FOUS30 KWBC 100038
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
738 PM EST Thu Jan 9 2025

Day 1
Valid 01Z Fri Jan 10 2025 - 12Z Fri Jan 10 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST...

Increased mid and upper level forcing is ongoing over portions of
TX caused by a potent ULL moving towards the Gulf Coast, with a 
surface low deepening near the Middle and Upper TX Coasts at the 
time of this discussion's writing. Warm air/moisture advection is 
occurring near a coastal front to the northeast of the cyclone 
which is allowing hourly rain totals to range between 0.5-1" as of 
late. A smidgen of mixed layer and most unstable CAPE is present 
near and ahead of the low, which should increase a little more 
tonight. When combined with increasing frontogenesis along the 
coastal front and a cooler air mass, there is concern for 
occasional organized convection as low-level inflow/effective bulk 
shear is sufficient for such should enough instability be present. 
Convective elements should try to build further along and near the 
coastal plain/swamp/bayous in this region. While hourly rain totals
to 1.5" and local amounts to 4" remain in the cards, recent 
dryness suggests that urban areas would be most impacted. IVT 
values with this system top 1,000 kg/m/s presently, which is well 
above the maximum observed IVT in the CFSR database for this time 
of year and implies some upward potential for rainfall efficiency 
despite the expectation of some forward progress with time. Some 
eastward extension to the Marginal Risk area was made to align with
10%+ chance of 5"+ noted in the 18z HREF probabilities, and the 
back edge carved back due to system progression.

Roth/Mullinax/Kleebauer


Day 2
Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

...20Z Update...

The inherited Marginal Risk remains in good shape as the region
will struggle to see a dearth of instability aloft and any
storms/showers will be progressive in their easterly movement.
That said, NAEFS shows that around 12Z Friday, IVT values aimed at
the central Gulf Coast will be as high as 1250 kg/m/s, which would
be above all observed IVT values in the CFSR database for this
time of year. Similar to the Upper Texas Coast on the Day 1 ERO,
soundings are highly saturated and warm cloud layers are around
12,000ft deep. This should result in efficient warm rainfall that
may support highly localized flash flooding potential, especially
in urbanized areas that drain poorly along the I-10 corridor. But
the progressive storm motions and lack of modest instability should
keep the areal extent and severity of the flash flood threat to a
limited number of at-risk urbanized areas through the early
afternoon hours on Thursday.

Mullinax

--Previous Discussion--

Our western Gulf disturbance will migrate east-northeastward with a
focus of heavier precip across southern LA through the urban zones
of southeast LA during the morning Friday. The system will be
fairly progressive and the trends have brought down storm totals
due to the progressive nature and limited instability factoring in
the local maxima relegated <2". The expectation is for there to be
a thin line of heavier rates along a developing cold rain band,
something not too common at these latitudes. There is a textbook
surface convergence pattern right within the north and western
flank of the low center as it cross through the southeastern
Parishes of LA and this aligns within a narrow tongue of elevated
theta_E's that get advected just ahead of the surface reflection.
This will lend of a period of potentially significant rainfall
rates bordering 1"/hr at peak intensity, a factor that could be
enough to generate some localized flooding within the urban zones
centered around New Orleans and points north, west, and east.

This is very much of a lower end MRGL risk considering the trends
of the forward propagation and such a skinny axis of the heavier
precip anticipated. This also aligns with more of a morning and
early afternoon risk with a quick decay behind the low as light
stratiform takes over, then dries out as drier air advects in
behind the departing low to the east. The previous MRGL risk was
maintained in full with an emphasis on flooding potential along
and south of I-10/12 with the best prospects located around urban
zones surrounding Lake Pontchartrain.

Kleebauer

Day 3
Valid 12Z Sat Jan 11 2025 - 12Z Sun Jan 12 2025

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

Mullinax

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