596
FOUS30 KWBC 050032
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
832 PM EDT Thu Sep 4 2025

Day 1
Valid 01Z Fri Sep 05 2025 - 12Z Fri Sep 05 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST CALIFORNIA AND CENTRAL AND SOUTHERN ARIZONA OVERNIGHT...

...Southwest...

The Slight Risk area remained in place with only minor adjustment
needed to its placement given the satellite and radar imagery as of
late afternoon. Showers and thunderstorms developed across southwest
AZ and the Lower CO River Valley today in response to strong
surface-based heating and elevated precipitable water values. The
soundings from Yuma Arizona throughout the morning and early
afternoon continued to show a moisture-laden atmosphere with
precipitable water values remaining around 1.95 inches and mixed-
layer CAPE on the order of 3000 J/kg...with other soundings across
Arizona at 18Z comparable to the 12Z soundings in terms of moisture
and instability. Mean wind flow within the 850-300mb layer was
very light (<5 kts) so storms will remain capable of producing
torrential downpours within a short time frame over affected areas
before dissipating. Some storms should persist into late this
evening in areas where residual outflows trigger additional storms
that can tap any lingering MU/ML CAPE. The signals were strong
enough to support trimming the portion of the Marginal risk area
that extended into central and even parts of northern California
with the impending loss of daytime heating.

...South Florida...
Removed the Marginal Risk area from South Florida given the trends
in satellite image for warming cloud tops as well as short term
trends in radar imagery of decreasing areal coverage of echoes.
Recent runs of the HRRR has shown a renewed shower and thunderstorm
development over the Gulf that approaches the southwest coast of
the Florida peninsula and the Florida Keys after 05/09Z. Even
so...the present indications are that the probability of rainfall
exceeding flash flood guidance is less than 5 percent in this area.


Bann


Day 2
Valid 12Z Fri Sep 05 2025 - 12Z Sat Sep 06 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
ARIZONA AND SOUTHWEST NEW MEXICO....

...Southwest...

Latest guidance has come into better agreement that Lorena will
steadily weaken while staying west of the Baja Peninsula. This is
good news as it keeps its more tropical air-mass and potent mid-
upper level vortex away from southern AZ and southern NM. That
said, its remnant moisture will be caught up in the mean 250-500mb
trough axis located over northern Baja and Southern CA and will be
directed across northern Mexico that is likely to bleed over into
far southern AZ, southern NM, and into western TX. PWATs will
remain at their highest in the Lower CO River Basin on east into
south-central AZ where PWATs will reach as high as 1.8". From
southeast AZ on east into southwest NM, PWATs will generally be in
the 1.25-1.5" range which is hovering near the 90th climatological
percentile. The expansive cloud cover east of the Lower CO River
Basin should limit instability enough to limit the intensity and
areal coverage of potential flash flooding. Still, some of these
areas are still sporting locally sensitive soils from the past
couple weeks of rainfall, not including what rainfall occurs today
(9/4). With the residual anomalous moisture from Lorena nearby,
opted to maintain the Slight Risk this forecast period as it will
not take much (1-hr FFGs generally less than 1.5") to produce flash
flooding.

The large Marginal risk area spans from Oregon and as far east as
Colorado and New Mexico. The overall synoptic pattern continues to
favor isolated to scattered thunderstorms from the Great Basin on
east to the central Rockies, and elevated PWs and meager
instability aloft support the potential for localized Excessive
Rainfall rates.

...South Florida...

The overall pattern described during the Day 1 period is likely to
be a near carbon-copy look for Day 2 with locally heavy rainfall
between 2-4" are forecast in the period. With upper level flow
becoming more southerly over south Florida and the upper jet
orientation favoring a bit more upper level divergence, there could
be a larger swath of storms across South FL and the FL Keys. While
PWATs will remain within the 2.0-2.2" range, extensive cloud cover
may play a role in limiting instability. Given warm cloud layers
remain rather deep and Upshear Corfidi Vectors are generally <10
knots, torrential downpours within training convection over South
FL is possible. The bulk of the heavier rainfall should occur over
the Everglades and more rural areas, but there is a chance storms
containing up to 3"/hr rainfall rates could occur within the Miami
metro area. A Marginal Risk remains in effect for southern Florida
and the Keys.

...Ohio Valley...

There was some thought on introducing a Marginal Risk for portions
of the OH Valley that stretched from eastern KY on north and east
through western WV, eastern OH, and southwest PA. This region has
been rather dry in recent weeks with NASA SPoRT-LIS showing 0-40cm
soil moisture percentile below 10% in some cases, and storm motions
should be progressive. It is worth noting there is likely to be
modest instability present (MUCAPE 1,000-2,000 J/kg) and PWATs
closer around 1.5" Friday afternoon. Should new CAMs guidance
support a stronger signal for excessive rainfall rates in future
forecast cycles, a categorical risk upgrade to Marginal may be
needed. At this moment, the rainfall looks more beneficial than
harmful, and thus a Marginal Risk was not introduced at this time.

Mullinax

Day 3
Valid 12Z Sat Sep 06 2025 - 12Z Sun Sep 07 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM TEXAS TO
THE PACIFIC NORTHWEST, SOUTHERN FLORIDA AND NEW ENGLAND...

2030Z Update: The Marginal Risks from overnight shift remain on
track and were tweaked in some areas to account for the latest
changes in WPC QPF. The Northeast will have an anomalous plume of
PWATs that surpass the 90th climatological percentile that produce
some localized Excessive Rainfall rates, but the region has been
quite dry in recent weeks and storm motions should be progressive.
The flash flood threat is likely to be localized with the majority
of the rainfall in the Northeast being more beneficial than
harmful. Similar anomalous PWATs will be present over the Northwest
and northern Great Basin, but model spread in where the heaviest
QPF and best instability shapes up remains unclear.

Mullinax

---Previous Discussion---

...West, Southwest and Southern Plains...

During this period a frontal boundary will stretch from the Gulf
westward into Texas. Some enhanced moisture left over from Lorena
and instability will be present over the Southern Plains, which
could allow for locally heavy rainfall to occur. A broad Marginal
is in effect for the western half of Texas into the southern High
Plains, Colorado, Southwest, Great Basin, Idaho and Oregon.

...Florida and New England...

The frontal system over Florida and northeast in the western
Atlantic will provide a focus for moisture to pool, leading to at
least scattered diurnal convection each day. Excessive rainfall
across Florida will likely focus across the southeast peninsula in
Miami-Dade County and vicinity. The Marginal Risk up north covers
most of Vermont, New Hampshire and Maine.

Campbell

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