FOUS30 KWBC 080803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Wed Jan 8 2025

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

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

Kleebauer


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...

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

$$
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)