Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
808 PM EST Fri Nov 27 2020

Day 1
Valid 01Z Sat Nov 28 2020 - 12Z Sat Nov 28 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH-CENTRAL TO SOUTHEAST TEXAS...

...Central/Eastern Gulf Coast...
One round of locally heavy rain continues to advance east across
areas of southern LA and stretching east across areas of southern
MS/AL and soon to be moving into portions of western GA. This
activity is associated with a broken axis of showers and
thunderstorms focused along and out ahead of a cold front along
with a wave of low pressure and supporting shortwave trough that
is crossing the central Gulf Coast region this evening. Generally
the rainfall rates have been well below FFG values, with
occasional rates over last couple of hours reaching upwards of 1.5
inches/hr over parts of central AL. Generally the strongest areas
of convection right now are across portions of central/southern AL
closer into the wave of low pressure. Overnight, this activity
will all continue to advance off to the east, and will result in
some additional areas of heavy rain, but generally the amounts
should continue to remain well below FFG values. A Marginal Risk
of excessive rainfall is maintained across portions of the central
and eastern Gulf Coast region, with only a very isolated concern
for any runoff problems.

...South-Central to Southeast Texas...
An upper-level trough and associated closed low ejecting out
toward the southern High Plains will increase the threat of heavy
showers and thunderstorms overnight (especially 06Z to 12Z) across
areas of south-central to southeast TX. This will occur as
increasingly divergent flow aloft interacts with a very
slow-moving frontal zone and a pool of fairly deep moisture with
PWs of 1.5 to 1.75 inches. A modest degree of instability, with
much of it elevated, will be also pooled along the front. The
latest hires model consensus, including the last few runs of the
HRRR and the HRRRp support locally as much as 2 to 3 inches of
rain with isolated heavier amounts. Overall, much of the rainfall
should be in the sub-FFG category owing to dry antecedent
conditions, but there will be some potential for some west/east
bands of convection to set up with some periodic training of cells
possible in vicinity of the front. Some of the more urbanized
areas may potentially see some increase in runoff problems as a
result. Given the increase in moisture transport and stronger
forcing that is expected here later in the night, the strongest
convective potential and heaviest rainfall will likely be in the
pre-dawn hours leading up to 12Z. A Slight Risk of excessive
rainfall is denoted across portions of south-central to southeast
TX as a result.

Orrison


Day 2
Valid 12Z Sat Nov 28 2020 - 12Z Sun Nov 29 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE WESTERN GULF COAST...

...West/Central Gulf Coast...

At the start of the forecast period (Sat-28.12z), ongoing activity
from today into tonight will be lingering in the vicinity of the
Western Gulf coast, along a well defined surface frontal zone. 
Strong southerly flow (West Gulf LLJ) along  responding to the
approach of strong closed low height falls out of the Southern
High Plains, but become increasingly weaker but still confluent
along the frontal boundary toward the LA coast.  The LLJ will be
pumping deep tropical moisture (1.75 to near 2" Total PWats)
providing even 1000-1250 J/kg of instability at 12z within the
axis of the LLJ toward the boundary.  As such ongoing activity may
back-build and regenerate across the Northwest Gulf of Mexico and
adjacent coastal SE Texas.  Uncertainty remains in the precise
location of the frontal zone due to strength of outflow boundary
today, yet the response for increased convection will pose another
round as the surface wave develops and deepens throughout the day
into Sunday morning, as it nears SW LA.  Hi-Res CAMs and global
guidance remain trending a bit more off-shore with the most
vigorous/surface rooted convection, however, there is ample signal
particularly from the slower guidance (12z ECMWF) for stronger
more northward shift and increased rainfall rates and totals
potentially over areas affected by today's rainfall.  As such,
very little has changed with the placement of the Slight Risk of
Excessive Rainfall covering much of Southeast Texas into
Southwest/South-Central LA to account for the continued model
spread; although highest confidence/greater risk appears to be
within the coastal counties in TX/parishes in LA.

Gallina/Campbell

Day 3
Valid 12Z Sun Nov 29 2020 - 12Z Mon Nov 30 2020

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

...Central to Eastern Gulf Coast...
At the start of the forecast period (Sun-29.12z), the mid to
upper-level low will continue to advance eastward supporting
continued surface low development along the coastal front.  Deep
moisture and instability will maintain ongoing convection
along/just ahead of the surface low across Southeast LA into
Coastal AL/Western FL panhandle.  As the shortwave/jet structure
pivots, low level warm conveyor belt will become more vertically
unidirectional to support individual elements with the ongoing
convection to momentarily train with slow eastward progression of
the pre-frontal trof.  Modest diffluence aloft and veering
low-level flow may also support further back-building along the
line, focused with stronger frictional surface convergence at the
coast.   Global guidance trends continue with steady increase in
rainfall efficiency and totals across the area with 2-4"totals
forecast, particularly focused across the western FL Panhandle.
Recently dry conditions of 5 to 50% of normal (per AHPS 2-week
anomalies), suggest high FFG values in the area are reasonable; so
these totals would be nearing this range, but not quite there,
especially given storm-scale nature to the potential flash
flooding.  As such will keep the area below categorization at this
time, but with increasing confidence/consistency in the guidance,
a Marginal Risk may be required.


...Lower Tennessee/Central Ohio Valley...
By 29.18z, the southern stream shortwave/closed low will begin to
shear  but swing negative-tilt toward the northeast under
influence of digging northern-stream trough across the Central
Plains.  The southern stream jet will press eastward, exposing the
area of concern to increased divergence aloft.  Surface and
low-level cyclogenesis will see a strengthening of the warm
conveyor belt/TROWAL bend back against the flow and sharpen the
low to mid-level deformation zone across western TN, providing
strong moisture flux convergence and isentropic ascent for
increasing rainfall efficiency.  Some embedded weak instability
(100-250 J/kg) may allow for some isolated vertical growth
increasing the rainfall rates too, but quite unlikely to exceed
short-term rates needed for flash flooding.  However, the duration
of this moderate rain suggest some pockets of 1-2" totals possible
per the guidance suite over a 6-12 hour period.  This area has
been very dry recently with AHPS precipitation anomalies well
below normal, however, ground conditions are dormant or
per-dormant to suggest some increased run-off could be a problem
in traditionally prone streams/areas.  All considered, this still
appears to be just below threshold for increase to a Marginal Risk
category at this time, with agreement from local forecast offices.


Gallina


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