Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
843 PM EDT Fri Apr 19 2024

Day 1
Valid 01Z Sat Apr 20 2024 - 12Z Sat Apr 20 2024

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

Chenard

Day 2
Valid 12Z Sat Apr 20 2024 - 12Z Sun Apr 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR CENTRAL AND
EAST TEXAS THROUGH PORTIONS OF THE ARKLAMISS...

2030Z Excessive Rainfall Discussion...
Except for extending the Slight and Marginal Risk areas a bit
eastward from the earlier issuance...no changes were needed to the
Excessive Rainfall Outlook issued earlier today. Numerical guidance
continues to advertise the approach of a large...but
weakening...shortwave trough that will support a growing area of
convection capable of producing excessive rainfall on Saturday.
Precipitable water values in the 1.5 to 1.8 inch range were already
in place...and timing of the shortwave trough remained similar in
the 12Z cycle compared with earlier runs.

Bann

0830Z Excessive Rainfall Discussion..
A large but weakening shortwave trough will eject into the
southern Plains from the Rockies on Saturday. Meanwhile, south to
southeasterly flow from the Gulf will advect increasing amounts of
Gulf moisture into south Texas ahead of the shortwave. At the
surface, building High pressure will move into the central Plains,
bringing a much cooler air mass into the Plains and Midwest. The
contrast of these clashing air masses along with the upper level
support from the shortwave will support shower and thunderstorm
activity Saturday and Saturday night across much of eastern Texas,
extending east into the Arklamiss.

Some light rain on Friday may help saturate soils a bit across
northern Texas, but for the most part as regards FFGs/antecedent
conditions, the storms will be on their own. However, the building
Gulf moisture will bring PWATs to 1.75 inches, which in some parts
of Texas will be 3 sigma above normal for this time of year. That
will be plenty of moisture to support strong, moisture-laden
thunderstorms across the Slight Risk area. Instability will be a
factor working against flash flooding for northern Texas...but as
the storms push southward, instability will increase enough to
support the storms. Portions of south Texas haven't had a soaking
rain in a while, so FFGs are quite high across this area. Meanwhile
the Slight Risk area was expanded east into the Arklamiss due to
relatively more recent heavy rainfall. By Saturday evening, the
upper level shortwave support will be somewhat diffuse, which will
result in multiple rounds of showers and storms, further increasing
the flash flooding threat due to increased probability of
overlapping storm tracks.

Portions of east-central Texas are in a high-end Slight category to
account for the increased likelihood of multiple rounds of storms
moving across this region. This area is little changed from
previous forecasts, as well as the broader Marginal and Slight Risk
areas. For now it appears the somewhat fast storm motions will
preclude any areas picking up enough rainfall to result in a
further upgrade to a Moderate, but the area will continue to be
monitored.

Wegman

Day 3
Valid 12Z Sun Apr 21 2024 - 12Z Mon Apr 22 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

...2030Z Excessive Rainfall Discussion...
A Marginal Risk of excessive rainfall remains in place along
portions of the central Gulf coast of Sunday as low pressure and an
associated cold front approaches from the north. With good run to
run continuity in the model mass fields and deterministic QPF from
the 12Z suite of numerical guidance and WPC...saw little reason to
make more than a couple of minor adjustments to the
northern/western boundary of the risk area.

Bann

...0830Z Excessive Rainfall Discussion...

Convection ongoing Sunday morning across far east Texas and the
Arklamiss will move eastward into the rest of Louisiana. While the
storms will be weakening in the most climatologically-hostile time
of day for the storms, expect the abundant moisture from the
eastward shifting LLJ will still support storms capable of locally
heavy rain. Recent rainfall events have made portions of eastern
Louisiana more sensitive to flooding, along with urban concerns
around New Orleans. The New Orleans metro is the highest concern
within the broader Marginal. The LLJ will move eastward more
quickly through Sunday afternoon, which with increasing westerly
flow will allow the storms to also move more quickly eastward with
time. Thus, the flash flooding threat Sunday night will be greatly
diminished into Alabama, the Florida Panhandle and Georgia. Thus,
the Marginal risk for southern Georgia and adjacent Florida and
Alabama was removed with this update. Any heavy rainfall will
likely be over or moving much too quickly across Georgia by Sunday
night to pose a flooding threat, so the Marginal is largely for the
daylight hours on Sunday.

Wegman


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt