Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Fri Apr 19 2024

Day 1
Valid 12Z Fri Apr 19 2024 - 12Z Sat Apr 20 2024

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

...Texas into southern Oklahoma...

The inherited Marginal Risk area from the Big Bend of Texas north
and east to the Red River Valley has been downgraded with this
update.

Starting south and working north, model guidance is in very good
agreement that any convection Friday afternoon and evening near the
Rio Grande will largely stay on the Mexico side of the river. While
it is probable that some remnant dying convection may make it over
the Rio Grande to the Texas side from Eagle Pass north, it appears
likely that much like today, the loss of terrain support should
ensure any convection that tries to move off the mountains fizzles
out quickly. However, these localized areas of convection can have
a mind of their own, which is to say may develop embedded
mesolows, which can allow the convection to persist for far longer
than expected. Since at the moment this appears to be a low
possibility, the combination of high FFGs and highly localized
convection should preclude flash flooding on the Texas side, so the
Marginal was removed.

Along the Red River, the area will be at the head of a rather
modest LLJ that will advect some Gulf moisture northward. Much of
the guidance shows an area of showers developing in response, but
lack of instability largely holds the shower activity to that
level, with relatively little embedded convection expected
overnight Friday night. Any showers and the embedded thunderstorm
or 2 should both move off to the east, and the limited instability
will cap the strength of any storms. FFGs in this area are also
quite high, so the flash flooding potential is low enough to not
meet Marginal criteria.

...Portions of the Southern Appalachians...

The remnants of the convection ongoing across portions of the Ohio
Valley will make their way into the southern Appalachians Friday,
particularly into the Virginias. Associated westerly flow will
upslope when the storms reach there, allowing them to maintain
their strength up until the crest of the Appalachians. Despite the
upslope, there will be little else supporting the showers...with
limited atmospheric moisture and instability. The one factor
causing just a hint of concern are the antecedent conditions, with
low FFGs into the southern half of WV and southwest VA. Since the
rain will really struggle to develop vertically into thunderstorms,
much less be strong enough to support heavy rain, the threat here
is also sub-Marginal. The area will continue to be monitored.

Wegman


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...

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...

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