FOUS30 KWBC 060804
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Fri Sep 6 2024

Day 1
Valid 12Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTHEASTERN
LOUISIANA...

...Gulf Coast & Southeast...

A very slow moving tropical low will track east along a stationary
front draped along the Gulf Coast today. Near record amounts of
atmospheric moisture in place across the Gulf will be drawn
northward into the front ahead of the low. PWATs currently are
around 2.6 inches and will not change much through the day. This
would be over 3 sigma above normal for this time of year...which
for southeastern Louisiana is no small feat.

The southerly flow ahead of the low will keep a steady supply of
deep Gulf moisture tracking into the Gulf Coast primarily...but
will spread east across portions of the Southeast in the Slight
Risk. The big question about heavy rain will be how much
instability will be available for any showers and thunderstorms.
Since it's currently raining over much of Louisiana and will
continue into tonight, any instability will need to be advected in
to the Gulf. That said, given the near record levels of moisture
available, it will not take much to result in 2+ inch per hour
rates with the strongest convection. The area's soils are quite
water-logged from multiple days of rains, so today's rainfall
should fully convert to runoff...greatly increasing the flooding
threat.

In coordination with LIX/Slidell, LA forecast office, the Moderate
Risk area was left unchanged with this update, with the greatest
threat for the greater New Orleans and Baton Rouge areas due to
lower FFGs. The Slight was nudged a row of counties north, and
removed from most of northeast Florida. Due to recent heavy rains
across much of the Peninsula however, the Marginal risk was
expanded to include the Orlando, Tampa, and Ft. Myers metros.

...New Mexico...

A small low-confidence Marginal Risk area was added for the 
Sacramento Mountains of south-central New Mexico with this update.
Afternoon showers and thunderstorms may form right over the very
flood prone burn scars near Ruidoso, which could cause local flash
flooding once again. However, it would require just that scenario
to unfold for flooding to occur which is low confidence since the
storm would have to form right over the correct spot, a microscale
challenge that's nearly impossible to know for sure well in
advance. The pattern supports one more day of area showers and
storms, prompting the localized upgrade.

Wegman


Day 2
Valid 12Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
EMERALD COAST OF THE FLORIDA PANHANDLE...

...Southeast...

The stationary front from Friday will drift southeastward across
the Southeast on Saturday. For the Central Gulf Coast, it would
probably be more accurate to call it a cold front, as the front
will make steady progress across southern Louisiana and into the
northwestern Gulf. Meanwhile across the Southeast, the front will
stall out over the Florida Panhandle, southern Georgia and up the
Carolina coast. It's in this region where training storms may cause
localized flash flooding. Moisture will not be a problem at all as
PWATs will be from 2 inches in the Carolinas to nearly 2.5 inches
in the Florida Panhandle. Thus, just as on Friday, the big limiting
factor for heavy rainfall will be instability, as very slow moving
fronts tend to be very lacking in that department due to extensive
cloud cover. 

In coordination with TAE/Tallahassee, FL forecast office, a Slight
Risk area was introduced with this update. Storms are likely to be
robust rain makers out over the northern Gulf on Saturday. With
very weak steering flow and limited instability inland however, 
the area with greatest likelihood for the storms to impact land 
will be around the Emerald Coast of the Florida Panhandle. The most
likely hazard area was highlighted in the Slight. The expectation 
is that any storms that move into the Emerald Coast will greatly 
struggle to maintain the heavy rainfall rates more than a few miles
inland from the coast, so the Slight is only for the immediate 
coast.

The Marginal Risk area was expanded several rows of counties north
with this update, while in the Carolinas it was more like a couple
states. The primary driving factor for these changes is the slowing
of the front in recent model runs. Thus, the rain may persist for a
bit longer than expected into Saturday morning on the northern edge
of the Marginal Risk area. Meanwhile Saturday will be another day
with potential strong thunderstorms capable of heavy rains across
the northern Florida Peninsula. This hard-hit area is likely to see
multiple days of afternoon thunderstorms capable of isolated flash
flooding.

...Northeast...

The inherited Marginal Risk area was removed in coordination with
BTV/Burlington, VT and ALY/Albany, NY forecast offices. Fast-moving
storms with this fast-moving front in this part of the country, 
very limited/no instability, and somewhat dry weather in recent
days should all preclude any flash flooding in this area. 

Wegman


Day 3
Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024

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

...Louisiana to the Atlantic Coast...

The stalled out front will weaken in place through the day on
Sunday. This once again should prevent most areas from getting
prodigious amounts of rain, despite abundant atmospheric moisture.
Further, there is increasing disagreement in the guidance as to
where the axis of heaviest rainfall associated with the front will
be on Sunday. Guidance trends have been towards the northwest,
resulting in a large expansion of the Marginal in that direction
due to the aforementioned uncertainty. It's likely given how wet
this entire area of the country has been that an area of Slight
Risk impacts will occur somewhere within the Marginal risk, but
where that will be is highly uncertain right now. Thus, it's likely
a Slight Risk upgrade will be needed with increased confidence.

...Deep South Texas...

A Marginal Risk area was added to the Brownsville to South Padre
Island area of Deep South Texas with this update. A tropical wave
will have moved through the Caribbean and into the western Gulf by
Sunday. Associated heavy rainfall may impact the immediate Deep
South Texas coast. Recent heavy rainfall has lowered FFGs here
which may not fully recover by the arrival of this heavy rainfall
due to potential additional shower and thunderstorm activity both
today and Saturday prior to this period.

Wegman
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