FOUS30 KWBC 180658
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Sat Apr 18 2026

Day 1
Valid 12Z Sat Apr 18 2026 - 12Z Sun Apr 19 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST AND LOWER MISSISSIPPI VALLEY AS WELL AS FOR PORTIONS
OF THE MID-ATLANTIC...

...Gulf Coast and Lower Mississippi Valley...

A cold front sinking south across Texas on Saturday will run into
an increasingly moist and unstable air mass located along the Gulf
Coast Sunday afternoon. As the front lifts that air mass, rounds of
showers and thunderstorms are expected to form and roughly parallel
I-10 from near San Antonio east through southwestern Louisiana. The
potential for multiple rounds of heavy rain across urban areas,
particularly Houston, could result in isolated instances of flash
flooding. Recent dry weather across southeast Texas and especially
into Louisiana and southern Mississippi should work to limit the
flash flooding potential there, whereas across central Texas, soils
are a bit wetter from more recent heavy rainfall. Thus, FFGs
precipitously increase with eastward extent along I-10. The
inherited Marginal Risk was trimmed on the northern side, as the
most persistent thunderstorms are likely to focus further south
towards the Gulf Coast. The Marginal was expanded a bit west down
I-10 towards San Antonio due to the aforementioned more favorable
soil conditions.

...Mid-Atlantic...

A progressive line of thunderstorms associated with a fast-moving
cold front will sweep eastward across northern Pennsylvania and
western and central New York this afternoon and evening. The
Marginal was introduced primarily because preexisting moist soil
conditions across this region and the topography will both support
the development of isolated instances of flash flooding despite
less than impressive rainfall totals across this area. The line of
storms will traverse the Marginal Risk region at peak heating late
afternoon through the evening, which should allow the storms to
also be at their peak in strength.

Wegman

Day 2
Valid 12Z Sun Apr 19 2026 - 12Z Mon Apr 20 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH TEXAS...

The cold front from Saturday night should be just off the coast
and into the Gulf Sunday morning, although a residual inverted
trough extending from the tail of this front is expected to linger
across South Texas much of the period. As the day progresses, weak
shortwave energy embedded within a shallow southern stream trough
will emerge from Mexico to produce modest ascent through divergence
and PVA, combining with convergence along the aforementioned
trough to create scattered showers and thunderstorms across the
region. Downstream of this feature, subtle ridging will halt the
progression of the surface trough, while additionally keeping PWs
pooled across South Texas, which is reflected by NAEFS ensemble
tables indicating that PWs will remain above the 90th percentile of
the CFSR climatology.

The overlap of this ascent into the moist column will manifest as
heavy rain producing convection, with both HREF and REFS indicating
a low end potential for 1"/hr rainfall rates. While the high-res
simulated reflectivity still features wide variation in coverage,
making confidence in the excessive rainfall potential a bit lower
than usual, storm motions within any of this convection will likely
be slow (0-6km mean winds of just 5-10 kts), so any of these heavy
rates could be sufficient to overcome the FFG. The greatest
potential may be along the immediate coast due to the overlap of
slower storms (pinned) and lower FFG, but anywhere these slow
storms develop, isolated instances of flash flooding could result.

Weiss

Day 3
Valid 12Z Mon Apr 20 2026 - 12Z Tue Apr 21 2026

...THERE IS A MARGINAL CHANCE OF EXCESSIVE RAINFALL ACROSS PARTS OF
NORTHERN CALIFORNIA AND ACROSS THE LOWER-MIDDLE TEXAS COAST TO THE
HILL COUNTRY...

...Northern California...

Deepening upper level low eventually closes off late in the weekend
off the northern CA coast. Afterwards, the guidance shows this
broad, deep low pinwheeling slowly eastward on day 3, thereby
allowing for a prolonged period of favorable forcing, i.e. upper
diffluence/deep layer ascent and anomalous low-mid layer moisture
transport. The latest GEFS in fact indicates 850-700 mb moisture
flux peaking between 2-3 standard deviations above normal from the
Bay area into the northern CA coastal ranges and Sacramento Valley.
Due to the nature of the upper system (closed mid-upper low), this
will not be a typical AR event, per the 'weak' AR classification
from CW3E. However, Integrated water vapor transport (IVT) values
do peak around 500 Kg/m/s, while the guidance (GFS-ECMWF-NAM) does
indicate elevated CAPEs of 100-250 J/Kg. Areal average QPFs per the
guidance on Day 3 (12Z Mon-12Z Tue) support widespread 1-2"
rainfall totals, though higher-res members (including the RRFS) do
show isolated higher totals closer to 3". Given the potential for
at least 0.50"/hr rainfall rates on Day 3, the Marginal Risk area
was maintained (and expanded) to account for the possibility of
some excess runoff over burn scars.

...Lower-Middle Texas Coast to the Hill Country...

Guidance continues to show subtle/flat southern stream shortwave
traversing this area Mon-Mon night, which will likely become
convectively-aided from any MCV(s) emanating out of northeast
Mexico. Uptick in deep-layer forcing along with moisture pooling
from the western Gulf (PWs increasing to 1.7-1.8") and some, albeit
weak instability (MUCAPEs ~500 J/Kg) will lead to a more favorable
environment for widespread rainfall with embedded heavier cores. We
maintained the Marginal Risk area inherited from yesterday's Day 4
ERO, while expanding a little farther north to include parts of the
Hill Country based on the latest guidance trends (deterministic and
probabilistic QPF). In addition, the latest ECMWF Extreme Forecast
Index (EFI) for QPF peaks between 0.6-0.7 during this event, with
a shift-of-tails close to 1 along the coast.

Hurley


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