FOUS30 KWBC 230852
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
452 AM EDT Wed Sep 23 2026

Day 1  Valid 12Z Wed Sep 23 2026 - 12Z Thu Sep 24 2026

..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL AND EASTERN NEW MEXICO...

...New Mexico and West Texas...
The ongoing heavy rainfall and flash flood threat over NM will
continue and is expected to peak today. Large scale forcing will
reach its maximum as a well defined mid level shortwave lifts
north-northeastward across the state, accompanied by robust upper
level divergence and the potential development of a weak surface
low. Scattered to numerous instances of flash flooding are
expected. Locally significant and life threatening impacts remain
possible, particularly across sensitive terrain, urban areas, and
burn scars. For the 24 hour window from 00z Wed to 00z Thu both the
HREF and REFS depict notable 50-60% probabilities of exceeding the
100 year ARI over portions of southern AZ.

With such strong forcing and widespread moisture in place, the
primary limiting factor for heavy convective rates will be the
extent of cloud cover and its impact on destabilization. While
widespread clouds are a concern, daytime heating should allow at
least the eastern edge of the rainfall shield to sufficiently
destabilize, aided by broad easterly upslope flow. As convection
attempts to propagate eastward into a growing pool of CAPE,
easterly inflow should help slow this forward progression,
promoting a threat for cell training within the deep southerly
flow. Given there will still be some eastward propagation and
instability is advecting in from the east, the heaviest convective
rates are expected to focus across the central and eastern portion
of the state. The 00z HREF and REFS mean are in good agreement on
this QPF axis, through the 00z HRRR and RRFS are shifted slightly
further east. This eastward evolution is plausible given the
instability gradient and convective propagation vectors, and may
extend the flash flood risk into portions of far west TX. Across
the eastern half of the MDT risk, HREF and REFS neighborhood
probabilities for exceeding 3" of rain are 40-70%. A widespread
1-2" of rain is generally expected, with localized swaths of 2-4
inches likely...falling in addition to rainfall received prior to 12z.

We were reluctant to trim the western extent of the inherited
Moderate risk at this time, as part of this area have already
received heavy rainfall. While the most significant flash flood
risk may ultimately focus over the eastern half of the MDT area,
these western locales will still experience a broad shield of rain
with embedded heavier convective cores. However, the western extent
of the surrounding Slight risk was trimmed back based on expected
cloud cover and HREF/REFS consensus.

A separate area of higher instability is forecast across portions
of AZ. Convective coverage and organization should be lower here,
but the instability/PW overlap supports a localized flash flood
risk. Southeast AZ may see a bit greater coverage, and with higher
FFG exceedance probabilities noted, the Slight risk was extended
into this area.

...Mid-Atlantic...
A complex synoptic pattern continues as a mid to upper low drifts
slowly across the OH Valley, while a separate coastal low develops
offshore. Meanwhile, surface high pressure positioned to the north
will advect drier and more stable air into the corridor between
these two features.

The primary threat for heavy rainfall over land should focus
closer to the the upper low, extending from OH southward into GA
and AL. Although PW values will have lowered across these areas,
over 1000 J/kg of CAPE should pool near a north to south oriented
trough axis. This instability, co-located with a slow moving
surface convergence axis, will support the development of slow
moving convective cells. Even with reduced deep moisture, locally
heavy rainfall rates are likely. Given that portions of this region
feature lowered FFG and heightened hydrologic sensitivity from
prior rainfall, a flash flood threat will persist.

A Slight risk will be carried across portions of eastern KY,
eastern TN, and far western NC, where the HREF and REFS indicate
the highest probabilities for FFG exceedance. Peak probabilities
are centered over eastern TN, where neighborhood probabilities
eclipse 40%, with notable spatial coverage. This aligns with the
forecast overlap of maximum instability and low level convergence,
supporting a scattered flash flood threat.

Further north and east, a Marginal risk covers areas from OH into
WV and western VA/NC. While less instability is forecast here,
which should keep rainfall rates lower, wet antecedent conditions
support a continued localized flash flood threat.

...Florida...
A Marginal risk continues across south FL where above normal PWs
and instability will continue to pose a localized flash flood
risk. A weak low pressure developing off the FL coast may initially
help focus heavier rainfall along coastal areas. Thereafter this
feature may actually help advect in some drier air behind it,
decreasing convective coverage by later today.

Chenard
 

Day 2  Valid 12Z Thu Sep 24 2026 - 12Z Fri Sep 25 2026

..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
NEW MEXICO INTO SOUTHWEST KANSAS...

...Southwest and Southern Plains...
Heading into Thursday the strongest mid level forcing and highest
PW anomalies will shift northward out of NM and into Colorado and
the Central Plains. However, in the wake of this exiting system,
daytime instability is actually forecast to increase over NM
compared to previous days. While PW values decrease some, they will
remain above the climatological 75th percentile. Additionally,
broad troughing will remain overhead, with embedded shortwave
energy continuing to traverse the region.

With less concentrated forcing and somewhat reduced moisture, the
overall areal coverage of heavy rainfall across NM should diminish
relative to prior days. However, the combination of stronger
instability and sufficient moisture will still support intense
localized convective rainfall rates capable of producing
significant rainfall totals. Given the hydrologic sensitivity built
up from preceding days of heavy rain, an additional flash flood
threat appears likely. While the threat may not be as widespread
as previous days, a Slight risk is maintained, as locally
significant and impactful flash flooding will still be possible.

The Slight risk also extends eastward across the TX and OK
Panhandles and into southwest KS. Convection may be ongoing across
this corridor at the start of the period. Later in the day, in the
wake of the lifting shortwave and surface low, a corridor of
enhanced low level convergence and instability may develop, which
could result in additional rounds of locally heavy convection.
Further west, isolated flash flooding will also be possible across
portions of AZ, southern UT and CO as the trough moves across these areas.

...Central Plains...
The highest PW anomalies and strongest synoptic forcing will
overlap across KS and NE. This forcing aligns well withe a 1-3"
consensus QPF swath depicted by a majority of the model guidance.
The primary limiting factor for a greater flash flood threat will
be the degree of available instability. Thus rainfall may take the
form of a broad stratiform rain shield with only embedded heavier
convective elements. Therefore, the risk level will be maintained
at Marginal for now, with trends continuing to be closely monitored
for any needed upgrades in subsequent forecast cycles.

Chenard
 

Day 3  Valid 12Z Fri Sep 25 2026 - 12Z Sat Sep 26 2026

..THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM NEW MEXICO
INTO THE PLAINS, AND OVER SOUTHEAST NEW ENGLAND...

...NM into the Central Plains...
A broad axis of anomalous PWs, low level moisture transport, and
CAPE will persist from portions of eastern NM northeastward through
NE and IA on Friday. The strongest and most persistent mid to
upper level forcing, coupled with the primary moisture transport
axis, is forecast to focus across the northern extent of this
corridor. Conversely, the most robust instability is expected to
pool further south, stretching from eastern NM into southern KS.

The spatial offset between the best large scale forcing and
greatest instability introduces uncertainty regarding exactly how
the flash flood threat will evolve. While it will undoubtedly be a
wet period across this corridor, the magnitude and extent of the
flash flood threat will depend on whether sufficient rainfall rates
can materialize further north, or if convection can organize and
become widespread enough further south where the instability is
more firmly established.

Given these lingering questions, a broad Marginal risk has been
maintained across the region. However, the available environmental
ingredients certainly support the potential for an embedded Slight
risk. Upgrades may be necessary in subsequent forecast cycles as
the event draws closer and confidence in mesoscale details increase.

...Northeast...
A Marginal risk has been maintained from RI into eastern MA as
uncertainty lingers regarding the exact track and evolution of a
developing coastal low. While the better chance for more
widespread heavy rainfall is likely to hold of until day 4, the
northern edge of the rain shield could push into coastal areas by later Friday.

Current indications suggest that the best instability and any
convective elements should remain offshore through the day 3
period, keeping rainfall rates predominately stratiform over land.
Consequently, the overall flash flood threat is expected to remain
relatively low. However, with 1-3 inches of rain possible and
ongoing uncertainty regarding the precise behavior of the coastal
low, continuity with the previous forecast is preferred for now,
and the Marginal risk remains in place.

Chenard
 

Day 4 and Day 5  Valid 12Z Sat Sep 26 2026 - 12Z Mon Sep 28 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN US/WESTERN PLAINS AS WELL AS EASTERN NEW ENGLAND...

...Southern Rockies to Central and High Plains...

The main focus of attention for possible heavy to excessive
rainfall remains over the Midwest/Central Plains on Saturday as a
shortwave energy ejects northeastward from the Rockies. The
envelope of solutions span the EC on the north side and the GFS
favoring a more southerly solution corresponding to Iowa and
Missouri/Kansas respectively. Maintained the Marginal in Iowa
where flash flood guidance is lowest after the recent rainfall
although confidence is waning about the need for one based on the
latest QPF. If the ECMWF verifies...the flash flood numbers are
higher with a correspondingly reduced threat of excessive rainfall.
A broadening mid/upper level trough to the east shunts deepest
moisture out of the Midwest by Sunday.

...New England...

The 23/00Z suite of numerical guidance has come into somewhat
better agreement that surface low pressure center and the
associated rain shield will track close enough to the New England
coast to bring moderate to heavy rainfall to the area over the
weekend. Some discrepancy remains with respect to placement of the
heaviest rainfall (ML vs NWP outputs) as the upper pattern shows an
occluding low with a drift to the west or southwest on Saturday.
Introduced a Slight Risk area over the eastern portion of New
England where ensemble members from both the GEFS and ECMWF
ensembles have contours of 2 and 3 inch amounts where moisture
transport and moisture flux convergence is maximized by strong
easterly surface and low level winds. Expectation is that the
system will begin to pull away on Sunday but that may not
necessarily mean an end to the flooding threat if the rainfall
shield is broad enough.

Bann
$$
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