338
FOUS30 KWBC 200100
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
800 PM EST Wed Nov 19 2025

Day 1
Valid 01Z Thu Nov 20 2025 - 12Z Thu Nov 20 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTH-CENTRAL TEXAS, INCLUDING PORTIONS OF THE HILL COUNTRY...

...Texas...
A targeted Moderate risk was maintained across portions of south
central TX, including portions of the Hill Country. The 18z HREF
and REFS show increasing probabilities of 1"/hr rainfall by
04z-05z, increasing further after 06z. The pattern is favorable
for localized areas of heavy rainfall by late tonight into the day
Thursday. PWs will be above the 90th percentile, with abundant low
level moisture advecting in from the Gulf, and mid and upper level
moisture out of the Pacific ahead of the approaching upper low.
Instability will be sufficient deep convection and high
rates...thus the extent of the flash flood risk will come down to
the longevity of these higher rates. Cells should initially be
rather quick moving of to the Northeast, however as low level flow
increases we should tend to see an increasing
backbuilding/training component to the convection. This should be
enough to result in an uptick in the flash flood risk.

Both the 18z HREF and REFS have high neighborhood probabilities of
exceeding 3" by 12z Thursday over the Moderate risk area. The most
recent 23z run of the HRRR as rainfall of 3-5" over this same
period. The rainfall continues into Thursday, with another uptick
in rainfall intensity and potential backbuilding convection by mid
morning into the early afternoon as large scale low level
convergence increases. Thus it still appears like the highest
chances for significant flash flooding is after 12z Thursday, when
this 2nd round convection potentially trains over areas that have
already received heavy rainfall and are thus saturated. In fact,
the REFS probabilities of exceeding the 24hr 100yr ARI are as high
as 25%, with the HREF around 10%. This is indicative of the higher
end impact potential Thursday should both rounds of convection
materialize over the same portion of the Hill Country.

Even though the higher probability threat is likely tomorrow, the
flash flood risk tonight should not be understated. While the model
consensus of upwards of 3-5" through 12z would result in some flash
flooding, there is certainly a possibility that enhanced
backbuilding results in more localized totals over 5" in just a
couple hours...which would likely be enough to produce significant
flash flooding over these more flood prone areas. Thus a Moderate
risk remains for both tonight and tomorrow across this corridor.
The Moderate risk areas are quite small, but in reality the axis of
most extreme rainfall and higher end impacts will likely be even
narrower than these risk areas. However predicting exactly where
this occurs is not feasible, warranting the slightly larger risk
areas to account for location uncertainty.

...Southwest...
A Marginal risk was maintained for portions of AZ and NM. The
approaching upper low will result in continued showers and embedded
thunderstorms into the overnight period. The highest rainfall rate
potential is likely this evening over central AZ, but this should
be isolated and generally on a downward trend into the overnight.
The highest areal averaged rainfall (~1") will be across portions
of southwest NM, but lower instability should keep these rates
mostly at 0.5"/hr or less. Thus overall the flash flood threat
should stay isolated in nature.

Chenard


Day 2
Valid 12Z Thu Nov 20 2025 - 12Z Fri Nov 21 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTH-CENTRAL TEXAS, INCLUDING PORTIONS OF THE HILL COUNTRY...

...20z Update...

A couple of significant changes to the outlook areas for this
update.

This first is to introduce a targeted Moderate Risk upgrade for
the same areas as detailed for Day 1. This is due to increasing
likelihood for localized 5"+ storm totals beyond the 12z Thursday
Day 1/2 cut- off, which may result in numerous instances of flash
flooding in a relatively small area (with higher confidence overall
in locally significant flash flooding as opposed to the total
areal coverage). While 6-hr FFG exceedance probs from the HREF are
below 20% through the end of Day 1, they increase meaningfully to
near 60% going into Day 2 (through 18z Thu).

The second significant change was to split up the larger Slight
Risk area into two distinct areas, 1) South-Central TX and 2)
eastern OK into northern AR and southern MO. This reflects the
consensus of the latest CAM guidance which suggests a large gap in
convection across North TX during the period (with questions still
lingering regarding the eastward extent of both the South-Central
TX and Ozarks clusters of convection into the later half of Day 2).

Churchill

..Previous Discussions..

...Southern Plains...

The environment continues to becomes more supportive of storms
capable of producing heavy to potentially excessive rainfall as the
flow aloft backs over portions of Texas and Oklahoma and becomes
increasingly diffluent as an upper trough approaches the region. At
lower levels...an airmass with precipitable water values gets
drawn northward an converges along a surface front over parts of
Texas and Oklahoma. Excessive rainfall is possible by means of
intense rainfall rates by individual storms in regions with the
highest amount of instability (presumably ahead of the approaching
cold front) and by training of cells/multiple rounds of convection
(more of a factor where synoptic scale forcing and mesoscale
forcing act in tandem farther north). The axis of highest
precipitable water values... 1.5-1.75"... sets up over the eastern
half of Texas into southeastern-most Oklahoma and into Arkansas.
Those values are well above average for mid-November. Localized
rainfall amounts up to 2.5" in an hour with additional local totals
of 5" possible within this set up which would result in scattered
instances of flash flooding.

...California...

Maintained a Marginal Risk area from continuity for portions of
coastal Southern California as the next area of low pressure over
the eastern North Pacific drops southward off the California coast
and brings another round of rainfall Thursday into early Friday.
Rainfall amounts at this point look to be pretty modest but the
rain will be falling on areas where antecedent conditions have
been wet for the better part of 5 or 6 days. That may result in
additional localized flooding on Thursday or Thursday night.

Bann


Day 3
Valid 12Z Fri Nov 21 2025 - 12Z Sat Nov 22 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY AND PORTIONS OF THE SOUTHWEST US...

20Z Update: Limited changes necessary from the previous forecast due
to general run-to-run continuity. Heaviest rainfall still trending
closer to the southern Peninsular Range with an extension of
convection back over the southeast CA deserts and southwest AZ for
the MRGL over the west. Moderate to locally heavy rain possible for
the Ohio Valley. Steady forward propagation of broad QPF shield
should limit the necessity for more than the current MRGL risk. Will
assess the southern portion of the risk area closely in the coming
days as this region, extending into the Southeast CONUS will be most
coincident with a purely convective regime as instability is
prevalent within a broad, but modest theta_E depiction. This area
would promote higher rate potential, but could be a case where
forcing is not enough to activate the better thermodynamic
environment. For now, maintain relatively continuity of the MRGL
with some minor adjustments overall.

Kleebauer

..Previous Discussions..

...Ohio Valley...

Rain will be spreading northward and eastward from the Plains on
Day 2 into the Ohio Valley on Friday and Friday night in
association with the surface low coming out of the mid-Mississippi
Valley. Between above normal levels of moisture being transported
into the region from the south and flow aloft becoming confluent...
there is still the possibility for isolated downpours that lead to
spotty problems from run-off despite the set up becoming less
favorable with time.

...Southwest US...

A mid- and upper-level trough with an embedded closed low will
continue to drop southward on Friday...with an embedded closed low
expected near the US border with Mexico by 00Z on Saturday. Showers
and embedded thunderstorms should be developing in the highly
diffluent flow to the east of the trough where precipitable water
values approaching an inch rotate into southern California and
western Arizona. Opted to remain at a Marginal threshold given the
timing of the best upper support but a Slight may still be needed.

Bann


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