FOUS30 KWBC 190745
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sun Apr 19 2026

Day 1
Valid 12Z Sun Apr 19 2026 - 12Z Mon Apr 20 2026

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

In coordination with CRP/Corpus Christi, TX and EWX/San Antonio, TX
forecast offices, the inherited Marginal Risk across Deep South
Texas was removed with this update. While there will be high
moisture content (PWATs around 1.75 inches) and slow moving cells
due to light steering winds across Deep South Texas, the lack of
instability and very high FFGs will effectively squelch any flash
flooding threat. MUCAPE values will struggle to exceed 300 J/kg
through the day, which is simply not enough instability to generate
the rain rates necessary to exceed the high FFGs.=20

Wegman


Day 2
Valid 12Z Mon Apr 20 2026 - 12Z Tue Apr 21 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA...

...Northern California...

A large upper level cutoff low located off the coast will drift
eastward into northern California on Monday. 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=20
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.

...South Texas...

The inherited Marginal Risk over a portion of this area was
removed with this update, in coordination with CRP/Corpus Christi,
TX, EWX/San Antonio, TX, and HGX/Houston, TX forecast offices. As
on Day 1/Sunday, the limiting factor once again will be a true lack
of instability, with MUCAPE values again struggling to exceed a
couple hundred J/kg anywhere in Texas. Thus, despite above normal
atmospheric moisture, and below normal forward motions for any
cellular convection that does form, the high FFGs and limited
rainfall rates should continue to prevent any flash flood development.

Wegman


Day 3
Valid 12Z Tue Apr 21 2026 - 12Z Wed Apr 22 2026

...THERE IS A MARGINAL RISK OF 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. Thus, the inherited Marginal
risk was expanded south down the coast to Santa Barbara, while the
Marginal in central California was left unchanged with this update.

...Southeast Texas...

The Marginal Risk across southeast Texas was adjusted a bit to the
northeast from inherited in keeping with the latest trends in the
guidance, suggesting increased potential for heavier rains into the
Houston area, and decreased potential around Corpus Christi. Unlike
both Sunday and Monday, by Tuesday there's better agreement that
the primary limiting factor, instability, will be less of a
limiting factor. MUCAPE values are expected to rise above 1,000
J/kg across much of the MRGL risk area. Somewhat more saturated
soils in the Houston area as a result of yesterday's rains could
also have some lingering effects into Tuesday. Thus, the inherited
Marginal was left largely unchanged with this update.

Wegman
$$
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