FOUS30 KWBC 160853
QPFERD

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


Day 2
Valid 12Z Fri May 17 2024 - 12Z Sat May 18 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTHERN MISSISSIPPI/ALABAMA...

By the start of the period 12Z Friday, the ongoing showers and
thunderstorms from the Day 1 period will be moving across the Day 2
Moderate risk area already. This first batch will quickly move off
to the north and east into central Alabama and Georgia through the
day, with occasional showers and storms following behind it.

The front that moved the LLJ advecting the impressive Gulf moisture
into the southern US will stall out across Louisiana, Mississippi,
and Alabama by Friday morning. With the LLJ no longer moving, the
low level southwesterly flow will align with the 90 kt upper level
jet streak. This will increase the potential for training storms,
if fast-moving, across the Moderate and surrounding Slight Risk
areas. Once the first batch of storms moves through in the morning,
there will be a relative break for much of the afternoon. However,
weak shortwave impulses tracking parallel to the jet streak may
still allow for occasional waves of storms to track across the
Moderate Risk area through the day.

Late Friday afternoon into the evening, the approach of a final and
strongest shortwave will reach the Slight and Moderate risk areas,
resulting in a "blossoming" of convection into the region. With
continued very high PWATs and instability, the storms will remain
capable of producing rates as high as 2 inches per hour, locally 
higher with the strongest storms. Multiple rounds of storms are
likely through Friday night, which will result in storm total
rainfall through early Saturday morning of 2-4 inches with local
totals to 6 inches. The strongest and most prolific rain-makers are
expected during the predawn hours Saturday as the aforementioned
strongest upper level shortwave moves northeast across the Slight
and Moderate Risk areas.

Soils in this area are also quite saturated from recent heavy 
rainfall, so it's expected that most of the rainfall through 
Saturday morning will convert to runoff, resulting in scattered 
instances of flash flooding with locally considerable flash 
flooding.

In coordination with JAN/Jackson, MS; BMX/Birmingham, AL; and
MOB/Mobile, AL forecast offices, the Moderate Risk was expanded
northward into Birmingham and southward into Mobile, AL with this
update.

...Mississippi and Ohio Confluence Region...

Little has changed in the forecast for this region. A secondary
shortwave trough on the northern side of the jet stream will take
some of the Gulf moisture streaming in from the south and support
convection across the area. This convection will likely be ongoing
at the start of the period Friday morning, with any resultant flash
flooding starting soon thereafter. The storms will be capable of 1
inch per hour rainfall rates. Recent heavy rainfall along the Ohio
and Mississippi Rivers in this region have locally saturated the 
soils, and additional rainfall amounts of 1 to locally 2 inches of 
rain from morning convection may result in widely scattered
instances of flash flooding.

Wegman

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