FOUS30 KWBC 040824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 AM EDT Tue Aug 4 2026

Day 1  Valid 12Z Tue Aug 04 2026 - 12Z Wed Aug 05 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN ARIZONA...

...Southwest/Southern High Plains...
The Slight Risk still looks reasonable across south-
southeast Arizona within a broader Marginal. Typical terrain
features and burn scars will be particularly vulnerable to flash
flooding concerns.

Weak energy aloft could undercut the broad Southwest upper ridge
to the south and provide support for enhanced convection Tuesday.
The peak instability of 1000-2500 J/kg MUCAPE will be in southern
Arizona. Above average moisture transport (4+ standardized
anomalies for 850-700 mb moisture flux from the GFS, 2+ from the
ECMWF) will be in place as well, allowing for heavy rain rates
above 1"/hour. CAMs including the Arizona WRF indicate slow
moving/westward moving convection potentially yielding increased amounts.

...Southeast to Southern/Central Appalachians into the Mid-
Atlantic States...

A wandering upper low over the Southeast/southern Appalachians
will be somewhat weak but still provide lower heights than normal
for early August and broadly support ascent. Large areas of
1000-2000 J/kg MUCAPE (locally higher) are expected in the
southeastern U.S., while above average PWs are expected at least in
eastern areas. Scattered convection is likely with high hourly
rain rates, so a Marginal Risk remains in effect for portions of
the Southeast, Appalachians, and southern Mid- Atlantic. Pockets of
wet antecedent conditions could exacerbate flooding concerns,
while rugged terrain and urban areas are also vulnerable.

...Central Plains into the Upper Midwest...

A moderately strong trough advancing through the northern tier will
send a leisurely cold front through the Midwest and Plains. Ahead
of this system moisture and instability will pool across the
region an when coupled with the right entrance region of the upper
level jet there will be enhanced ascent for rain/thunderstorms
development. The front's axis of orientation will generally be west
to east which should be supportive of training convection. Hi-res
guidance has a general consensus of 24 hour accumulations around 3
inches; however given the recent drier period the threat for
excessive rainfall and flash flooding impacts may be minimal thus
maintained a Marginal Risk.

Campbell/Tate
 

Day 2  Valid 12Z Wed Aug 05 2026 - 12Z Thu Aug 06 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS TO MIDDLE MISSISSIPPI VALLEY...

...Central Plains to Mid/Upper Mississippi Valley...

The upper trough and associated cold front described in the Day 2
discussion will reduce to a snail's pace while providing an axis
for moisture to pool and thunderstorms to focus. A Slight Risk for
an elevated threat for excessive rainfall was maintained given
some potential overlap with rain the previous day affecting
antecedent conditions. There has been more model spread in the
position of heaviest rain, but general consensus is along line from
northeast Kansas to the Chicago vicinity.

...Southern to Central Appalachians into the Northern Mid-Atlantic...
Whatever is left of the upper level trough across the eastern U.S.
should devolve into a shear line by midweek. However, a moist and
unstable environment (PWs greater than the 75th percentile in an
already moist season of the year) will continue and expand
northwestward as the upper energy shifts. Scattered rainfall with
high rain rates can be expected thus maintaining a Marginal Risk
for excessive rainfall and local flash flooding potential. Convection
may once again be enhanced over portions of the Florida Peninsula,
but given a likely rainfall focus over the less sensitive areas of
the Everglades westward, will not introduce a Marginal at this point.

...Southwest/Southern High Plains...
There will be a persistent influx of moisture along the southern
periphery of the Southwest upper high, although its been noted that
there is a less focused area for where higher QPF will occur. A
Marginal Risk remains for much of Arizona and New Mexico for
isolated flooding and runoff issues.

Campbell/Tate
 

Day 3  Valid 12Z Thu Aug 06 2026 - 12Z Fri Aug 07 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST THROUGH THE CENTRAL/SOUTHERN PLAINS, MIDWEST, & EAST...

...Central Plains into the Lower Missouri Valley...
Another round of organized convection is expected to fire up along
the southward moving cold front. During the overnight hours,
moist, southerly flow will be increasing over the frontal boundary
which will lead to locally heavy rainfall, potentially accumulation
around 2.5 inches.

...Lower Lakes, Ohio Valley into the Northern Mid-Atlantic...
A slow-moving frontal boundary expected to persist as shortwave
energy track east along the southern edge of the Westerlies,
helping to accentuate lift in the high PW axis in the vicinity of
this front. Moderate to locally heavy rain totals expected with
hourly rain amounts up to 2.5" possible, supporting at least
isolated runoff issues in urban areas and places with some degree
of soil saturation.

...Southern to Central Appalachians...
A broad Marginal Risk covers the Central to Southern Appalachians
for widespread scattered convection. Not a lot of confidence in
details, but with PW values expected to remain in the 1.5-1.75"+
range, hourly rain amounts to 2.5" could lead to runoff issues,
particularly in urban areas and within sharp relief.

...Southwest/Southern High Plains...
A Marginal Risk was maintained for much of the Southwest and
Southern Rockies for monsoonal convection. The ridge placement
across the southern Great Basin during the time frame along with
some backing off in the moisture return on the guidance led to some
reduction in the Marginal Risk area across southern Nevada. While
there is continued low confidence with details, locally heavy rains
with hourly amounts up to 2" could lead to isolated runoff issues.

Campbell/Roth
 

Day 4 and Day 5  Valid 12Z Fri Aug 07 2026 - 12Z Sun Aug 09 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE LOWER
MISSISSIPPI VALLEY, NORTHEASTWARD INTO NEW ENGLAND, ACROSS THE
UPPER MISSISSIPPI VALLEY AND OVER THE SOUTHWEST...

...Lower Lakes, Ohio Valley into New England...
An axis of high PW values...1.50-1.75"+ expected to continue to
stretch from the Lower MS Valley, northeastward into New England
in the southwest flow between the western Atlantic upper ridge and
the broad mean trof across the Great Lakes into the OH Valley.
Shortwave energy in this southwest flow will help accentuate lift
in this high PW axis, supporting widespread scattered convection
across these areas. There may be better organization to the
convective activity across the Lower Lakes/OH Valley region in the
vicinity of a slow moving frontal boundary expected to stretch west
southwest to east northeast across this area. The risk level was
maintained at marginal, but with FFG values relatively low across
the eastern Lakes/Northern Mid-Atlantic, an upgrade is possible,
especially as more hi res guidance becomes available.

...Southern to Central Appalachians...
A broad marginal risk is depicted through the Central to Southern
Appalachians for the last week period for widespread scattered
convection. Not a lot of confidence in details, but with PW values
expected to remain in the 1.5-1.75"+ range, localized hourly rain
amounts of 1-2"+ could lead to runoff issues, particularly in
urban areas and within areas of steep terrain.

...Upper Mississippi Valley...
Additional shortwave energy riding west to east along the southern
edge of the westerlies will increase convective activity day 5 in
the vicinity of a strengthening west to east oriented frontal
boundary stretching from the Northern Plains into the Upper
Mississippi Valley. The low level southerly to southwesterly flow
is expected to strengthen into this front, increasing PW values to
1.50-1.75"+ and supporting heavy to locally excessive rainfall
totals in what may be an area of organized convection. The
marginal risk area was drawn to encompass the current spread in
model qpf axes.

..Southwest...
No significant changes to the broad marginal risk area across the
Southwest for monsoonal convection both day 4 and day 5. While
there continues to be low confidence with qpf details, locally
heavy rains and isolated runoff issues will continue in the axis of
above average PW values expected across this area.

Oravec
$$
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