Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
834 PM EDT Sat Apr 20 2024

Day 1
Valid 01Z Sun Apr 21 2024 - 12Z Sun Apr 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR CENTRAL AND
EAST TEXAS...


01Z Update...

Only changes needed to the outlook area were based on trends in
radar and satellite imagery. Removed areas of the Marginal and
Slight Risk areas where radar showed the rain has ended and
realigned the Slight Risk area over the eastern portion of Texas
where heavier rainfall rates remain possible given the placement
of the deeper moisture immediately upstream, the better upper level
difluence and pockets of lower Flash Flood Guidance. Farther east,
expanded the Marginal a bit to the east now that radar trends were
beginning to mirror the HRRR.

Bann

16Z Excessive Rainfall Discussion...

East TX will be the main focus for heavy rainfall today. Currently
we have ongoing elevated convection based near the 850mb front
from west TX into northeast TX. This activity is well into the
cool sector of this system, which will impact the degree of
instability and rainfall rates associated with it. Given this
activity is expected to continue through the day, conditions may
saturate enough for a gradual uptick in the flood threat,
especially over north central to northeast TX where the duration
will be longer. So while exceedance of the current 1 and 3hr FFG is
unlikely, some flood risk could eventually evolve warranting a
continuation of the Slight risk.

The higher rainfall rate potential today will be further south
closer to the surface boundary. This is where stronger low level
convergence and instability will focus, allowing for the potential
of slower moving/training convection capable of 2"+/hr type
rainfall. The greatest signal for this is over south central to
southeast TX, where 12z HREF neighborhood probabilities of
exceeding 3" are over 70% ,and exceeding 5" around 50%. While this
indicates 3"+ amounts are likely, EAS 3" probabilities are only
~15%, indicating amounts of this magnitude, while likely, should
stay pretty isolated. There is some uncertainty with regards to the
organization and coverage of this convection, which makes this
Slight risk more conditional in nature...especially given the high
FFG. However there is some threat of more organized FFG exceedance
should convection grow upscale along this low level stationary
boundary.

We did remove the Slight risk across most of northern LA into
central MS, as this area is well within the cool sector of the
system, and thus not anticipating we'll see rainfall rates high
enough for flash flooding. With that said, steady rain with
embedded higher rates may still cause isolated flooding concerns,
which is handled by the Marginal risk. The better chance for higher
rainfall rates is actually further south over southern MS. It is
here, closer to the front, where heavier rates and 3"+ totals are
a possibility. However given the high FFG, currently not expecting
enough coverage to justify a Slight risk, but localized flash
flooding (especially in any more sensitive urban areas), is
certainly possible today/tonight.

Chenard


Day 2
Valid 12Z Sun Apr 21 2024 - 12Z Mon Apr 22 2024

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

Chenard


Day 3
Valid 12Z Mon Apr 22 2024 - 12Z Tue Apr 23 2024

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

Chenard

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