591
FOUS30 KWBC 081953
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
253 PM EST Wed Jan 8 2025

Day 1
Valid 16Z Wed Jan 08 2025 - 12Z Thu Jan 09 2025


Given the lack of any regions with deep moisture across the country
during this time, the probability of rainfall exceeding flash
flood guidance is less than 5 percent.

Hamrick


Day 2
Valid 12Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025

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

20Z Update: The current Day 2 Marginal remains in good shape
overall. The western portion of the outlook area was trimmed back a
little to account for a slightly faster trend in the 12Z guidance
suite, including the CAM guidance. It appears likely that the best
instability should remain generally along and south of the
Interstate 10 corridor. Simulated reflectivity from the FV3
supports the potential for some rounds of convective training,
mainly between Victoria and Beaumont, where rainfall rates could
exceed an inch per hour. Even though widespread 1-2 inch rainfall
totals are likely to the north of the Marginal Risk area, the rates
should be held in check and thus limit any flooding problems.
/Hamrick

------------------------

Large scale ascent ahead of an open, yet vigorous upper trough
will help initiate surface cyclogenesis over the western Gulf with
a modest push of warm unstable air in-of the TX coast, eventually
into southwestern LA. Current guidance is coming into agreement on
a skinny axis of heavier rainfall near the middle and upper TX
coasts propagating east-northeast into the Lower Sabine by the end
of the period with local amounts of 2-4" plausible within the
heavier rain footprint. The driving component of the potential is
still very much the increasing frictional convergence pattern on
the western flank of the surface low as easterly winds bisect the
TX coast up into LA with the core of better theta_E`s remaining at
the immediate coast and over the Gulf waters. The key is the
onshore flow mechanism taking some of the convective cores offshore
and moving them inland causing some heavier rainfall to protrude
inland before running into a wall of cooler, more stable air thanks
to an Arctic push that occurred prior to the evolving surface
pattern.

Recent NBM prob fields are >50% for locally 2+" of rainfall within
some of the urban footprint in coastal TX which allows the
opportunity for local flood concerns within those urbanized zones
thanks to higher runoff potential. The saving grace from this being
a higher risk is the agreement on minimal surface based
instability with the mean SBCAPE in relevant ensembles hovering
between 50-100 J/kg with some deterministic maybe exceeding that
at the immediate coast. With the cells likely to struggle being
rooted at the surface, the prospects of more widespread flash
flooding are lower than normal creating a threat that is more
localized and within the lower to mid bounds of the MRGL risk
threshold. The previous forecast was generally maintained outside
some adjustment further on the northern periphery of the risk area,
aligning with the probability and mean instability fields.

Kleebauer

Day 3
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...

Our favorable longwave trough and accompanying surface pattern will
move eastward with the surface low over the Gulf moving onshore
within LA by the first half of the period. Increasing surface
convergence within the periphery of the surface low will generate
an axis of heavier precipitation aligned within the modest
frontogenetic setup within the northern periphery of the surface
circulation. There`s growing support for a heavy rain footprint
over south-central LA, moving eastward into southeastern LA where
the urban corridors centered around New Orleans come into play.

Like the prior period, the instability pattern is meager,
especially at the surface with most of the ascent driven within the
top of the boundary layer and above where the area lies in the
right-entrance region of the upper jet focused to the north. There
is enough consensus on the location of heaviest rainfall within the
that frontogenetic evolution, coinciding with a strong IVT pulse
(~1000 kg/ms) being depicted within the NAEFS, a solid 4+ standard
deviation interval for the time of year. This should help mitigate
the negligible instability pattern in place and maintain a solid
heavy rain footprint in that focal area mentioned above. Another
consideration is the much colder grounds occurring prior to the
arrival of the rainfall as Arctic air will navigate southward and
allow for a period of near and sub-freezing air to harden the top
soil layer creating a slightly higher potential for runoff.

The MRGL risk from D4 was maintained and expanded to include much
of south-central LA through southeast LA and coastal MS. There is a
potential for a targeted SLGT risk in future updates, mainly within
the corridor extending along and south of I-20 and just north of
the coastal Parishes as the best flood threat will exist within the
more urban zones.

Kleebauer

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt