FOUS30 KWBC 160816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Wed Jul 16 2025

Day 2
Valid 12Z Thu Jul 17 2025 - 12Z Fri Jul 18 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL
GULF COAST AND PORTIONS OF THE FOUR CORNERS STATES...

...Gulf Coast...
All eyes will be on Invest 93L for which NHC has placed a 40% 
chance of development across the northern Gulf. The guidance is
converging on the track of this invest, although uncertainty
remains high as the system has not developed yet. Still, there is
increasing confidence in the track of this system, and the mid-
level circulation should begin to track NW on Thursday into a
weakness which develops in the ridge to the north. This will allow
for more widespread precipitation to spread onshore as the northern
subsidence/dryness begins to erode and the accompanying tropical
moisture plume pivots northward. This will support training of
tropical rain showers lifting into Louisiana, with rainfall rates
2-3"/hr supported by deep column saturation noted in forecast
soundings and PWs approaching 2.5 inches. Additionally, forecast
soundings indicate that the mean winds through 300mb are quite
weak, and this indicates that if the system can organize more
quickly and intensify, low-level moist inflow could quickly exceed
the mean wind to enhance ascent, which would additionally create
more widespread heavy rainfall. GEFS and ECENS probabilities both
indicate a low-end risk (10-15%), as does the LREF from DESI, but
this may under-forecasting the true threat in the tropical 
airmass, as several deterministic models show 3-6" across southern 
Louisiana on Thursday. At this time opted to maintain the SLGT risk
for this area, but should the system begin to organize and models 
converge on a more aggressive solution, an upgrade may be needed 
for D2 (a MDT risk is already in effect for D3).

...Central Plains through the Northeast...
A cold front will sag slowly southeast, pushed by a broad but
subtly amplifying trough along the Canadian/United States border.
This trough will impinge into broad mid-level ridging which will
remain elongated west from the Carolinas, leading to pinched flow
generally aligned WSW to ENE from the Central Plains into the Mid-
Atlantic and Northeast. Through this pinched mid-level axis,
multiple vorticity impulses are progged to cross from west to east,
each one interacting with the low-level baroclinic boundary to
enhance ascent leading to scattered to widespread thunderstorms. 

Convection which develops on Thursday will likely contain intense
rain rates as thermodynamics in the vicinity of this front remain
impressive. PWs are progged to exceed 2 inches, which is above the
90th percentile from NAEFS, and be collocated with a MUCAPE axis of
2000-3000 J/kg, or higher. These intense thermodynamics will
support rainfall rates that could reach 2-3"/hr (as high as 30%
chance from the HREF through 00Z Friday), with cell training likely
as 0-6km mean winds align parallel to the front. Additionally, bulk
shear magnitude of 20-30 kts will support storm organization,
especially beneath any of the more powerful impulses moving through the flow.

While at this time there is no clear signal, and hence
too much uncertainty, for any local SLGT risk areas, it is possible
that some targeted SLGT risk upgrades may be required with later
issuances. This would be most likely across parts of KS/MO where a
morning MCS decays and leaves a lingering boundary to enhance
thunderstorms with training, or potentially farther east into the
Ohio Valley/Central Appalachians where training atop more sensitive
soils may occur.

...Four Corners... 
Elongated ridge extending from the Carolinas through the southern 
High Plains will interact with a closed low lifting northward from 
Baja into southern California on Thursday. This will squeeze 
intensifying deep-layer southerly flow out of Mexico and the Gulf 
of California, driving impressive moisture advection northward into
the Southwest. PWs within this plume will exceed 1.25 inches as 
IVT surges above the 90th percentile, at least locally, according 
to NAEFS, but collocated MUCAPE is progged to be modest as 
reflected by SREF 500 J/kg probabilities peaking around 40% in AZ, 
although higher across northeast NM. While many of the 
deterministic models predict at least scattered convection during 
the day, the heaviest rainfall may be across northern AZ/northern 
NM as the 850mb inflow convergences into weaker wind speeds, which 
is more effectively overlapped by upper diffluence to enhance 
ascent. This suggests that the current MRGL risk is still 
appropriate for most of the area, however, a targeted SLGT risk has
been collaborated for northeast NM and far Southeast CO where a 
vorticity lobe lifting N/NE will interact with the greater 
instability and the tail end of the lingering front to provide a 
focus for more organized convection.

Weiss


Day 3
Valid 12Z Fri Jul 18 2025 - 12Z Sat Jul 19 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL GULF COAST...

...Gulf Coast...
Concern for flash flooding Friday revolves around Invest 93L which
will likely begin its turn to the NW, regardless of how it 
develops. While guidance has tended to back off on the intensity 
and associated precipitation with this feature, there continues to 
be a lot of uncertainty into this forecast and NHC has maintained a
40% chance of development over the next few days. Regardless of 
development, the associated tropical moisture plume will support 
heavy rainfall rates of 2-3"/hr, with some training likely on the 
upwind side of this feature. The GEFS and ECENS ensembles are 
modest with the rainfall Friday(only 10% chance for 3" of rain), 
but after coordination with the affected WFOs, the MDT risk was 
maintained with only minor cosmetic adjustments for consistent 
messaging and the potential if this system does become more organized.

...Mid-Mississippi Valley...
A stationary front Thursday will gradually lift northward on
Friday, moving across the Mid-Mississippi Valley and into the Upper
Midwest. This boundary will surge north in response to both the
expanding ridge to the southeast and an intensification of the
850mb LLJ reaching 30+ kts out of the Gulf and returning through
the southern Plains. At the same time, a shortwave impulse,
potentially convectively reinforced from prior thunderstorms, will
move along the front to enhance ascent, resulting in an axis of
thunderstorms which may congeal into an MCS Friday night. There
remains some uncertainty into both the placement and speed of this
MCS, but PWs elevating to above 1.75 inches and MUCAPE pushing
above 3000 J/kg will support rain rates of 1-2"/hr, if not locally
higher. This could produce more than 3" of rain where any training
can occur, and the SLGT risk was tailored to best match the GEFS
and ECENS probabilities for 3"/24hrs. 

...Southwest...
An active monsoon day is expected Friday as a potent vorticity
maxima lifts northward over the Four Corners, embedded within
pinched southerly mid-level flow between a ridge to the east and
closed low over southern CA to the west. The key difference in the
evolution of the synoptic pattern Friday is a push west of the
mid-level ridge, which should deflect higher moisture westward,
noted by a plume of 1.25" PWs lifting into southern NV, coincident
with a dramatic increase in SREF CAPE>500 J/kg probabilities as far
as southern UT. There is a lot of uncertainty in the potential for
convective coverage on Friday, but increasing bulk shear and
chaotic/weak storm motions will result in at least an isolated
flash flood risk as storms contain rainfall rates above 0.5"/hr. It
is possible, as well, that a targeted SLGT risk may be needed for
portions of the area with later issuances.

...Central Appalachians and Mid-Atlantic...
A cold front and wave of low pressure will dig slowly southward
from the Great Lakes into the region on Friday. This front will
move into an environment that will be favorable for heavy rainfall
noted by PWs above the 90th percentile according to NAEFS
collocated with MUCAPE of 1000-2000 J/kg. This will likely result
in scattered to numerous thunderstorms developing during the
aftn/eve, with rain rates of more than 2"/hr possible at times. The
guidance is highly variable both with the placement of the
strongest convection, as well as whether or not it can organize
beyond typical pulse type thunderstorms. Due to the uncertainty, no
SLGT risk has been raised at this time. However, increasing bulk
shear Friday evening and the potential for training along the front
could result in 2-3" of rain with locally higher amounts as
reflected by SREF/GEFS probabilities. If this occurs atop the most
vulnerable soils from recent rainfall, a targeted SLGT risk may be
needed for portions of the region.

$$
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