FOUS30 KWBC 030809
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Mon Aug 3 2026

Day 1  Valid 12Z Mon Aug 03 2026 - 12Z Tue Aug 04 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NEW JERSEY TO
MASSACHUSETTS, FOR THE COASTAL CAROLINAS, THE BIG BEND OF FLORIDA,
AND FAR SOUTHERN ARIZONA...

...East Coast...

A shortwave trough rotating through the longwave feature will help
close off a mid-level low center over northern GA, driving subtle
height rises to its north, but with subtle impulses continuing to
rotate cyclonically around it. Meanwhile, a jet streak pivoting
into New England will intensify as it arcs poleward, leaving its
favorable RRQ for ascent over the Mid- Atlantic/Northeast. This
setup supports broad deep layer ascent that will help trigger
widespread convection capable of heavy rainfall intensities. The
slow nature of the front will help organize some convection along
it. This convergence combined with bulk shear of 20-25 kts will
lead to clusters/multi- cells that will train thanks to boundary-
parallel mean flow to lengthen the duration of heavy rain rates.
With both HREF and REFS probabilities indicating a 30-60% of 1"/hr
or more rates, where any convection can organize or train, this
could produce 3-5" of rain. The most likely locations for this
heavier rainfall will be from southern New Jersey to central
Massachusetts, which is where the Slight Risk was maintained.

...Southwest...

A shortwave tracking beneath the monsoon ridge will squeeze the
ridge causing briefing lowered heights over southern Arizona,
while the ridge itself expands a bit farther east into the Four
Corners and Central High Plains. Although instability will be
somewhat limited across the region, the areal coverage of
convection will be increased for this period. Higher concentration
of heavy rainfall is expected to focus over southern Arizona where
a plume of PWs exceeding +1.5 sigma will work together with MUCAPE
as high as 2000 J/kg and a pocket of enhanced bulk shear of 25-35
kts to organize thunderstorms with hourly rainfall reaching or
exceeding 1.5" (HREF/REFS/UA WRF). A Slight Risk was maintained for
southern Arizona with a Marginal Risk covers southern and central
portions of Arizona and New Mexico.

...Northern Plains...

Showers and thunderstorms are expected to fire up across the
Northern Plains as a potent shortwave tracking from Montana to
Minnesota drags a wave of low pressure and its associated cold
front across the region. A steady stream of moisture will be drawn
northward ahead of the approaching system and will aid in the
enhance of rainfall. Hourly rain rates have the potential (10-15%
chance) to reach 2"/hr, this training/repeating could produce as
much as 3-4" of rain, leading to FFG exceedance reaching 5-25% from
the HREF/REFS. A Marginal Risk covers the eastern Dakotas and
northern half of Minnesota.

Campbell/Weiss
 
Day 2  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...

Shortwave energy rotating around the base of the ridge will briefly suppress
it and cause lowered heights. The combination of this locally
intense ascent into favorable thermodynamics will again result in
scattered to widespread thunderstorms, with some organization
possible through 25-35 kts of bulk shear. Slow storm motions and
heavy rain rates above 1"/hr will produce at least a local flash
flood risk over vulnerable terrain features and burn scars from
southern Colorado through southern Arizona, with the greatest
concentration across south-southeast Arizona with a Slight Risk
area was maintained for this period.

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

Cool air aloft along with ample moisture and instability could lead
to hourly rain amounts up to 2.5 inches, which would be
problematic in rugged terrain, sensitive soils, and urban areas.
Recent rain will keep soils sensitive for potential flooding
issues. The Marginal Risk area was maintained across the eastern
Tennessee Valley/Southern Appalachians to the Mid-Atlantic coastline.

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

Showers and thunderstorms will fire up along and ahead of a slow
moving cold front sinking southward into the Central Plains and
extending into the Upper Midwest. PW values up to 1.75 inches will
be drawn into the system. Mean winds parallel to this front and a
shortwave tracking along it will help to spawn training
thunderstorms with rainfall rates of 1+"/hr, leading to a narrow
corridor of as much as 3" of rain which may result with instances
of flash flooding.

Campbell/Weiss
 

Day 3  Valid 12Z Wed Aug 05 2026 - 12Z Thu Aug 06 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID TO UPPER MISSISSIPPI VALLEY/MIDWEST...

...Mid to Upper Mississippi Valley/Mid West...

Heavy rainfall will continue to spread along and ahead of the
cold front sinking southeastward from the Upper MS Valley/Upper
Lakes region. The presence of a strong upper level jet within the
right entrance region will foster local enhancement of rainfall
rates potentially up to 2.5 inches/hour in proximity to the
boundary. The latest guidance continue to focus the highest axis of
QPF southeast Nebraska to southern Wisconsin/ eastward into
northern IL including the Chicago metro area.

...Southern to Central Appalachians into the Northern Mid-Atlantic...

Whatever is left of the upper level trough across the eastern U.S.
mid to late week should devolve into a shear line. An axis of high
PW values...1.5-1.75"+, will remain across much of the eastern
U.S. High PW values will continue to transport northward into the
region which will support locally heavy rain, with potential for
hourly rates reaching 2.5 inches/hour. A Marginal Risk area was
maintained across the Southern to Central Appalachians into the
northern Mid-Atlantic for this period. Much of this region will be
sensitive given multiple days of rainfall and lowered FFG values.

...Southwest/Southern High Plains...

A Marginal Risk was maintained to cover persistent monsoonal
activity across the Southwest and Southern Rockies where heavy
rain and isolated runoff issues in the above average PW values are expected.

Campbell/Roth
 

Day 4 and Day 5  Valid 12Z Thu Aug 06 2026 - 12Z Sat Aug 08 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST, EAST NORTHEASTWARD THROUGH THE CENTRAL/SOUTHERN PLAINS,
MID MS VALLEY, OH VALLEY, NORTHERN MID ATLANTIC AND ACROSS THE
CENTRAL TO SOUTHERN APPALACHIANS...

...Central Plains into the Lower Missouri Valley...
By Thursday...additional shortwave energy moving along the
southern edge of the Westerlies into the Central High Plains may
ignite another round of organized convection along the southward
moving cold front from KS into MO.  With low level southerly flow
expected to strengthen Thursday night into early Friday morning
into this front, locally heavy rains are possible. Given the
moisture available, hourly rain amounts to 2.5" should be possible.
The risk level near the front over the Central Plains toward the
Lower MO Valley was kept at Marginal for Thursday night/early Friday morning.

...Lower Lakes, OH Valley into the Northern Mid Atlantic...
A slow moving frontal boundary expected to persist across the
Lower Lakes into the Northeast both day 4 and 5. Shortwave energy
expected to push east both day 4 and 5 along the southern edge of
the westerlies across these regions, helping to accentuate lift in
the high PW axis in the vicinity of this front. There is fairly
good model qpf agreement on moderate to locally heavy precip totals
across these areas, supporting isolated runoff issues.

...Southern to Central Appalachians...
For both day 4 and day 5, a broad marginal risk is depicted
through the Central to Southern Appalachians for widespread
scattered convection. Not a lot of confidence in qpf details, but
with PW values expected to remain in the 1.5-1.75"+ range, locally
heavy rains and runoff issues possible.

...Southwest/Southern High Plains...
For mid to late week, a Marginal Risk was depicted across much of
the Southwest and Southern Rockies for monsoonal convection.
While there is continued low confidence with qpf details, locally
heavy rains and isolated runoff issues possible in the above
average PW values expected across this area.

Oravec
$$
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