FOUS30 KWBC 201912
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
312 PM EDT Mon Apr 20 2026
Day 1
Valid 16Z Mon Apr 20 2026 - 12Z Tue Apr 21 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH-CENTRAL TEXAS INCLUDING PARTS OF THE HILL COUNTRY...
1900Z Special Update...
Very efficient and relatively concentrated areas of heavy showers
and thunderstorms have been tending to expand in coverage across
areas of south-central TX over the last couple of hours. Rainfall
rates of up to 1.5 inches/hour have been noted with activity more
recently in between the San Antonio and New Braunsfel vicinity, and
is well aligned with a corridor of stronger 850/700 mb moisture=20
convergence. Broadly diffuent aloft and modest instability coupled=20
with some uptick in the low-level jet going into the evening hours=20
suggests at least some potential corridors of persistent and=20
focused convection with high rainfall rates. The latest CAMs=20
generally are doing a poor job with the current activity. Given the
satellite and radar trends, and the increasingly sensitive=20
conditions on the ground near the Hill Country and adjacent areas=20
of south-central TX, some areas of flash flooding will be possible.
Potential for additional 2 to 4+ inch rainfall amounts will exist
which will include additional rounds of heavy rainfall potential
tonight. As a result, a Slight Risk of exceesive rainfall has been
introduced for these areas.
Orrison
1600Z Update...
The morning 12Z HREF guidance continues to support a threat for
mainly elevated, but also rather efficient areas of convection
impacting portions of the TX Hill Country and adjacent areas of
south-central TX going through this afternoon and tonight. A
combination of modest MUCAPE values of around 500 J/kg along with
850/700 mb frontogenesis and some divergent flow aloft ahead of
upstream shortwave energy/troughing should continue to support
areas of locally repeating rounds of moderate to occasionally heavy
rainfall. The early-morning RAP data shows a weak, slow-moving 500
mb vort center over the TX Hill Country playing a role as well
aside from the deeper layer modest ascent downstream of troughing
over northwest Mexico and the Southwest U.S. The soil conditions
are moistening up and locally some streamflows are more elevated
after some of the heavy rainfall from last night, and this suggests
some increasing instability that may foster a somewhat stronger
concern for at least localized flash flooding going through this
evening and overnight. The early-day CAMs and AI guidance suggests
some locally concentrated areas of rainfall with some additional
totals of as much as 2 to 3+ inches by Tuesday morning. As such,
the Marginal Risk has been expanded a bit, and there is an outside
chance that a targeted Slight Risk could be introduced given the
relatively persistent nature of the rainfall.
Only very minor mainly timing related adjustments were made to the
Marginal Risk area over northern CA. A rather strong upper low
offshore of the West Coast will edge east and drive a band of
heavier rainfall tonight across northern CA, with generally low-end
small stream and some localized urban flooding possible for the Bay
Area and into the Sacramento Valley.
A Marginal Risk was introduced to portions of southern Florida as
a cold front will be dropping south and combining with localized
seabreeze convergence for areas of heavy showers and thunderstorms.
A combination of the 12Z HREF guidance along with recent HRRR and
RRFS solutions suggests a localized potential for concentrated
heavy rainfall along the I-95 corridor of southeast FL and also the
southwest part of the state near and adjacent to the Naples area.
Some pockets of 3 to 4+ inches will be possible which may favor an
isolated urban flash flooding threat.
Orrison
Previous discussion...
...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.
...South-Central Texas...
A new Marginal Risk was reintroduced for a portion of south-central
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 south-central 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 IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA AND SOUTHEAST TEXAS...
1900Z Update...
Generally no changes to the previous thinking with the deep closed
low/trough coming into CA. Locally heavy rains are expected for=20
the coastal ranges and especially into the Sierra Nevada foothills=20
where locally a few inches of rain will be likely for this period.=20
Minimal changes were made to the Marginal Risk area here where
localized urban flooding concerns and an isolated threat for burn
scar flash flooding will exist.
Meanwhile, the mid-level shortwave energy/troughing over central=20
TX that will be gradually shifting eastward will not have much in=20
the way of instability to work with for most of the period.
However, the environment will be rather moist which coupled with=20
low-level convergence near an inverted surface trough and at least
modest DPVA aloft will support at least broken areas of heavy=20
showers and thunderstorms. Relatively efficient rainfall processes
may foster some heavy rainfall totals of locally 2 to 4 inches
across southeast TX where the convection becomes more concentrated
and slow-moving. Therefore, an isolated concern for runoff=20
problems/flash flooding will exist which will mainly be urban in=20
nature.
Orrison
Previous discussion...
...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 Sierra Nevada, where the greatest rainfall rates and amounts=20
are expected. Due to the instability however, the more populous=20
valley locations may also see periods of heavier rains, including=20
across the Bay area and Sacramento. Burn scars and heavier rains=20
into some of the coastal ranges and western Transverse Ranges could
also result in localized flash flooding. The forecast remains on=20
track across California, and no changes were made. However,=20
increases in forecast rainfall along the Sierra Nevada in the=20
Central Valley may require a Slight Risk upgrade with future=20
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...
1900Z Update...
Some modest eastward adjustment to the Marginal Risk area was
accommodated for this update given some slightly more progressive
trends (especially with the AI guidance) concerning the gradually
ejecting shortwave energy across the region. Interaction with a
fairly well-defined plume/nose of moisture and instability off the
Gulf may favor some locally concentrated areas of heavy showers and thunderstorms across area near the Upper TX Coast and southwest LA.
FFG values are generally pretty high, but there will be pockets of
slow-moving storms capable of producing high rainfall rates that=20
may exceed 2 inches/hour. This would pose a concern for the more
urbanized locations for some runoff problems.
Orrison
Previous discussion...
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
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-TfkTz4ZB2dDFkpwuHD5geNvV8whd9NgY1wav27DoOKu= P5rj556eNTlEklbRGcUIeHAEflLnbZbv_X1jDll8t4CrbmE$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-TfkTz4ZB2dDFkpwuHD5geNvV8whd9NgY1wav27DoOKu= P5rj556eNTlEklbRGcUIeHAEflLnbZbv_X1jDll8EQRjU58$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-TfkTz4ZB2dDFkpwuHD5geNvV8whd9NgY1wav27DoOKu= P5rj556eNTlEklbRGcUIeHAEflLnbZbv_X1jDll8QTz_iKg$=20
=3D =3D =3D
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