications suggest that while robust
convection will initiate over the terrain, it will propagate
westward quickly, potentially limiting the threat to isolate
instances of flash flooding.

Chenard
$$
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