Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
723 PM EST Wed Nov 29 2023

Day 1
Valid 16Z Wed Nov 29 2023 - 12Z Thu Nov 30 2023

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

Hurley


Day 2
Valid 12Z Thu Nov 30 2023 - 12Z Fri Dec 01 2023

...THERE IS A MARGINAL RISK AREA FROM THE CENTRAL GULF COAST TO
THE MID-MISSISSIPPI/OHIO RIVER VALLEY...

...20Z Update...

Primary change to the previous D2 issuance was trimming back on
the northern extent of the MRGL risk across central and eastern MO
into southern IL. Consensus on warm front approach late in the
period will allow for some shallow convection to develop, but the
lack of appreciable rates worthy of flash flood concerns will be a
deterant with the latest 12z HREF 1"/hr probabilities totally void
over the aforementioned area. Totals will be generally within the
0.75-1.5" territory, and likely over an extended period of time.
Thus, brought the northern extent further south into southern MO
where there's a slightly better risk for convection and locally
higher rates.

The trend continues to have a focal point of convection and flash
flood concerns across east TX into the western half of LA where
instability maximum for the period will be co-located within the
best ascent from our passing shortwave trough to the north and
surface convergence within the confines of a propagating cold
front. Rates will be generally between 1-2"/hr with perhaps a few
isolated storms capable of up to 2.5"/hr, mainly within the
southwest LA coastal areas where instability is forecasted to be
highest. The antecedent conditions present across the Lower
Mississippi Valley will deter more widespread flood prospects and
much of this rain will be beneficial. However, the prospect for
supercell generation across east TX given the deep shear profiles
and backing low-level wind field off the Gulf will yield some
enhanced rainfall within the SPC Enhanced/Slight severe storm
risks respectively. Later in the period, convegent signature along
the leading edge of an advancing cold front will act as a focal
point for a line of heavy thunderstorms to move eastward across
the LA/TX border into the central portion of LA. CIPS analogs were
very much in line with the current forecast with multiple analog
from the 90s and 80s during stronger el ninos depicting a very
similar setup with an outcomes well within the means of current
guidance. Those events yielded more localized flash flood impacts
with the primary concerns aligned with the urban settings around
east TX into LA. This was also another reason to maintain the MRGL
risk with less widespread focus and more localized impacts
anticipated.

Kleebauer


...Previous Discussion...

Surface low over the plains will evolve with a developing cold
front pushing eastward within the time frame. The latest guidance
continues to show a swath of QPF of 1 to 3 inches lifting from the
Gulf Coast to the Ohio River Valley.
Moisture return from the Gulf will have the PW values upwards of 2
to 2.5 standard deviations above normal over a large portion of
the Lower and Middle Mississippi Valley. One this deep moisture
plume meets the greatest instability hourly rates will likely max
out near 2 inches/hour, leading to local maximums near 4 or more
inches during this period across Louisiana and the immediate
vicinity. The Marginal Risk that was already in effect from the
Central Gulf to southwest Indiana remains in effect. Minor
adjustments were made along the eastern boundary to trim it back
to the west reflecting the latest model trends and WPC forecast.

Campbell

Day 3
Valid 12Z Fri Dec 01 2023 - 12Z Sat Dec 02 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL/EASTERN GULF COAST AND THE SOUTHEAST...

...20Z Update...

Little to no change necessary from the previous forecast as the
expected evolution of our shortwave trough and frontal alignment
across the Southeast U.S has maintained continuity from previous
guidance with only small mesoscale based details wavering from
run-to-run. Convergence along an approaching cold front within the
confines of a anomalously moist environment will generate locally
heavy rainfall with flash flood concerns over any urbanized
settings. The primary focus will likely be over southern AL into
the FL Panhandle where the instability axis will be greatest and
convective regimes could produce 1-2"/hr rainfall with locally up
to 3"/hr near the coastal areas. A growing consensus of somewhere
within the MRGL bounds could pick up 3-5" of rainfall over the
course of the period, enough for a MRGL risk placement
encompassing the forecast best axis of convective potential.

Kleebauer



...Previous Discussion...

The low pressure system will continue to advance east during this
period with its warm front lifting trough the Deep South and
Southeast. Convection is expected to be the most concentrated near
the Gulf Coast and tracking northeast into southern Alabama and
Georgia ahead of the warm front. With anomalous moisture
continuing to feed northward into this boundary, enhanced rainfall
can be expected. Most of the guidance has 1 to 2+ inches spreading
from south-central Louisiana into central Georgia/northern
Florida. There is suggestions that a narrow axis of 3 to 4 inches
could setup within this flow but the exact location remains
uncertain. It could span from far southwest Alabama to southwest
Georgia per a couple of solutions, or further south across the
Florida panhandle and inland. A Marginal Risk area was maintained
from the previous Day 4 period as it reflected the part of the
country were localized flash flooding problems could arise.

Campbell


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