FOUS30 KWBC 070823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Mon Oct 7 2024

Day 1
Valid 12Z Mon Oct 07 2024 - 12Z Tue Oct 08 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE GOLD
COAST AND THE UPPER FLORIDA KEYS...

Very few changes were made to the inherited EROs for this period.

A leading low, front, and surface trough over the eastern Gulf and
Florida well ahead of Hurricane Milton continues to spread rain
over nearly all of the Florida Peninsula this morning. The low will
shift east over the Everglades and South Florida today. Ahead of
the low, additional showers and thunderstorms capable of very heavy
rainfall rates due to the extremely moist environment will
continue. Portions of the Gold Coast saw 2-4 inches of rain
yesterday, resulting in localized flash flooding, so the potential
for that much rain again today supports the continuance of the
Moderate Risk. The trailing front south and west of the low will
remain stationary near Southwest Florida and the Keys. This will
keep the threat for heavy rainfall ongoing through tonight. The
Slight Risk area remains in place for these regions as far
southwest Florida is the swampy Everglades and the Keys present
very small targets for heavy rain. If training cells were to move 
over the Keys during high tide then flooding is probable.

HREF probabilities suggest the heaviest rain today all over south
Florida are more likely to remain offshore, so the heavy rain
threat is slightly lower today compared to yesterday/Sunday.
Convergence along the Atlantic coast remains prevalent today, so
the Slight Risk remains in place to the north through the Space and
Treasure Coasts. With the center of the low tracking over south
Florida and off to the east towards the Bahamas, this should give
the likely landfall areas of the Gulf Coast north of Ft. Myers
through Tampa a general reprieve from heavy rain for most of today.

Wegman


Day 2
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...

There will be a general lull in the overall general rainfall across
Florida on Tuesday. The "calm before the storm". The low impacting
most of the state Monday will be well east of the Bahamas by
Tuesday, while the rainfall directly associated with Hurricane
Milton will likely remain off or just arriving at the Gulf Coast
before 12Z Wednesday. Nevertheless, widespread light to moderate
rainfall will impact the state Tuesday and Tuesday night. Thus, any
resultant flooding will be largely due to favorable antecedent
conditions due to generally widespread saturated soils across the
Peninsula. Further, continued extremely moist atmospheric
conditions consistent with the deep tropical air mass with PWATs
around 2.25 inches will still support locally heavy rainfall with
any convection that manages to form. 

On the Atlantic side, a convergence zone may remain over the Space
and Treasure Coasts on Tuesday, which could locally organize any
convection and result in locally heavier rainfall over these mostly
urban areas. Meanwhile over the Gold Coast, a local minimum of
rainfall may develop, but the heavy rain there the past couple days
as well as some chance of shower and thunderstorm activity may
cause widely scattered localized flooding. No changes were made to
the ERO risk areas.

Wegman


Day 3
Valid 12Z Wed Oct 09 2024 - 12Z Thu Oct 10 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR CENTRAL AND
NORTH FLORIDA AS WELL AS FAR SOUTHEASTERN GEORGIA...

The main circulation of Hurricane Milton will cross the Florida
Peninsula on Wednesday. There remains considerable uncertainty as
to where the center will make landfall, which will of course play a
huge role as to where the heaviest rainfall amounts will be. See
the NHC forecast for the latest track. The storm is expected to be
undergoing increasing shear, beginning extratropical transition,
growing in size, and ingesting dry air. Thus, there is decent 
agreement that at landfall, the storm will be weakening. This 
should help to reduce the rainfall south of the circulation center 
as dry air ingestion associated with extratropical transition 
should quickly erode the rain-producing storms southwest of the 
center. Very dry air behind a southward moving front will greatly
cut into rainfall amounts on the northern fringe of the
circulation. Thus, expect there to be a tight gradient of rainfall
both on the south and north sides of the circulation, but for 
different reasons. The ERO risk areas have been expanded in both 
directions due to track and storm size uncertainties, especially 
towards the north, with the Moderate Risk expanded into far 
southeastern Georgia, and the surrounding Slight and Marginal 
expanded into the Low Country of coastal southern South Carolina. 
This may be optimistic, but these expansions were made primarily 
due to uncertainties, and will likely be refined in the coming 
days.

For the moment, there are several reasons a High Risk was not
considered in no particular order: 1) The storm will be weakening 
prior to landfall which may temper rainfall amounts a little bit, 
2) Milton's small size should reduce the areal coverage of the 
heaviest rain, 3) The primary rain footprint of Milton over north 
Florida will largely miss where the hardest hit areas of the PRE in
south Florida are. Of course there is some overlap, but not nearly
as much as with Helene, 4) The core of the heaviest rain at the 
moment is expected to track from north of Tampa northeast through 
around Jacksonville. This area has been relatively dry in recent 
days compared to areas further south, 5) Milton will be moving 
quickly and accelerating. This will reduce the time the heaviest 
rain has along and north of the track to result in widespread and 
numerous flash floods, 6) The inclusion of CAMs and higher
certainty with the track may help to narrow down where a potential
High Risk may be needed with future forecasts. For now with Day 3
being outside the CAMs time range, the large Moderate will
suffice.

Wegman
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