FOUS30 KWBC 140802
QPFERD

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

Day 1
Valid 12Z Mon Jul 14 2025 - 12Z Tue Jul 15 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC STATES...

...Mid-Atlantic Through Northeast...
A cold front slowly advancing from the Ohio Valley will advance
into New England and the Mid- Atlantic states during the evening.
The guidance is in good agreement that convection will develop on a
pre-frontal trough downstream of the front and become numerous
during the afternoon. This will be in response to the accompanying
ascent (through height falls, a potent vort max swinging out of the
Ohio Valley, and low- level convergence) acting upon impressive
thermodynamics characterized by PWs that will exceed 2.25 inches (a
daily record) and MUCAPE of 1000-2000 J/kg. With warm cloud depths
also progged to reach 14,000 ft, efficient warm-rain processes
should dominate all convection this afternoon.

Although storm motions may remain progressive today as 0-6km mean
winds are progged to be angled sharply to the front (some better
training potential farther north into New England), intense rain
rates are expected as reflected by HREF 2"/hr probabilities that
reach 30-50%, suggesting locally 3+"/hr rates are possible. During
storm initiation, motions could be a little slower than once they
become more organized, which would lengthen the duration of any
heavy rain rates, and bulk shear of 20-25 kts could organized cells
into multi-cells, leading to subtly longer and more enhanced
rainfall as well. With extreme rain rates expected everywhere that
develops convection, rainfall could reach 2-4" with locally higher
amounts above 5" possible (HREF/REFS 40-50% chance).

While the CAMS are in good agreement with storms along the pre-
frontal trough pushing eastward, there remains some uncertainty into
pre-trough convective development. If the solutions of the ARW/ARW2
verify, which appear reasonable depicting scattered air-mass type
storms in the region of broad large scale ascent, then these slow
moving storms ahead of the line would enhance the flash flood risk,
and develop the greater potential for the 5+ inches of rain
progged by the neighborhood probabilities. Additionally, the CSU
First- Guess field (both UFVS and non UFVS versions) indicate a 25%
risk area across the I-95 corridor from near Richmond, VA to near
New York City, which is aligned with the highest ECMWF EFI
probabilities as well. Since this heavy rain signature is
consistent among the various ensembles, and will likely occur atop
primed soils from recent heavy rain, an upgrade to a MDT risk was
coordinated with the affected WFOs.

...Florida...
A westward advancing inverted mid-level trough accompanying a
slowly intensifying tropical wave will continue to push westward
today, reaching the east coast of the Florida Peninsula by the end
of D1. Although the primary trough axis will just clip Florida
today, downstream vorticity lobes swinging out ahead of it will
cross the peninsula, leading to robust ascent which will otherwise
be enhanced by upper divergence and a westward advancing sea
breeze. This lift will occur into intense thermodynamics as PWs
surge towards 2.25 inches, which is above the 90th percentile
according to the SPC sounding climatology, collocated with MUCAPE
of more than 2000 J/kg. As the trough axis shifts west, low-level
flow will veer more to the east, leading to enhanced moisture
advection onshore while additionally producing low- level
convergence axes in a region already experiencing impressive deep
layer lift. The result of all of this will be widespread showers
and thunderstorms today, with rainfall rates of 2-3+"/hr.

Coverage across the peninsula will vary considerably based on time
of day today. Overnight (before 12z Monday) and early on Monday,
storms will likely be confined along the Atlantic coast. However, as instability climbs through the day, convection will spread across
the area and then move S/SW on 0-6km mean winds of 15 kts. While
this will keep cells generally progressive, some short duration
training is possible along convergence axes and where any
organization can occur through 20-25 kts of bulk shear. Then late
tonight, generally after 03Z Tuesday, as the trough axis moves
onshore, a secondary enhancement of convection may accompany a
developing surface wave and move back onto the Atlantic coast. Where
repeating rounds or any training can occur, total rainfall of 3-5"
with locally higher amounts (40-60% chance of 5+ inches from the
HREF) possible. This will produce at least scattered flash flood
impacts today, especially across any urban areas.

...Desert Southwest through the Mid-Mississippi Valley...
Complex upper level pattern will lead to another day of scattered
but intense convection, with clusters of locally more organized
activity, from the Desert Southwest through the Mid-Mississippi
Valley. The driver of this activity will be a strung out mid-level
ridge which will maintain a west-to-east oriented axis from Nevada
into Oklahoma, and a secondary ridge over the Southeast. Between
these features, a shortwave trough will be trapped within a col and
move slowly over the Ozarks, while additional shortwave impulses
rotate westward around the equatorward side of the ridge from West
Texas through New Mexico and Arizona. Ascent accompanying these
features will act upon robust thermodynamics to support heavy
rainfall, as PWs rise to 1.75-2.0 inches from the Gulf Coast of
Texas into the Mid-Mississippi Valley, with a secondary maximum
approaching 1.25-1.5 inches in southern Arizona. Additionally,
MUCAPE, especially during peak aftn/eve heating, will rise to 1000-
2000 J/kg across much of the area.

In this environment, the CAMs suggest generally scattered
thunderstorm development, but with locally enhanced coverage and
intensity. Regardless, any cells that develop will likely contain
rainfall rates of at least 1"/hr (generally 30-60% chance), with
local rates of 2-3"/hr likely, especially from Texas into the
Ozarks. Despite the scattered nature of thunderstorms, there may be
two focused areas today:

1) Across the Desert Southwest a potent shortwave undercutting the
ridge will drift southwest into southern NM and AZ, overlapping a
surge of elevated PWs drawing northward from the Rio Grande Valley
this evening. Rainfall rates may touch 1"/hr at times as CAPE
increases, especially where any storms can organize into clusters
through 0-6km bulk shear of 20-25 kts and storms drop SW off the
terrain. The HREF and REFS 24-hr probabilities for 1+" rise above
60%, highest in SE AZ/SW NM, with the HREF indicating a low end
potential (10% chance) for 3" in this same vicinity. Briefly
considered a SLGT risk for this region, but current satellite
suggests cloud cover will be expansive this morning and CAPE may
struggle to regenerate, so the MRGL risk was left although
isolated impacts are likely.

2) The other area will be from the TX Hill Country northeast into
the Ozarks along a decaying stationary front which will help to
focus slow-moving thunderstorms with the aforementioned intense rain
rates this afternoon/evening. Additionally, an MCV noted in
satellite imagery lifting northward from Coahuila, Mexico this
morning, is likely to provide additional focus for ascent across
south-central Texas. Although the CAMs feature a large variation in
QPF amounts and spatial footprint, there is an increasing signal
for heavy rainfall across the Hill Country, which is additionally
supported by low-level PW (from CIRA LPW products) reaching above
the 95th and even 99th percentile between 850-700mb which will be
supported by strengthening winds within that layer (actually the
strongest winds of the column) providing additional lift through
convergence. This region has been exceptionally wet recently as
well (AHPS 7-day rainfall more than 600% of normal) leading to
extremely vulnerable soils and FFG of just 0.25-1.5"/3hrs. This is
also where the ECMWF EFI is 0.6 to 0.7, indicating an increasing
threat for a heavy rain event, which overlaps a SLGT risk forecast
from the CSU first guess field. After coordination with WFO
SJT/EWX, a SLGT risk was added, focused across the TX Hill Country.

...Northern Plains...
Generally flat/zonal 500mb flow aligned along the Canada/United
States border will gradually amplify into a broad trough this
aftn/tonight as a potent shortwave digs out of Alberta. This
evolution will push a wavy cold front southeastward into an airmass
with PWs above the 90th (locally 97th) percentile according to
NAEFS. A piece of this shortwave and its accompanying vorticity
will swing eastward along the front as the low-level baroclinic
gradient strengthens due to a surging southerly LLJ reaching 25-35
kts to enhanced warm air advection. This will draw even more
favorable PWs northward, and where the most impressive ascent
overlaps the favorable thermodynamics, convection with heavy
rainfall will result. 0-6km mean winds of 20-30 kts suggest storms
will be progressive, but also track parallel to the front which
indicates a training potential. Additionally, some modest
organization is possible which could help enhance rain rates to 1-
2"/hr, especially later this evening. 6hr/5yr RI exceedances peak
this evening as the LLJ ramps upward, suggesting at least an
isolated excessive rainfall/flash flood risk later today.

Weiss
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