FOUS30 KWBC 210846
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Mon Sep 21 2026

Day 1
Valid 12Z Mon Sep 21 2026 - 12Z Tue Sep 22 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL APPALACHIANS...

...Central Appalachians and the Mid-Atlantic...

The Slight risk has been expanded to include a larger portion of WV
and southwest VA. The primary driver of heavy rainfall will be
embedded shortwave energy interacting with a backdoor cold front
and inverted surface trough.

Model guidance indicates a moderately to highly unstable airmass
will develop near the boundary over WV, with 2000-3000 J/kg of
CAPE expected. As a distinct shortwave/MCV currently positioned
over IL/IN progresses into the region, robust convective
development is anticipated. While most model solutions keep these
cells moving steadily across WV, a quick 1-3" of rain over
increasingly saturated soils should result in an additional flash
flood threat.

By this afternoon convection should propagate into southwest VA
along/near the backdoor front. In this region more persistent low
level convergence should increase the duration of convective
elements and locally enhance rainfall totals. While some guidance
pushes the front and best convective focus slightly further south
into northern NC, current model consensus supports southwest VA as
the primary axis for heavy rain. Recent high res models highlight
the potential for localized excessive totals. Both the 00z and 06z
HRRR depict a small but impressive swath of rainfall as high as
3-6". Given the available instability, mesoscale forcing, and
favorable convergence near the front, these elevated totals appear
plausible, justifying the expansion of the Slight risk.

...New Mexico and Nearby Southern High Plains...

Both the HREF and REFS indicate the potential for rainfall locally
exceeding 3" as isolated convection develops this afternoon over
eastern NM and west TX. A widely isolated flash flood risk is possible.

...South Florida...

A Marginal Risk has been maintained for portions of South Florida
as a plume of anomalous moisture will help to fuel heavy downpours
and potentially lead to some isolated flooding/ponding in urban areas.

...CA/NV...
Convection underneath a closed low may pose a localized flash
flood risk today across portions of eastern CA into NV.
Instability is forecast to be weak, but with cold air aloft under
the low and PWs forecast to increase over the climatological 90th
percentile, an isolated flash flood risk could evolve.

Chenard
 

Day 2
Valid 12Z Tue Sep 22 2026 - 12Z Wed Sep 23 2026

..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN NEW MEXICO...

...New Mexico and West Texas...

A potent, multi-day heavy rainfall event is set to begin across NM
and far southwest TX Tuesday. An upper level trough approaching
from the west will help strengthen an upper level jet,
persistently placing the region in the favorable right entrance
region. Deep southerly flow will transport tropical moisture
northward, while embedded mid level shortwave energy advances north
out of Mexico and into NM by Tuesday night. Low level easterly
upslope flow will also enhance lift in this setup.

PW values are forecast to approach mid to late Sep. record levels.
With sufficient instability expected, this deep moisture profile
will support heavy rainfall rates. Scattered flash flooding is
expected across much of NM and far southwest TX, with a more
concentrated area of slow moving convection possible over southern NM.

Guidance is in pretty good agreement regarding high QPF potential
over southern NM. The 00z RRFS and 3km NAM depict swaths of 3"
rainfall totals, with the GFS/AIGFS/Gem reg depicting higher QPF
across a similar axis. With a day 3 Moderate risk already in
effect, and the flash flood threat expected to ramp up by Tuesday
night, we opted to upgrade the day 2 to a Moderate as well.
Scattered to numerous instances of flash flooding are anticipated,
some of which could be locally significant. This threat includes
the sensitive burn scar areas within the Sacramento Mountains.

...Ohio Valley and Mid-Atlantic...

An approaching mid-level trough and favorable positioning within
the right entrance region of an upper level jet will support
widespread rainfall across portions of the OH Valley and Mid-
Atlantic. At the surface, a backdoor cold front with strong
easterly flow will enhance low level convergence, further amplified
by a developing inverted surface trough.

Well to the north of the frontal boundary, limited instability
will keep rainfall primarily stratiform, with rain rates remaining
below flash flood thresholds. Considerable model spread remains
regarding where the strongest convergence and sufficient
instability will overlap, complicated by dry air advecting in from
the northeast. While the 12z EC and 00z GFS were previously in
agreement with a heavy QPF axis along the coastal plain from
eastern VA into the Delmarva, the 00z EC has trended further
inland. Meanwhile, the 00z RRFS and REFS favor a further inland
and southward axis stretching from south central VA into central NC.

Ultimately, the forecast hinges on far south cold air damming
forces the instability axis and also how far inland the surface
trough gets nudged. A localized sweet spot may emerge where
persistent convergence intersects sufficient instability to drive
a heavy rainfall and flash flood threat. Where these ingredients
overlap, the environment will be conducive for some low topped
convective cores and efficient warm rain processes. However given
the current uncertainty, we will keep the risk level at Marginal
for this update, and we will reevaluate later today when we have
the full suite of high res guidance available.

...South Florida...

A Marginal Risk has been maintained for portions of South Florida.
Locally heavy downpours will remain possible with anomalously high
moisture in place, which could lead to some isolated
flooding/ponding in urban areas.

Chenard
 

Day 3
Valid 12Z Wed Sep 23 2026 - 12Z Thu Sep 24 2026

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

...New Mexico and West Texas...

The heavy rainfall threat established on day 2 will continue and
potentially peak over NM on day 3 as large scale forcing gets even
stronger. A well defined mid level shortwave is forecast to lift
northward across the state, accompanied by robust upper level
divergence. PW values will remain near record levels for this time
of year, with an anomalously deep moisture profile.

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. Thus while some
areaS may see predominately stratiform rain, a concentrated swath
of intense convective rainfall remains probable.

Guidance continues to show some spread regarding the placement of
this heaviest rainfall axis. The 00z EC remains the furthest west
solution, with the GFS, AIGFS, AIFS and RRFS clustered a bit
further east. The current forecast favors this eastern consensus,
though given we are still three days out, additional spatial
adjustments are probable.

Based on this setup the inherited Moderate risk remains justified,
and it was expanded to encompass a larger portion of eastern NM.
Scattered to numerous instances of flash flooding are
anticipated, some of which could be locally significant. This
threat includes the sensitive burn scar areas within the
Sacramento Mountains.

...Mid-Atlantic...

A complex and somewhat disjointed synoptic pattern will be present
over the Mid-Atlantic on day 3. A mid to upper trough will will
drift slowly eastward from the OH Valley into the Mid-Atlantic
while a separate coastal low develops offshore. Meanwhile, surface
high pressure to the north will advect drier and more stable air
into the corridor between these two systems.

While some instability may materialize directly beneath the cold
core of the mid level low, PW values will have generally decreased
by this time. The bulk of the deeper moisture and instability may
remain offshore with the coastal low.

Despite the intrusion of dry air, there remains enough
uncertainty regarding the exact evolution of these features to
warrant a continuation of the Marginal risk. An embedded Slight
risk may be needed in subsequent updates, contingent upon the exact
track of the upper low, position of the moisture/instability axis,
and the state of antecedent soil conditions from previous days rainfall.

...Florida...
A Marginal risk continues across south FL where above normal PWs
and instability will continue to pose a localized flash flood risk.

Chenard
$$
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