Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Sat Apr 20 2024

Day 1
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...

The primary change to the ERO from the previous forecast was to
expand the Slight a bit to cover all of central Mississippi all the
way to the Alabama border. The latest guidance and CAMs suggest
that convection that forms at the nose of a robust LLJ will track
along to the east a bit more quickly than previous forecasts.
Training convection will be likely in this scenario as the storms
track along a nearly stationary boundary. Unfortunately in the
Arklamiss, the convection will track near or over an area that was
quite hard hit a few days ago, covering the AR/LA/MS tripoint along
the Mississippi River and extending across central Mississippi.
Thus, despite the fact the forecast remains wetter across portions
of east Texas, the antecedent conditions and saturated soils in the
Arklamiss should make up for the lesser amounts of rainfall
expected to still result in widely scattered instances of flash
flooding. Depending on how well the convection holds together in
this area, localized considerable flash flooding is possible,
especially if the stronger storms track in the same areas hard hit
a few days ago.

Across Texas, the Slight risk area is largely unchanged. As a
strong shortwave ejects out into Texas today, increasing Gulf
moisture moving north along a strong LLJ will raise PWATs above 1.5
inches in many areas, which is about 2-2.5 sigma above normal. This
will support a large area of rain over much of north Texas, which
will quickly expand east with time. The shortwave and resultant
cyclogenesis will shift the LLJ east with time. Meanwhile, strong
Canadian high pressure will dive south down the Plains, increasing
the thermal gradient...along which the convection will form and
move east. The shortwave and strongest moisture flux will support
the strongest convection across east Texas this afternoon at peak
heating, where a higher-end Slight is considered in effect. As the
rainfall pushes east into the Arklamiss this evening, the
thunderstorms should gradually weaken, resulting in slightly less
rainfall, but as mentioned above the antecedent conditions are more
favorable for flash flooding development.

While changes to the Slight and surrounding Marginal Risk areas
were minimal, they were nudged southward with this update in
keeping with both guidance trends and typical climatology of these
kinds of events, where stronger storms and heavier rains tend to
form a bit south of guidance where both instability and moisture
availability are both greater.

Wegman

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.

The inherited Marginal Risk area for the central Gulf Coast was
removed with this update, in coordination with LIX/Slidell, LA and
MOB/Mobile, AL forecast offices. A bit faster evolution of the
rainfall and dry air encroachment from the central Plains will
allow remaining rainfall and potentially some embedded
thunderstorms to move east quickly by Sunday morning. As a result,
forecast rainfall in this area continues to rapidly decrease, as
has been the trend in the guidance for the past few days.

Wegman

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.

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