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FOUS30 KWBC 040113
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
913 PM EDT Sun Aug 3 2025

Day 1
Valid 01Z Mon Aug 04 2025 - 12Z Mon Aug 04 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/GULF COAST...

...Southeast/Gulf Coast...

01Z Update...Pared the Slight and Marginal Risk areas quite a bit,
particularly the northern periphery, based on the latest
observational and mesoanalysis trends, along with the most recent
HREF (18Z) and RRFS (12Z) 1/3/6 hr rainfall exceedance
probabilities. Despite some deep-layer QG forcing along the tail-
end of the upper trough, weak kinematics (0-6km bulk shear
generally below 20 kts) and diminishing deep-layer instability
following sunset (with the weak mid-level lapse rates between
5.5-6.0 C/Km) will cause the convection to diminish in areal
coverage and intensity overnight.

Hurley

...Plains...

01Z Update...Only Minor changes were made to the Marginal Risk
area. As per the 16Z update and based on current satellite and
radar trends, additional MCSs are anticipated this evening as
vorticity impulses swinging through the mid-level W/NW flow
interacts with both a stationary front and an increasing LLJ late.
While any of these MCSs will contain heavy rain rates, they should
also remain progressive as they dive SE into the moisture, so
unless they overlap saturated soils from antecedent rainfall, the
flash flood risk appears more isolated than scattered. This is
reflected by HREF probabilities for 3-hr FFG exceedance that reach
only 5-10%, suggesting just small adjustments to the large MRGL
risk area are needed.

Relevant portion of previous discussion:
The stationary front stretching from Montana to Texas is expected
to weaken, leaving behind a weak convergence boundary as it
decays. Mid-level flow upstream of this feature will gradually
become more W/NW as it gets squeezed between an amplifying ridge
over the Southwest and a trough across Canada. Within this pinched
flow, multiple shortwaves/vorticity impulses will shift eastward,
interacting with the front to cause another day of convective
development across the High Plains and into the Plains, with
several MCSs possible beneath any of the stronger impulses.

The exact location and track for any of the MCSs that develop
remain uncertain however there is a decent signal for storms to
have rainfall rates of 1-2 inches/hour. While the CAMs are
generally suggestive of forward propagating features that will
limit the duration of heavy rainfall, locally any of these could
produce some short-term training, especially where any MCVs and
lingering boundaries can track.

Weiss/Campbell


Day 2
Valid 12Z Mon Aug 04 2025 - 12Z Tue Aug 05 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE
SOUTHEAST...

...Southeast...
An active and unsettled period continues across the Southeast as a
mid-level trough axis lingers to the west allowing for moisture to
spill northeast from the Gulf to encompass the region. During
Monday, the trough draped from the Great Lakes to the Arklatex will
weaken slightly as ridging from the Atlantic expands northwestward.
This will have little impact to the overall thermodynamic
environment which will remain favorable for heavy rain as PWs
remain 1.75 to 2.25 inches, highest within a narrow channel from
the FL Panhandle into coastal SC, overlapping MUCAPE that is
progged to rise above 1000 J/kg. Into these thermodynamics, forcing
will become enhanced as an upper jet streak intensifies to the
north leaving favorable RRQ diffluence overhead, while the surface
front remains draped across the region, and a shortwave ejects
northeast from the Gulf. Together, this will result in expanding
showers and thunderstorms Monday, with just a slight shift NW
expected in the axis of heaviest rain due to the expansion of the
mid-level ridging from the east.

The inherited SLGT risk appears reasonably well aligned with the
recent guidance so is maintained, although a subtle shift NW was
made to account for the new REFS/HREF probabilities for 3"/24
hours, the region of highest EFI, and both the WPC PQPF and CSU
UFVS first guess fields. Additionally, some of this heavy rain
could fall across areas that are becoming increasingly sensitive
due to recent heavy rain Saturday (and anticipated Sunday)
reflected by NASA SPoRT 0-10cm and 10-40cm RSM. With rainfall rates
likely eclipsing 2"/hr at times, any of this heavy rainfall
falling atop these vulnerable soils could quickly become runoff
leading to instances of flash flooding.


...Northern Plains...
Mid-level impulses lifting slowly northeast from the Northern
Plains into the Upper Midwest will weaken Monday. However, forcing
for ascent will continue, despite being in a weaker state, as PVA
from the accompanying vorticity maxima pivots across the area and
interacts with persistent WAA on 20-25 kts of 850mb southerly flow
demarcating the LLJ. This will push PWs to above 1.25 inches, or
around the 90th percentile according to NAEFS. MUCAPE will be
somewhat limited, but the same WAA could drive CAPE to above 500
J/kg, supporting HREF neighborhood probabilities for rainfall rates
above 1"/hr to 10-20%.

In general, cells should be somewhat progressive to the northeast
on 0-6km mean winds of 10-15 kts. However, a region of enhanced
bulk shear nearing 30 kts close to the MN/SD border will also be
within an area of deformation which could result in regenerating
and backbuilding convection. This will lead to slower net motion
and repeating heavy rainfall, which is likely to be the focus of
heaviest rainfall accumulation reflected by HREF 3"/24hr
neighborhood probabilities of 20% (displaced south of the REFS but
seemingly more likely in position). This could occur atop soils
that become primed by rainfall on Sunday, but the excessive rain
risk still appears MRGL so no upgrades appear needed at this time.


...Lower Ohio Valley...
An inverted surface trough will lift slowly northwest into the
lower Ohio Valley during Monday. Low-level convergence along this
boundary will combine with broad thickness diffluence and a
shortwave pivoting northeast to drive pronounced lift into a region
of high PWs (above 1.5 inches) and a collocated plume of MUCAPE
above 1000 J/kg. Very weak flow across this area suggests that as
convection blossoms, cells will move slowly, generally from south
to north, with repeating rounds of 1-2"/hr rainfall rates leading
to pockets of 3+ inches of rainfall. The MRGL risk was drawn to
cover the highest probabilities from the HREF and REFS for 3-hr FFG
exceedance probabilities above 15%.


...Northern Rockies...
A sharpening shortwave emerging from the Pacific Northwest will
advance into the Northern Rockies by the end of D2. The
amplification of this feature will result in impressive ascent,
primarily through height falls and increasing mid-level divergence,
helping to spawn a wave of low pressure tracking into the High
Plains by Tuesday morning. The response to this ascent will be
increasing showers and thunderstorms moving across Montana, with
low-level easterly flow wrapping greater moisture into the region
supporting rainfall rates that may exceed 1"/hr at times. Storms
should generally be progressive on 15-20 kts of mean 0-6km wind,
but 25-35 kts of bulk shear to help organize cells into clusters
could enhance the rain rates and temporal duration, at least
briefly, in some areas. This supports the inherited MRGL risk which
was tailored cosmetically for new guidance.


Weiss


Day 3
Valid 12Z Tue Aug 05 2025 - 12Z Wed Aug 06 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN THE SOUTHEAST...

...Southeast...
The pattern remains mostly unchanged going through Tuesday with a
ridge extending from the Atlantic across Florida and a trough
extending down into the lower Mississippi Valley. Between these two
features, moisture will continue to surge northeast beneath
shortwave impulses that are being squeezed by this synoptic
pattern. The result of this will be waves of convection lifting
northeast, with rainfall rates of 1-2" continuing to be supported
despite a modest forecast reduction in PWs.

There continues to be uncertainty into the latitudinal gain of
precipitation as the models feature a variety of speed solutions.
However, much of this area will be vulnerable due to several days
of heavy rain over mostly the same areas by the time Tuesday
arrives. This will enhanced the flash flood risk, so despite model
uncertainty leading to modest 24-hr rainfall probabilities, the
potential for training of cells across saturated soils has prompted
an expansion of the SLGT risk for D3.


...Northern Plains and Minnesota...
Mid-level impulse closing off over Saskatchewan will shed vorticity
downstream to drive height falls and PVA from Montana through North
Dakota and into Minnesota. This mid-level ascent will help deepen a
surface low moving across southern Canada, with triple-point
development/occlusion pushing a complex frontal structure
southeastward. Ascent across this area will impinge upon favorable
thermodynamics surging northward, especially in the vicinity of the
warm front, leading to widespread showers and thunderstorms
pivoting northeast through the day. With PWs progged to exceed 1.5
inches (between the 90th and 97th percentile according to NAEFS)
and MUCAPE reaching 1000-2000 J/kg, convection should support
rainfall rates of 1-2"/hr. The QPF output from the various models
features a variety of solutions with a considerable longitudinal
spread in footprint, and cells should be progressive to the
northeast. However, Corfidi vectors aligned increasingly anti-
parallel to the mean winds should promote backbuilding into the
higher instability which will allow for some training along the
frontal boundaries. The MRGL risk was adjusted to account for the
highest probabilities for 1" and 3" from the GEFS/ECENS ensemble
systems, and at least isolated flash flooding is possible.

Weiss


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