FOUS30 KWBC 220826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Thu Oct 22 2026

Day 1  Valid 12Z Thu Oct 22 2026 - 12Z Fri Oct 23 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHWEST COLORADO AND SOUTHERN UTAH...

...Southwest...
A broad Slight risk remains in place across the Southwest as a deep
mid to upper level low tracking into the West Coast provides robust
large scale scent. Dynamic forcing is further enhanced by strong
upper level divergence within the favorable right entrance region
of an upper jet streak at the base of the trough. Ahead of the
trough deep southerly flow will continue to advect anomalous
tropical moisture northward, resulting in a broad footprint of near
to record PW values for the time of year to go along with near record
IVT values as well.

Instability will be maximized from southern to central AZ into
west central NM, where MUCAPE value upwards of 2000 J/kg are
expected. This corridor of greater instability is where the
greatest potential for intense convective rainfall rates exists,
with rates locally as high as 1-2"/hr likely. Several CAMs signal
the potential for brief cell training within the unidirectional
southerly flow this morning across central AZ, with the corridor
between Phoenix and Flagstaff favored along the nose of the low
level moisture transport axis. This higher rainfall rate potential
will gradually shift eastward across central AZ and into west
central NM later today into this evening. Scattered instances of
flash flooding are expected along his corridor, with locally
significant impacts possible.

Further north, the MDT risk centered over the San Juan Mountains
has been expanded westward into south central and southeastern UT.
Instability will be comparatively lower over this northern flank,
but portions of southern UT have already received heavy rainfall
yesterday, leaving hydrologic sensitivity elevated. A prolonged
duration of scattered showers and thunderstorms is expected today.
While quick cell motions and marginal instability will generally
cap hourly rates around 0.5"/hr, the higher CAPE axis (~1000 J/kg)
is forecast to nudge northward towards southern UT by later this
morning and afternoon. This destabilization, combined with brief
cell training, should support localized swaths of 1"/hr rain rates
over sensitive terrain, slot canyons and washes, leading to
scattered to numerous instances of flash flooding.

The San Juan Mountains will see the heaviest rainfall accumulations
from this event, with model consensus supporting around 3-4", and
localized totals potentially exceeding 5". Rainfall of this
magnitude generally equates ot a 10-50yr ARI for 24hr totals.
Instability will struggle to reach this far northeast, keeping
high res ensemble neighborhood probabilities for 1"/hr rain
subdued at roughly 10-15%. However, strong orographic ascent
within the deep moisture plume will generate persistent moderate
to at times heavy rainfall, with rates periodically reaching
0.5"/hr. While some snowmelt is expected to enhance runoff and the
flood threat, temperature at higher elevations should remain in
the 30s and 40s, allowing existing snowpack to absorb some of the
rain. Overall this event is a solid MDT risk, with numerous
instances of flooding likely, some of which may produce significant
impacts. The lack of higher convective rates and storm totals
largely holding under 5 inches keep this just below High risk
levels...nonetheless, this is still expected ot be a high impact
hydrologic event for the region.

...CA into ID...
Along the CA coast, the initial band of heavier rainfall should
have weakened by 12z. However, with the main mid to upper level low
still offshore, additional isolated to scattered convective cells
are likely to redevelop through the day, supporting a localized
flash flood risk.

Further inland, modest destabilization at or below 500 J/kg is
expected into portions of NV/OR/UT. This weak instability paired
with quick cell motions off to the north northeast should keep any
flash flood threat localized and primarily over sensitive terrain
and burn scars.

A localized corridor of higher rainfall (1 to 2", locally up to 3"
is expected from far northwest NV into far northeast CA and
portions of southeast OR. Totals of this magnitude represent a
10-25yr ARI for this region and may be sufficient to induce
hydrologic concerns. A Slight risk was considered given the
anomalous rainfall magnitudes for the local climatology, but
limited instability should result in an increasingly stratiform
character of the rain following a period of initial convection.
This may help keep the hydrologic impacts more isolated in nature.

...Southeast Florida...
A Marginal risk continues for the southeast FL urban corridor. Low
level coastal convergence within prevailing northeasterly flow
will remain the primary focus for convection, with the potential
to anchor slow moving cell either along the immediate coast or
just offshore. Several CAMs highlight localized swaths of 5"+ of
rain. However, uncertainty remains whether these heavy cores will
make it onshore or stay just offshore. In addition, PW values are
currently around 1.4", and while these values may increase some
today, a significant increase is not expected. Thus some of the
more aggressive high res guidance could very well be a bit
overdone with peak rainfall magnitudes. A Marginal risk remains
appropriate to cover the localized conditional flash flood risk.

Chenard
 

Day 2  Valid 12Z Fri Oct 23 2026 - 12Z Sat Oct 24 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE SACRAMENTO
MOUNTAINS SOUTH THROUGH THE EL PASO METRO...

...NM and southern CO...
Scattered convection is expected to develop across portions of NM
into southern CO downstream of a mid to upper level trough
tracking eastward from CA into AZ. In the broader evolution of the
system, day 2 (Friday) appears to represent a relative minimum in
convective coverage and overall intensity compared to more active
periods expected on both Thursday and Saturday. Nonetheless, PWs
will remain anomalous and collocated with 500-1500 J/kg of CAPE.
This environment will support scattered convection capable of
locally high rainfall rates and an isolated flash flood threat.
While the latest QPF guidance remains modest and generally
supportive of a Marginal risk, the presence of better instability
over southern NM could yield an uptick in convective rainfall as we
get more into the high res model timeframe. Considering the
hydrologic vulnerability, the inherited Slight risk was maintained
across the terrain and burn scars of the Sacramento Mountains into
adjacent urban corridors.

...Southeast FL...
A setup similar to day 1 will persist across southeast FL, where
low level northeasterly flow and coastal convergence could focus
slow moving cells along the immediate coast. The heavy rainfall
threat remains conditional, with a possibility the heaviest cores
remain just offshore. Model differences also introduce some
uncertainty regarding the moisture profile, with the GFS showing
notably higher PW values than the ECMWF. Given the lingering
uncertainty with these details the inherited Marginal risk was left unchanged.

Chenard
 

Day 3  Valid 12Z Sat Oct 24 2026 - 12Z Sun Oct 25 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN NEW
MEXICO INTO SOUTHEASTERN COLORADO, AND OVER SOUTHEAST FLORIDA...

...Eastern NM into southeast CO...
An uptick in the flash flood threat compared to day 2 is
anticipated across eastern NM into southeastern CO on Saturday.
Synoptic scale ascent will strengthen as the mid to upper level
trough and associated jet streak approach the region, which will
also help amplify the low level moisture transport axis. PWs will
remain anomalous, holding well above normal and near climatological
max values for late October. Model guidance also indicates upwards
of 500-1500 J/kg of CAPE within this moisture plume. Deep
unidirectional southerly flow will favor some cell training and
backbuilding potential. Given the favorable ingredients, not
surprising that model QPF values are notably higher compared to day 2.

The combination of strong forcing, deep and anomalous moisture,
and sufficient instability, will support convective rain rates
locally as high as 1-2"/hr. Overall model agreement on the axis of
heaviest rainfall is above average for a day 3 forecast. Higher
end Slight risk probabilities are justified across much of eastern
NM with scattered flash flooding expected.

...Southeast FL...
The Slight risk has been maintained across the southeast FL urban
corridor. The mesoscale setup remains similar to days 1 and 2,
with northeasterly low level flow promoting coastal convergence
and possible slow moving convection. By this time, dynamic ascent
may be aided by a distinct mid level shortwave dropping southward
into the area. While model guidance is in relatively good agreement
synoptically, model spread persists regarding the magnitude of
moisture present. The ECMWF and AIFS remain noticeably drier
overall, depicting an environment less conducive for heavy
rainfall. Conversely, the GFS/AIGFS/UKMET/CMC all depict a deeper
moisture profile and consistently produce a coastal heavy rain
signal. Given this lingering uncertainty and the conditional
probability for slow moving convection to anchor along the
sensitive coastal corridor, there is no reason to deviate from the
inherited Slight risk. Trends will continue to be monitored in
subsequent forecast cycles.

Chenard
$$
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