FOUS30 KWBC 200804
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Mon Apr 20 2026

Day 1
Valid 12Z Mon Apr 20 2026 - 12Z Tue Apr 21 2026

...THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHCENTRAL TEXAS...

...Northern California...

A large upper level cutoff low located off the coast will drift
eastward into northern California today. The cutoff low will not
have a connection to the tropics, which will limit both amount of
moisture and any instability that can be transported into northern
California. Despite that, as is typical of cutoff lows, the
movement will be slow, which could allow for an extended duration
of light to moderate rainfall. That rain will impact an area that
has burn scars, urban areas (Bay Area), and steep terrain which
favors localized flash flooding. Thus, the Marginal Risk is left
unchanged with this update.

...Southcentral Texas...

A new Marginal Risk was reintroduced for a portion of Southcentral
Texas with this update in coordination with EWX/San Antonio, TX
forecast office. A subtle shortwave in the upper levels will add
forcing to an already highly moist environment with relatively slow
storm motions. Instability will be the primary limiting factor for
heavy rainfall rates, but the shortwave is expected to overcome
that instability a bit with the increased forcing. This solution is
depicted in several of the CAMs, which show a series of training
storms developing well after sundown, and tracking slowly eastward
across Southcentral Texas. Many of those CAMs highlight the storms
tracking right over the San Antonio metro area. The primary flash
flooding threat involves urban concerns in the San Antonio metro
Monday night.

Wegman


Day 2
Valid 12Z Tue Apr 21 2026 - 12Z Wed Apr 22 2026

...THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA AND SOUTHEAST TEXAS...

...California...

A cutoff large upper level low over the coast Tuesday morning will
progress east/inland through Tuesday night. This will direct the
southwesterly onshore flow further south and east across central
California. A pocket of instability with MUCAPE values as high as
500 J/kg may move into central California during the day Tuesday as
well. The primary forcing however will remain the steep slopes of
the Sierras, where the greatest rainfall rates and amounts are
expected. Due to the instability however, the more populous valley
locations may also see periods of heavier rains, including across
the Bay area and Sacramento. Burn scars and heavier rains into some
of the coastal ranges and western Transverse Ranges could also
result in localized flash flooding. The forecast remains on track
across California, and no changes were made. However, increases in
forecast rainfall along the Sierras in the Central Valley may
require a Slight Risk upgrade with future updates.

...Southeast Texas...

The Marginal Risk across southeast Texas remains unchanged with
this update. Onshore flow of deep Gulf moisture will advect across
southeast Texas along the periphery of a large area of high
pressure across the Southeast. MUCAPE values are expected to rise
above 1,000 J/kg across much of the MRGL risk area. Slow-moving and
training thunderstorms could cause isolated instances of flash
flooding, especially in urban areas such as Houston.

Wegman


Day 3
Valid 12Z Wed Apr 22 2026 - 12Z Thu Apr 23 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA...

Persistent onshore flow of deep Gulf moisture and instability will
continue into southeast Texas and southwest Louisiana on Wednesday.
This is a continuation of the moist, unstable flow off the Gulf
from prior days, just drifted a bit to the east. Instability across
this region continues to increase compared to prior days, with
areas of over 2,000 J/kg in MUCAPE. Showers and thunderstorms will
continue to move northwestward amidst the southeasterly flow off
the Gulf. Urban areas such as Houston and Beaumont will have a
higher threat of isolated flash flooding, which will occur where
the storms with the heaviest rainfall rates are most persistent.

Wegman
$$
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