FOUS30 KWBC 050745
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sat Oct 5 2024

Day 1
Valid 12Z Sat Oct 05 2024 - 12Z Sun Oct 06 2024

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

...Deep South Texas...

Easterly flow to the north of a developing but weak low over the
western Gulf that is the focus for widely scattered thunderstorm
activity over the Gulf is forcing storms to move inland from the
Gulf over Padre and South Padre Islands this morning. A cold front
will push south across Deep South Texas today, which will push the
tropical moisture south into Mexico, so the threat for flash
flooding is confined to the immediate coast down to Brownsville for
the day today. The flood threat should abate by tonight.

...South Florida...

The urban corridor of southeast Florida, including Miami may see
occasional heavy rain from westward-moving thunderstorms this
afternoon and evening. The storms should continue moving and weaken
as they move inland, so the threat while non-zero, should remain
under Marginal Risk levels. Better agreement in the CAMs may
necessitate a Marginal Risk issuance for this area with the midday
update.

Wegman


Day 2
Valid 12Z Sun Oct 06 2024 - 12Z Mon Oct 07 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
SOUTHERN FLORIDA PENINSULA...

A trough developing ahead of a slowly developing tropical low over
the Gulf will form over the southern Florida Peninsula on Sunday.
The air mass the trough will be developing in will be incredibly
moisture-rich...with PWATs anywhere from 2.2 to 2.6 inches. This
will be close to the climatological maximum for this time of
year...between the 95th and 99th percentile and more than 3 sigma
above normal. The predominant mid-level wind flow will switch from
northeasterly to southwesterly during the period, resulting in
chaotic storm motions. The heaviest rainfall totals will be along
both coasts, from Tampa south through Cape Coral on the Gulf Coast 
as well as from the Treasure Coast south through Miami on the 
Atlantic side. This bimodal distribution of QPF suggests 
organization will be somewhat lacking, depending more on mesoscale 
phenomena such as sea breezes or differential heating between the 
urban areas and their surroundings. 

This will likely be the first day of several where heavy rain will
be common and widespread across the Florida Peninsula. In the
increasingly likely event a tropical cyclone eventually develops in
the Gulf and moves towards the Florida Peninsula mid to late week,
this will be the first day of the predecessor rain event, or PRE. 

Given high FFGs areally and this being the first day of the PRE, a
Marginal Risk was left intact for this area, but further increases
in forecast rainfall may require a Slight Risk for some of the
urban areas with future updates.

Wegman


Day 3
Valid 12Z Mon Oct 07 2024 - 12Z Tue Oct 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
FLORIDA PENINSULA...

Conditions across the Florida Peninsula will continue to
deteriorate on Monday. The PRE is expected to continue to
intensify, making for a second full day of periodic heavy rainfall
across the Peninsula. Both overall rainfall amounts and coverage
north up the Peninsula will increase on Monday as compared with
Sunday. All but the fastest guidance suggest that should a tropical
cyclone form over the Gulf, it will remain well west of the Florida
Peninsula through Monday night. This means the PRE will remain in
full force across the Peninsula. Once again, there will be a
bimodal distribution of the precipitation...but much more
pronounced. The focus will remain along both coasts, albeit heavier
and spread north. Given the expected heavy rainfall from Day
2/Sunday, by Monday most soils should be at or near saturation.
This means most of the day's rains will convert to runoff in most
areas.

With heavier rainfall expected in the urban areas along both
coasts, a Slight Risk upgrade was introduced with this update, in
coordination with MFL/Miami, FL; TBW/Tampa, FL; and MLB/Melbourne,
FL forecast offices. With better definition as to how and where any
tropical cyclone may develop over the Gulf, this will improve the
forecast for the PRE on Monday. This will apply both to where the
heaviest rain may occur as well as how heavy the amounts will be.
Further, with the hindsight of where Sunday's rainfall will occur,
it's possible additional upgrades may be needed with future
forecasts...though a preponderance of the guidance suggests any
direct effects of a potential tropical cyclone will hold until 
after this period. Depending on how the heaviest rain areas align
on Monday with Sunday, some areas may be approaching 2 day totals
of 10 inches of rain by Tuesday morning. These amounts of rain in
any urban areas could approach Moderate Risk level impacts. 

Wegman
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