FOUS30 KWBC 140802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Mon Jul 14 2025

Day 2
Valid 12Z Tue Jul 15 2025 - 12Z Wed Jul 16 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA
PENINSULA, PARTS OF THE MID-ATLANTIC, THE UPPER MIDWEST, AND SOUTHEAST ARIZONA.

...Florida...
Mid-level trough and possibly an accompanying surface reflection
will move across the FL Peninsula Tuesday and emerge back over the
northern Gulf by Wednesday morning. This feature will provide
sufficient ascent for widespread convection across the state, and
with PWs likely hovering around 2.25 inches combined with tall-
skinny CAPE profiles and MUCAPE above 1000 J/kg, rainfall rates of
2-3"/hr are likely (>50% chance from the HREF). In the presence of
the strong forcing accompanying this wave, and within the robust thermodynamics, convection will once again be widespread across the
area, both near the mid-level center, but also in surrounding
convergence bands as 850mb flow of 10-15 kts pivots to become more
E/NE through the day, before eventually shifting again to the south
as the vorticity swings into the Gulf. Storm motions will generally
be slow and chaotic, driven by storm interactions and boundary
collisions, leading to total rainfall on D2 that could reach 3-4",
highest along the W and SW coast, and the inherited SLGT risk was
only adjusted cosmetically for the recent guidance.

...Northern Rockies through the Upper Midwest...
A compact but potent shortwave and associated vorticity maxima will
drop across Montana Tuesday while it becomes embedded in pinched
westerlies draped from MT into the Great Lakes. This increasingly
progressive flow will shed spokes of vorticity eastward across the
region, helping to push broad height falls into the Northern Plains
and driving a cold front south and east. The interaction of these
vorticity spokes with the low-level baroclinic gradient will yield
waves of low pressure developing along the boundary, to enhance the
slowly intensifying synoptic ascent. Additionally, as the front
drifts southward, it will gradually encounter a more intense LLJ at
850mb, with speeds rising to 20-25 kts from NE/SD into MN, leading
to modest isentropic ascent but stronger convergence as post
frontal winds surge out of the N/NE at 25 kts as well. This will
result in widespread showers and thunderstorms from MT all the way
to MN, with rainfall rates of 1-2"/hr likely at times within the
anomalous PWs (broadly above the 90th percentile from NAEFS) and
increasing MUCAPE, especially Tuesday night when it eclipses 2000
J/kg across MN.

While there will be widespread rain and thunderstorms across much
of this area, the excessive rainfall risk appears higher across eastern/downstream portions of the region. While the ECMWF EFI
across MT is impressive (>0.9 in many areas), instability there is
quite weak, suggesting more widespread stratiform rain with just
modest embedded convective elements. Downstream from NE into MN,
MUCAPE surges Tuesday night which will support the more intense
rain rates, and the excessive rainfall risk is enhanced by 0-6km
mean winds that are aligned parallel to the front, and in a region
of 25-35 kts of bulk shear. This suggests that storms will become
more organized in this region, with some training likely from SW to
NE to enhance the rainfall potential. FFG exceedance probabilities
are modest from the HREF, likely due to still temporal and spatial
variation among the various high-res CAMs, but the inherited SLGT
risk remains with just some cosmetic adjustments to match the
higher 24-hr neighborhood rainfall probabilities.

...Northeast, Mid-Atlantic and Southeast...
A moist and unstable environment will persist from New England
southward along most of the Atlantic Seaboard downstream of a
decaying front that is expected to weaken and dissipate Tuesday
from New England into the Central Appalachians. This front will
provide a focus for convective development, with ascent across the
region aided by subtle impulses/ripples moving within the flow and
around a ridge centered off the coast. Broad SW return flow around
this ridge will pump plentiful moisture (PWs above 2 inches) into
much of the area, although a sharp gradient will exist along and
north of the front into New England, with MUCAPE during peak
afternoon heating cresting over 1000 J/kg.

The front and the multiple shortwaves traversing the flow and
impinging into the favorable thermodynamics will support widespread
convective development which is reflected by the simulated
reflectivity in the available guidance. A lack of bulk shear will
keep storms generally of the pulse variety with limited temporal
lifespans, but local enhancements are expected along storm
mergers/boundary collisions, which will support rainfall rates of
2-3"/hr at times as warm-rain processes dominate within warm cloud
depths above 15,000 ft. Storm motions will be slow and chaotic,
additionally enhancing the temporal duration of heavy rain, and
some short duration training or locally enhanced organization is
possible from the Southern Appalachians into the Mid-Atlantic
states due to upslope flow/isentropic ascent. This region is also
the most vulnerable to rapid runoff due to recent rainfall pushing
0-10cm soil moisture to above the 95th percentile according to NASA
SPoRT. Any slow moving or repeating storms across this area will
enhance the flash flood risk, and a SLGT risk has been added where
24-hr probabilities for 3" maximize from the SREF, and 1-hr FFG
exceedance probabilities peak above 25%.

...Southwest...
A more active monsoon day is likely Tuesday as mid-level ridging
weakens a bit and shortwaves traverse east to west beneath the
ridge. This will occur in tandem with a PW surge to 1.25-1.5 inches
(+1 standard deviation above the climo mean) as 850mb winds surge
to 20-25 kts from the south, advecting moisture northward up the
Rio Grande and out of Mexico. These anomalous PWs will combine with
SBCAPE of 1000-2000 J/kg, which has been shown to be the best
discriminator for significant monsoon days, to produce an
environment favorable for widespread showers and thunderstorms,
especially in southern Arizona. Storms will likely fire in the
higher terrain and then drift slowly southward on mean winds of
just 5-10 kts, with some organization into clusters supported by
0-6km bulk shear of 20-30 kts. With rain rates potentially
exceeding 1"/hr at times leading to short-duration rainfall of
0.5-0.75 inches in less than 1 hour, the flash flood risk appears
to be increasing across southern AZ, which is also where the HREF
1-hr FFG exceedance probabilities are highest (20-40% through
00Z/Wednesday). After coordination with WFO TWC, a targeted SLGT
risk was added, embedded within a broader MRGL risk area.

...Mid-Mississippi Valley...
Weak impulses lifting northeast out of Texas will interact with a
weakening stationary front over the Mid-Mississippi Valley. While
forcing for ascent will remain generally modest, enough lift into
robust thermodynamics (PWs 1.5 to 2.0 inches overlapped with MUCAPE
of 1000 J/kg) will support scattered thunderstorms with rain rates
of 1-2"/hr. 0-6km mean winds of 15 kts suggest cells will remain
generally progressive, but some weak organization in the vicinity
of any mid-level impulse combined with some training along the
weakening boundary could produce some excessive rainfall related impacts.

Weiss
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