FOUS30 KWBC 250828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sat Jul 25 2026

Day 1
Valid 12Z Sat Jul 25 2026 - 12Z Sun Jul 26 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE EASTERN
CAROLINAS...PORTIONS OF ARKANSAS...AND THE SOUTHWEST...

...Southeast through the Tennessee Valley...

Deep moisture will continue to pool along and near a stalled
frontal boundary, which leads to precipitable water values reaching
nearly 3 standardized anomalies above climatology for late July and
in excess of the 95th percentile of climatology extending eastward
from the Tennessee Valley to the Carolinas. Guidance from the
25/00Z model cycle continued to point to heavy rainfall potential.
Localized 2-4 inch totals with higher instantaneous rainfall rates
appear in this portion of the Slight Risk. Changes made to the
outlook areas were largely confined to the Carolinas and to ensure
that they capture the effects of shortwave energy crossing the area
later this afternoon and the lowering of mid- and upper-level
heights as the larger scale trough amplifies along the coast.

Over portions of Arkansas, the 25/00 UTC guidance backed away as
strongly and quickly as the previous run strongly and quickly
showed agreement concerning the potential for a heavy rainfall-
producing thunderstorm complex developing today. The HREF
neighborhood probabilities for 3-5 inches in 24 hours fell back to
35 percent or less ending at 12Z Sunday. The decrease was
comparable for hourly rainfall accumulations exceeding 2-3 inches
in an hour. Given the large magnitude of run to run
differences...the confidence to upgrade was gone. But the
ingredients for heavy rainfall rates and amounts remain present so
a Slight risk area was kept and realigned a bit to fit the HREF and
RRFS guidance.

...Southwest...

The monsoon will get re-invigorated across the Four Corners today
with an uptick in coverage and intensity of rainfall expected in
the region. QPF amounts did trend up overall and precipitable water
values still being forecast at or above the 90th percentile.
Convection developed in the overnight hours and guidance such as
the HRRR have picked up on an MCV in the vicinity that was
anticipated on Friday. It still appears that this becomes a key
player in enhancing the rainfall which leads to greater/higher
rain rates. Concern is heightened given the possibility that the
higher rainfall rates/amounts overlaps with areas which have
recently dealt with heavy rainfall and where soils will be less
capable of handling the influx of rain water. The Slight Risk was
maintained with some adjustments to the latest QPF and across west-
central NM, the risk remains skewed toward the higher end of the
Slight Risk probability range.

Bann/Taylor
 

Day 2
Valid 12Z Sun Jul 26 2026 - 12Z Mon Jul 27 2026

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS REGION...UPPER MIDWEST...AND PORTIONS OF FLORIDA...

...Four Corners States onto the Central Plains...

Monsoonal flow from the Desert Southwest to the central Rockies
continues into early next week. Precipitable water values and
available instability will again support isolated to widely
scattered thunderstorms capable of producing intense rain rates
capable of producing flash flooding. While future outlooks may need
to embed a Slight Risk, there was little in the way of consistency
in the details of the 25/00Z model cycle that helped define the
location of a Slight risk area.

...Upper Midwest-Great Lakes-Ohio Valley-Interior Northeast...

Global and AI guidance continue to show convectively-aided
shortwave activity riding down the top side of the upper ridge axis
over the western Dakota border in northwest flow. Any kind of a
mean QPF had too little QPF spread over a large area due to the
spread in QPF placement. A nudge towards Wisconsin made last
outlook still has some merit given where the axis of highest
precipitable water appears in the ECMWF and GFS. This can sometimes
signal a preferred track for storms that produce the heaviest
rainfall but concern is that a upper level convergence associated
with a broad arching anticyclonic flow at the jet level might
alter the location of convection. However...a Marginal risk
category was appropriate for the uncertainty and the ingredients
for heavy rainfall were present in the lower levels so only minor
modifications were made.

Bann/Taylor
 

Day 3
Valid 12Z Mon Jul 27 2026 - 12Z Tue Jul 28 2026

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL MONDAY ACROSS
THE FOUR CORNERS STATES EXTENDING INTO THE CENTRAL PLAINS...

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
CENTRAL AND EASTERN GREAT LAKES INTO THE OHIO VALLEY...

...Four Corners States into the Central Plains...

Elevated amounts of moisture will linger over the Four Corners
region given a persistent monsoon pattern...although guidance still
points to a gradual southward shift in the placement of the upper
high center. That high will be directing activity from the central
plains back toward the Intermountain region. Maintained a
Marginal risk area in/the typical terrain of western/northwest
Arizona and Southwest Utah and extending eastward as far as the far
western High Plains. Plots of ECMWF PW anomaly plots continue to
prove helpful in highlighting areas of increased concern.

...Great Lakes and Ohio Valley into the Northeast US...

Shortwave energy rounding the top of the High Plains high is
forecast to trek from Minnesota into parts of the Great Lakes by
the end of the day. Difluent flow ahead of the shortwave will
eventually lead to the development of a low pressure system late
in the period. This low is expected to help forcing for heavier
rainfall over the eastern Lakes/Ohio Valley. Given the progressive
convective activity...maintained the Marginal risk area and not
particularly close to upgrading with this outlook cycle.

Bann
 

Day 4 and Day 5
Valid 12Z Tue Jul 28 2026 - 12Z Thu Jul 30 2026

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS STATES EXTENDING INTO THE CENTRAL PLAINS, AND ACROSS THE
CENTRAL APPALACHIANS AND MID ATLANTIC TO THE NORTHEAST...

...Four Corners States into the Central Plains...

Monsoonal flow bringing elevated moisture over the Desert
Southwest through the central Rockies continues with a southward
trend in the upper high center directing activity back onto the
central Plains early next week. Areal extent appears to be more
limited on Day 5 per the guidance, including the latest EC-AIFS and
GFS and AIGFS, as the PW anomaly decreases a bit. The more
suppressed ridge will allow multiple mesoscale/convectively aided
shortwaves to push over the top/across the Rockies into the Plains.

...Central Appalachians-Mid Atlantic to the Northeast...

Upper level jet streak riding down the ridge on Tuesday (D4) will
lead to amplification of the longwave trough across the Eastern
U.S. by the middle-later part of next week. Increasing deep-layer
forcing ahead of this amplifying trough and strengthening surface
front will coincide with the uptick in pre-trough/pre-frontal
low-level theta-e advection, as the PW anomaly expands across the
Eastern Seaboard. Whenever favorable (typical) warm-season
thermodynamics (deep-layer instability and moisture) are present
ahead of a more robust, cooler-season trough with strong deep-
layer shear, chances for more widespread convection are much
higher. For now, will maintain a Marginal Risk in the Days 4-5 ERO
because the guidance continues to move the front through fairly
quickly, with a fairly quick transition to strong negative theta-e
advection behind it. Over the weekend and early next week we will
reassess the need for a targeted Slight Risk, especially once the
event moves into the high res (CAM) windows.

Hurley
$$
--- MultiMail/DOS
 * Origin: capitolcityonline.net * KY, USA (618:250/10)