Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Sat Sep 7 2024

Day 1
Valid 16Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND WESTERN U.S...

...16z Update Summary...

The SLGT risk was removed from the previous update in coordination
with the impacted offices. A MRGL risk remains for parts of the
Southeastern U.S, with a focus along the coastal plain of Southeast
LA and the Emerald coast of FL. MRGL risks remain steadfast across
portions of the Western U.S.

Kleebauer

...Gulf Coast and Southeast...

16Z Update: The pattern from this morning have devolved into
something less robust with continued rainfall, but paltry rates to
suffice much of a flash flood concern. Recent mesoanalysis has only
furthered the argument for less of a risk of heavy rainfall as
MUCAPE indices remain relatively benign with the coastal plain of
Southeastern LA over into the FL Panhandle only subject to ~500
J/kg of MUCAPE, at best. Analysis of radar trends indicate a limit
to the overall convective output this morning, correlating well
with the <1" totals over the past 6 hrs for areas it has been
raining through that short window. Some diurnal destabilization
will be had for portions of FL and Southern GA, but the coastal
plain will be bordering on low-end probability for any instability
regime to really overcome the prevailing cloudiness and the
convergence axis shifting off the coast as drier air advects in
from the north. Despite continued rainfall this afternoon with
scattered thunderstorm concerns for the immediate coast, the
prospects for anything more than a rogue over-performer will limit
the threat to a mere MRGL risk, thus necessitating the removal of
the previous SLGT risk in place.

The northern extent of the MRGL was also trimmed away to tighten
the gradient of the zone for best convective development. The setup
may yield more flare further east as a better diurnal
destabilization regime could yield convective pattern with stronger
cores capable of localized heavy rainfall >1"/hr as signaled
within the 12z HREF probs and associated CAMs. This setup is best
aligned across the FL Big Bend into parts of Southern GA. This is
still a lower end probability for flash flood concerns due to the
higher FFG indices in place over the proposed area of interest.
This was enough to maintain the MRGL risk across the northern half
of FL.

Kleebauer

...Portions of the Southern Rockies...

16Z Update: Little to no change occurred from the previous MRGL
risk forecast across portions of the Western U.S. Scattered
convective signals in-of the Southern Rockies, Mogollon Rim of AZ,
and the Peninsular Range of Southern CA will all benefit from a
persistent mid-level ridge axis with a slightly higher than normal
PWAT accompaniment as it migrates around the western and northern
flank of the ridge. Modest instability coupled with the better
moisture presence will provide just enough to create an isolated
flash flood threat within the terrain, and over remnant burn scars
located within both MRGL risk areas.

Kleebauer

..Previous Discussion..

The Marginal risks in these areas remain largely the same, other
than trimming out areas of southern CA and northwestern NM where
less topographic influence should generally preclude any flash
flooding. The Marginals thus concentrate from the Peninsular Ranges
of southern California over to the western Mogollon Rim of western
Arizona, as well as for the Sangre de Cristo Mountains of southern
Colorado and northern New Mexico. Any burn scars will locally
enhance the flash flooding potential due to a much greater
percentage of any rainfall converting to runoff.

Wegman

Day 2
Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN ROCKIES, DEEP SOUTH TEXAS, AND THE SOUTHEAST...

...Southern Rockies...

Southerly monsoonal flow out of the Gulf of California will push
abnormally high moisture into portions of the Southwest on Sunday.
By Sunday afternoon with diurnal heating, expect widespread shower
and thunderstorm development across the Marginal Risk area, in
particular on south facing slopes of the mountains. The fact the
area has been baking in recent days should only increase the amount
of instability that any storms in the area can feed on to increase
their potential ability to produce heavy rains. Urban areas,
arroyos, slot canyons, and other low-lying flood prone areas are
where any isolated flash flooding will be most likely.

...Deep South Texas...

An approaching tropical wave and potential surface low will move
across the western Gulf and impact deep south Texas on Sunday and
especially Sunday night. The strong front draped along the coast
will act as a focus for heavy rain. Coastal areas of Deep South
Texas have seen rounds of heavy rain in recent days, resulting in
some depression of FFGs in the area. A tropical wave and its
associated heavy rainfall when combined with those antecedent soil
conditions could result in isolated flash flooding.

A major point of uncertainty in the forecast is how far west the
slug of heavy rain will get to. There is significant disagreement
in the guidance as to the answer to that question. A notable trend
has been to shift the heaviest rain a bit to the east and more
offshore. This in turn is both increasing uncertainty and lowering
the flash flooding threat. For now, the Marginal risk area is
maintained unchanged, but even a small additional eastward nudge
would require a downgrade of the risk area.

...Southeast...

The eastward extent of the same front as impacting Deep South Texas
will remain across northern Florida (both Panhandle and Peninsula)
as well as southern Georgia. The same pattern will remain on Sunday
as on today, in that instability will be greatly lacking with any
distance from the coast, so the greatest threat for flooding will
be along the immediate Gulf Coast of Florida.

Wegman

Day 3
Valid 12Z Mon Sep 09 2024 - 12Z Tue Sep 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP
SOUTH TEXAS...

...Coastal Deep South Texas...

A tropical wave moving across the Gulf will continue to spread
heavy rains along much of the Gulf Coast on Monday, continuing from
Sunday. The bulk of the heaviest rain associated with the wave is
likely to impact coastal Deep South Texas and adjacent coastal
northeast Mexico. Thus, there is higher confidence that on Monday
very heavy rainfall will occur along the coast of Deep South Texas
as compared with Sunday. However, as mentioned in the Day 2
discussion, there is increasing uncertainty as to the track of the
tropical wave, which greatly impacts the forecast and the potential
for flash-flood-producing heavy rains. Guidance has been trending
east/offshore with the heaviest rain, so as on Sunday, any further
eastward shifts will require downgrades to the ERO risk areas.

For now, the Slight risk upgrade is contingent on at least 2 days
(today and Sunday's) rains further lowering FFGs in advance of the
main rainfall event in the area on Monday. If Monday is the third
consecutive day with heavy rainfall, as it's highly likely to be
the wettest of the 3 days, then it stands to reason compounding
influences by increasingly saturated soils will quickly increase
the potential for flash flooding.

...The Rest of the Gulf Coast...

No change to the pattern is forecast for the Gulf Coast from upper
Texas east into Florida. As on previous days, most of the heaviest
rain should remain offshore of these areas, with a predominant
light ENE flow along the front allowing some of the heavier rains
to move into FL and GA from the Gulf, though quickly weakening with
increasing distance from the Gulf. Once again, this area will be
swamped with multiple days of light to occasionally heavy rainfall.
Thus, it's quite possible a Slight risk upgrade may be needed with
future forecast in the areas hardest hit with heavy rains both
today and on Sunday.

The inherited Marginal was greatly expanded to the north with this
update to cover for all intents and purposes the entirely of the US
Gulf Coast.

Wegman


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