FOUS30 KWBC 060758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Sun Oct 6 2024

Day 1
Valid 12Z Sun Oct 06 2024 - 12Z Mon Oct 07 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE MIAMI-
FORT LAUDERDALE METRO...

In coordination with the MFL/Miami, FL forecast office, a Moderate
Risk upgrade was introduced with this update.

A weak surface low is developing over the eastern Gulf well east of
T.S. Milton, but in the same plume of incredibly deep tropical 
moisture. The low will drift southeastward toward south Florida.
This will increase the easterly flow over the Straits of Florida
between the Bahamas and Florida. This increased easterly flow in an
air mass with PWATs between 2.25 and 2.5 inches will both increase
the concentration and intensity of resultant thunderstorms over the
Gold Coast.

Portions of the Ft. Lauderdale and Miami metros saw up to 3 inches
of rain on Saturday. Rainfall amounts today and especially tonight
may accumulate to over 6 inches in that time. With saturated soils
and urban effects, the threat for ponding and flooding in the area
has greatly increased. Expect numerous areas of ponding and
flooding from today's rainfall, which necessitated the Moderate
Risk upgrade.

Elsewhere, the surrounding Slight Risk was expanded to include the
Gulf Coast from Tampa south and the Atlantic coast from Cape
Canaveral south. The aforementioned low will enhance rainfall rates
along much of the Gulf Coast from Tampa south, where heavy rainfall
Saturday caused localized flash flooding. This additional rainfall
on saturated soils will cause renewed flash flooding, mostly in
urban areas. Further north up the Atlantic coast, similar
convergence as further south is expected, but with lesser forcing,
resulting in lower rainfall amounts for the Treasure and Space
Coasts. Widely scattered instances of flash flooding are expected.

Finally, for the middle of the Florida Peninsula, the area will be
furthest from the strongest forcing, resulting in a local minimum
of rainfall. Since the area will not have tidal flooding concerns
and is somewhat less developed, only isolated flash flooding is
expected. The threat for heavy rainfall will continue up to the
Georgia coast however, where a local convergence zone may develop
as indicated in several of the CAMs. The Marginal was expanded
north to account for this possibility.

Wegman


Day 2
Valid 12Z Mon Oct 07 2024 - 12Z Tue Oct 08 2024

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

The weak surface low that will bring heavy rain to south Florida on
Sunday will continue eastward over the Peninsula and into the
Bahamas by Monday afternoon. Heavy rain will likely continue over
the southern Peninsula through at least Monday morning. There is
considerable disagreement on the track, strength and the speed of
the low, which will play a major role in how long the heavy rain
persists into Monday afternoon. With somewhat better agreement that
rainfall amounts will at least be lower than on Sunday, the Slight
risk for the area that was inherited was largely maintained, albeit
including some of the interior of the Peninsula as soils become
saturated area-wide, resulting in close to 100% conversion of heavy
rain to runoff. An internal higher-end Slight is in place for the
Gold Coast, for the potential that should amounts from Sunday
verify, then another targeted Moderate Risk upgrade will be
necessary. Meanwhile, a second day of very heavy rain and
thunderstorms is expected to impact all of the Keys, which at high
tide times may cause flooding due to poor drainage.

With some of the other CAMs moving the low across the southern
Peninsula and east to the Bahamas, portions of the Gulf Coast,
especially north of Ft. Myers/Cape Coral up through Tampa are in
the rare decreasing trend in the forecast rainfall amounts on
Monday. Thus, the Slight that extended up through Tampa was
downgraded to a Marginal north of Ft. Myers/Cape Coral with this
update. 

Wegman


Day 3
Valid 12Z Tue Oct 08 2024 - 12Z Wed Oct 09 2024

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

The PRE (predecessor rain event) will enter its third and final day
over the Florida Peninsula on Tuesday. The trough that occasionally
spins up weak surface lows will remain in place across the
Peninsula. The circulation of what will then be Hurricane Milton
will make its approach to the Gulf Coast of Florida, likely near
Tampa very late Tuesday night/early Wednesday morning. At this
time, expect little of the rainfall directly associated with
Milton's core to yet impact the Florida Peninsula. Of course, with
future updates and forecast changes from NHC, this may change.
Given the inherent uncertainty with the track and speed of this
small hurricane, the Slight over the Peninsula was maintained,
albeit with some filling in in the middle of the Peninsula near the
Orlando area. There remains a bimodal distribution of the rain with
much more expected along the Gulf coast from Tampa down through
Naples, but with a secondary maximum for the Space and First
Coasts. 

With the latest guidance, there is some offset of the axis of
heaviest rainfall ahead of Milton. The northward trend will reduce
amounts in portions of the hard-hit Gold Coast in favor of more
rain for the northern Peninsula towards the First Coast. By Tuesday
the greatest overlap will be over the Gulf Coast. It is here than
an internal higher-end Slight was introduced for the Tampa through
Naples area due to increasing amounts of heavy rain moving into the
coast, being moved along by the predominant southwesterly flow in
the trough ahead of Milton's core circulation. Heavy rainfall both
the past couple days as well as expected both today and Tuesday
should keep all of the soils on the Florida Peninsula at or near
saturation, resulting in nearly all of the rainfall converting to
runoff. Thus, progressively less rainfall will be needed to result
in flooding.

Despite Milton and directly associated rains largely holding to the
Day 4/Wednesday period, it appears that similar to Helene, most of
the rain over the Florida Peninsula will be associated with the
PRE, while the heaviest rain (as well as wind and storm surge) will
conclude the multi-day rain event with Milton moving through,
causing much more widespread flooding.

Wegman
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