FOUS30 KWBC 011911
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
311 PM EDT Sat Aug 1 2026

Day 1
Valid 16Z Sat Aug 01 2026 - 12Z Sun Aug 02 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE UPPER
MIDWEST TO THE GULF COAST AND FOR NEW MEXICO INTO THE SOUTHERN
HIGH PLAINS...

...Upper Midwest to the Gulf Coast...
An anomalously strong upper low (NAEFS 500mb heights falling below
the 2.5 percentile, 850mb heights to below the climo minimum) will
pivot slowly eastward today, reaching central Indiana by the end
of the forecast period. This closed low will drive pronounced
height falls and mid-level divergence for ascent across the area,
while the accompanying occluded surface low and trailing cold front
push into the Ohio and Tennessee Valleys. Downstream of these
features, robust thermodynamics will be drawn northward from the
Gulf, resulting in a corridor of MUCAPE above 1000 J/kg overlapped
with PWs of 1.75 to 2.00 inches extending from the Florida
Panhandle all the way into Indiana.

Within this broad plume of impressive thermodynamics, all convection
which develops ahead of the system will have the potential to
produce excessive rain rates. This is supported by both HREF and
REFS neighborhood probabilities that indicate a 30-70% chance of
at least 1"/hr rates within a broad area where thunderstorms are
expected today. With mean 0-6km winds aligned to the front,
especially from Florida through Kentucky, at least short duration
training is expected, and this results in notable spatial coverage
of probabilities above 40% (almost continuous) for at least 3" of
rainfall today from Florida to Illinois. Some of this will be
moving across vulnerable soils due to natural sensitivity of
terrain features or from recent heavy rains, but within this
broader area there are two focused regions with a slightly higher
threat today for flash flooding.

1) Illinois into Indiana: Here, an impressive deformation axis will
setup N/NW of the mid-level low as it pivots E/SE, and will drive
ascent into an impressive (especially for August 1) TROWAL as the
accompanying theta-e ridge pivots cyclonically around the system.
This will result not only as a region of increased instability and
moisture, but also a greater training risk as propagation vectors
and mean winds become increasingly anti-parallel. This suggests a
high threat for backbuilding and repeating cells, through which the
1-2"/hr rain rates could cause locally more than 5" of rain (30-50%
chance). Although the SBCAPE may be somewhat muted here, the
elevated CAPE should offset that limitation, and an enhanced slight
risk continues with only modest cosmetic adjustments.

2) Florida Gulf Coast and Panhandle: Impressive thermodynamics
acted upon by a notable shortwave digging across Alabama will spawn
multiple rounds of thunderstorms today producing rainfall rates as
much as 3"/hr. Storms will likely develop over the Gulf through
today and then train to the E/NE moving shore the central/eastern
Florida Panhandle before lifting northeast into southern Georgia.
The cold front slowly approaching from the NW will additionally
contribute ascent and increase shear, so organization into clusters
is likely, and this could result in 5+" of rain, with locally up to
8" possible (10-20% chance). Although there is some uncertainty in
placement of the heaviest rain among the various deterministic
models, both the HREF and REFS have the greatest signal from the
Emerald Coast through the Nature Coast, and then just inland.
Although there is consistency in this ensemble signal, the
variability in the guidance across a region that has some of the
highest FFG in the country, suggests the current SLGT risk remains
the correct level for this event, but after coordination with WFO
TAE a targeted enhanced slight was added where heavy rain is
ongoing and both the REFS and HREF focus the greatest 24-hr rain
probabilities.

...Southern Rockies/High Plains to the Texas Hill Country...
Monsoon ridge gets suppressed to the SW even further today as the
anomalous closed low and longwave trough dig across the
Mississippi Valley. This re-structuring of the ridge will squelch a
lot of monsoon activity today, but weak shortwaves rotating around
the ridge and south/southeast into NM/AZ will provide sufficient
ascent for convection this aftn/eve. At the same time, a cold front
dropping through the middle of the country will bank into the High
Plains of NM, providing an additional focus for ascent through
accompanying upslope flow. Together, this will create scattered to
widespread thunderstorms for eastern NM.

This activity will likely begin on higher terrain, typical, but
increased effective bulk shear (30-40 kts by this aftn) will help
organized cells into clusters which will then drop steadily
southward on mean 0-6km winds of 10-15 kts. Despite the steady
progression of cells today, hourly rainfall may exceed 1" in places
(supported by HREF, REFS, and the UA WRF), which could cause
excessive rainfall leading to flash flooding, especially atop
vulnerable terrain, burn scars, or urban areas. The greatest
coverage of activity is expected in eastern NM today where the
inherited SLGT risk was only cosmetically adjusted.

The other area of interest is associated with an increasing signal
for heavy rainfall for portions of West Texas into the Hill
Country. Thunderstorms developing along the southward sagging cold
front will tap into robust thermodynamics characterized by PWs of
1.75 to 2.00 and MUCAPE of 2000 J/kg. This will be most impressive
in a narrow channel immediately ahead of the cold front, and will
support heavy rainfall rates of 2"/hr or more. The high-res CAMs
have become more aggressive with convective coverage today, and
although cells should move steadily S/SW, any short term training
through modest shear along the front could produce 2-3" of rain
with locally higher amounts. Considered an upgrade to a SLGT risk,
especially noting the continued elevated 0-40cm soil moisture and
general sensitivity of this region, but after coordination with
the impacted WFOs, and due to uncertainty in storm coverage, opted
to maintain the MRGL risk at this time.

Weiss


Day 2
Valid 12Z Sun Aug 02 2026 - 12Z Mon Aug 03 2026

..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA BIG
BEND REGION AND THE EASTERN GREAT LAKES...

...East Coast States...
Widespread showers and thunderstorms are expected from the Gulf
Coast of Florida through the Mid-Atlantic States, and into the
interior Northeast on Sunday.

An anomalously strong upper closed low (NAEFS 500mb heights down to
the 1st climatological percentile and 850mb heights below the
climatological minimum) will be positioned over IN Sunday morning.
This feature will begin to open during D2 as a piece of vorticity
energy swings through the base, but this will result in a still
highly amplified/short wavelength trough digging all the way to the
Gulf Coast. This evolution will help strengthen a meridionally
arcing jet streak into the Northeast and Canada such that the
favorable diffluent RRQ will overlap the broad height falls
downstream of the opening trough. Together, this will create a
large expanse of impressive synoptic ascent from the Gulf Coast
all the way into New England.

This ascent will act upon impressive thermodynamics as tropical
moisture gets drawn northward ahead of the system. PWs as high as
2.25" are progged to flood northward as far as Massachusetts, with
still impressive PWs above 1.75" as far west as the St. Lawrence
River Valley. Together with a collocated plume of 1000+ J/kg
MUCAPE, this will provide an environment for which thunderstorms
will be widespread, and create numerous 1-2"/hr rain rates.
Generally fast 0-6km mean winds of 20-30 kts should limit the
duration of any individual cell in most areas, but this flow will
additionally be aligned to an approaching cold front beneath the
upper trough, suggesting the potential for repeating/training of
cells. This has prompted a large MRGL risk expansion to encompass
most of the eastern states, with isolated flash flood potential
beneath any of these heavier rain rates.

However, there appear to be two area of a more focused threat. The
first is across the Florida Gulf Coast/Big Bend and just inland
where heavy rain will occur again after heavy rain that is
currently falling during D1. An additional 3-5" of rain is possible
(30-50% chance of at least 5") due to backbuilding of cells across
the Gulf repeating onshore with 2-3"/hr rates. Within this region,
an enhanced slight risk was added for Taylor, Dixie, and Lafayette
counties which is where the 2-day rainfall is expected to be
heaviest.

Another area of locally higher excessive rainfall risk exists along
the eastern Great Lakes from around Cleveland, OH through Buffalo,
NY and into the Northern Adirondacks. While instability may be a
bit less here, this is also the region where a pivoting
deformation axis and accompanying mid-level TROWAL may exist north
of the closed low. This will produce some higher thermodynamics
into which storms, which will be slow moving and training, can
repeat with 1-2"/hr rain rates. The inherited SLGT risk was
cosmetically adjusted for new guidance, and the urban areas will be
most susceptible to impacts on Sunday.

..Southwest...
The monsoon ridge begins to expand back to the east once again on
Sunday, but activity is still expected to be suppressed to just
southern AZ/NM. This will be partially due to a shortwave rotating
anticylonically around the base of the ridge to enhance ascent,
impinging into areas with favorable thermodynamics characterized by
PWs of 1,25 to 1.5 inches, and MUCAPE above 1000 J/kg. The
challenge Sunday is that it appears there will be some drying that
occurs from NE to SW as a decaying front drops across Texas,
followed by a weak surface trough as well. This suggests that
during peak heating, there will actually be somewhat of a
deterioration of the environment, further limiting convective
coverage. Still, scattered thunderstorms are likely for the
southeast portion of AZ, with some more isolated activity as far
east as the Sacramento Mountains of NM. With sufficient bulk shear
of 20-25 kts for modest organization, any of these storms could
produce locally 1"/hr rain rates and instances of flash flooding.

Weiss


Day 3
Valid 12Z Mon Aug 03 2026 - 12Z Tue Aug 04 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE TRI-STATE
AREA OF NEW YORK, NEW JERSEY, AND CONNECTICUT..

...United States East Coast...
The impressive upper trough/low moving across the Ohio Valley is
progged to cutoff once again as it digs into the Southeast by
Tuesday morning. Downstream of this anomalous feature, robust PWs
of more than +2 sigma will flood the Atlantic Seaboard, combining
with a ribbon of elevated instability within the deep layer WAA to
produce an environment supportive of heavy rainfall for nearly the
entire coast. While specific locations for heavier rainfall and a
greater excessive rainfall risk are difficult to discern at this
time range, it appears nearly any location that received repeating
showers and thunderstorms with 1-2"/hr rain rates could experienced
impacts, so the inherited MRGL risk was adjusted for newer
guidance.

The narrow SLGT risk for the Tri-State area of CT/NJ/NY was
curtailed a bit on the SW side (closer to Philadelphia, PA) due to
a slightly more progressive/eastward signal in the QPF from the
guidance. Did not remove the SLGT risk all together due to
extremely vulnerable soils from recent heavy rainfall in this area,
but if the trends continue for lesser/faster QPF, this SLGT may
also be removed with later updates.

Additionally, there may be the need for a small SLGT across the Big
Bend of Florida where additional heavy rain may pivot onshore
Monday after 2-days of heavy rain this weekend. However, there is
just enough uncertainty as to the amount of rain lingering D3 that,
after coordination with WFO TAE, opted to maintain just the MRGL
risk for now and re-evaluate the need for an upgrade with later
forecasts.

...Southwest...
Convection associated with the monsoon will become even less
suppressed Monday as the ridge expands even farther east, and weak
vorticity impulses rotate around it. Moisture and instability will
gradually lift northward during this time, allowing for an
expansion north of an environment supportive of heavy rain. The
limiting factor Monday appears to be instability moreso than
moisture, so the inherited MRGL risk was curtailed a bit from the
north, but at least isolated instances of excessive rainfall
leading to flash flood related impacts are expected for parts of
AZ/NM.

...Northern Plains...
A potent shortwave diving along the Canadian/United States border
will race eastward Monday bringing height falls into the Dakotas
and Minnesota. Downstream of this feature, brief but pinched SW
flow will draw moisture and instability northward to create an
environment supportive of heavy rainfall, into which a cold front
beneath the shortwave will create enhanced ascent for convective
development. Strong 0-6km mean winds suggests storms will move
rapidly northeast ahead of the front, but with flow aligned
parallel to the boundary, at least short duration training could
occur. As rainfall rates climb to 1+"/hr, any training could create
locally excessive rainfall as reflected by the ECMWF EFI reaching
above 0.7 coincident with low-end probabilities for 3"/24 hr
rainfall.

Weiss


Day 4 and Day 5

Valid 12Z Tue Aug 04 2026 - 12Z Thu Aug 06 2026

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, PLAINS, MIDWEST, & EAST...

...Upper OH Valley/Cumberland Plateau/East...
There has been a substantial change to most of the model guidance
at 12z, with the UKMET holding serve with a more northern solution
to the mid- to upper-level low while the other guidance has trended
southwest. Conceptually, the non-UKMET guidance makes the most
sense based on the position/strength of the ridge forecast to move
over the top of the feature. However, the concern is sensitivity
over portions of NY from recent heavy rains. Taking into account
continuity, NY's sensitivity, and the southwest trend of the
guidance led to a bit of southwest broadening of the Marginal Risk
area. No matter where the system is, some cool air aloft along with
ample moisture and resultant instability could lead to hourly rain
amounts up to 2.5", which would be problematic in rugged terrain,
sensitive soils, and urban areas. Should the UKMET shift southwest
towards the remainder of the guidance later, we can shave off the
northeast side of the Marginal Risk area at that time.

...Southwest/Southern High Plains...
For both Tuesday and Wednesday, a marginal risk was depicted for
monsoonal convection across southern Arizona into New Mexico. Not a
lot of confidence with qpf details here, but locally heavy rains
and isolated runoff issues possible in the above average PW values
expected across this area.

...Central Plains into the Mid to Upper MS Valley...
A surface frontal boundary is expected to be slow-moving Tuesday
and Wednesday from the Central Plains, east- northeastward into the
Mid to Upper MS Valley into the Great Lakes. This front will be
situated along the southern edge of the westerlies where shortwave
energy pushing east northeastward near the front will support the
potential for organized convection both days. PW values expected
to be slightly above average...1.5 to 1.75"+ in the vicinity of
this front, supporting the potential of hourly rain amounts to 2.5"
where convection does organize and isolated runoff issues. For the
moment, coverage/amounts appear shy of a Slight Risk, so let the
Marginal Risk continue from continuity.

Roth


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt



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