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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
453 AM EDT Thu May 16 2024
Day 1
Valid 12Z Thu May 16 2024 - 12Z Fri May 17 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PINEY WOODS REGION OF EASTERN TEXAS AND WESTERN LOUISIANA...

...DANGEROUS AND LIFE-THREATENING FLASH FLOODING LIKELY...

In coordination with HGX/Houston, TX; SHV/Shreveport, LA; and
LCH/Lake Charles, LA forecast offices, a High Risk upgrade was
introduced with this update. Dangerous and life threatening flash
flooding is likely in the High Risk area of the Piney Woods.

A vigorous shortwave trough leading a 110 kt zonal jet will move
across Texas today into tonight. Meanwhile, a moisture laden air
mass from the Gulf will advect north into eastern Texas today. The
air mass is characterized by PWATs that may exceed 2.25 inches,
which is above the 95th percentile for PWATs for this time of year,
and is over 3 sigma above normal. In portions of Louisiana this
exceeds the 99th percentile. All this to say, there will be plenty
of moisture for storms to work with as they move east. Instability
advecting northward with the moisture from the Gulf will approach
4,000 J/kg south of the line, but that level of instability will
further support very deep convection. The result is the expected
storms will be capable of extreme rainfall rates that may exceed 3
inches per hour with the strongest storms, but even with much of
the convection, 1-2 inch per hour rainfall rates will be common.

An impressive theta-e gradient over south-central Texas will push
northward on the low-level jet this morning. Some convection has
already fired across the Moderate and High Risk areas already this
morning, but it will take the arrival of the deep Gulf moisture
characterized by surface dewpoints will into the 70s to get the
storms going with daytime heating late this morning. An MCS is
likely to develop across north Texas late this morning with
embedded storms producing extreme rainfall rates. The line will
push southeastward rather quickly, but more convection developing
out ahead of the line, as well as training segments will make for a
rather small-width, but extended length line of convection that
could produce rainfall totals of 3-5 inches with local amounts as
high as 8 inches.

As the storms move into the High Risk area, a push of even deeper
moisture will greet the storms and allow them to further intensify
as they push east across the area. There has been better than
normal agreement in the overall guidance suite for multiple inches
of rain to occur as the storms move through, highlighting the Piney
Woods region for multiple days. With the full CAMs suite now
actualizing the event in the same place, confidence has increased
for the issuance of the High Risk.

AHPS data shows that the High Risk area has seen over 600% of their
normal rainfall for the past 2 weeks alone. Moderate to Major river
flooding is already ongoing in this area. Soils are not expected
to retain any of the rainfall expected today. Rates of 2 inches
per hour with locally higher rates will very quickly re-raise
levels in smaller creeks and streams, while the major rivers
already in flood stage will also see rises. Widespread flash
flooding with locally significant and life-threatening flash
flooding is likely as a result.

A couple cons still adding a bit of uncertainty to the forecast:
1) The cells will be fast-moving so any one area may only see the
extreme rainfall rates for a fraction of an hour, but training of
multiple storms capable of the high rates is expected over the High
Risk area. 2) Rain will only fall for 3-6 hours over the high risk
area as the strong storms move through. Thus, the 3-5 inches of
rain expected will fall over that much shorter time period. This
certainly raises the chances for flash flooding, but with no rain
expected following the storms, this does limit the severity of the
flooding expected somewhat.

As the storms push east across Louisiana tonight, they will weaken
a bit due to the loss of daytime heating and associated
instability, so amounts will drop off as the storms approach
Mississippi. This area has been hard hit with recent rains and
nearly saturated soils, but not as hard-hit as portions of Texas.
Thus, with decreasing rates, the risk areas trail off to the east.
However, there is considerable uncertainty as to how fast that will
happen and certainly where the storms will train towards the east
across Louisiana. Thus, the eastern boundaries of the Moderate and
Slight risk areas are more uncertain.

...North Central Texas...

Better agreement in the CAMs for an axis of heavy rain developing
along the aforementioned theta-e gradient this morning will allow
convection capable of very heavy rains to develop in the vicinity
of the Dallas-Fort Worth Metroplex. Most of the guidance suggests
the strongest convection will be on the west/Fort Worth side of the
Metroplex, so the threat is a little bit lower for Dallas. Nonetheless,
low FFGs characteristic of large cities will allow flash flooding
to develop much more quickly and with less rain than surrounding
areas. In coordination with FWD/Fort Worth, TX forecast office, a
Moderate Risk upgrade was introduced with this update. The
strongest storms will quickly shift south of the Metroplex this
afternoon, but lingering light to moderate rain may continue into
the evening, lengthening the duration of any flash flooding.

Wegman