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FOUS30 KWBC 052318
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
718 PM EDT Sat Oct 5 2024

Day 1
Valid 01Z Sun Oct 06 2024 - 12Z Sun Oct 06 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR
PORTIONS OF FLORIDA...


...South Florida...
Low-level convergence near a front/baroclinic trough combined with
onshore flow should keep the possibility of heavy rainfall in the
offing for the Keys, South FL, and Southwest FL overnight.
Precipitable water values are expected to remain 2.25-2.5", more
than sufficient for efficient warm rain process backed by a
wet bulb zeroes well above 15,000 feet. ML CAPE over the offshore
waters remains 1000-2000 J/kg. Hourly rain totals to 3" and local
amounts to 6" remain possible where cells show little movement,
merge, or train.


...In and near northeast Florida...
A bit to the north of the front/baroclinic trough, low-level inflow
is expected to remain in the 15-20 kt range, and be opposed by the
upper level flow, yielding effective bulk shear of 25+ kts which
could organize convection. Precipitable water values remain 2" or
so, and inflow off the warm Atlantic would allow 1000-2000 J/kg to
advect inland. Hourly rain totals to 2.5" and local amounts to 5"
would be possible, particularly near and after 06z, where cells
show little movement, merge, or train.

Roth


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

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

20Z Update: The previous MRGL was relatively maintained, but there
is now an additional SLGT risk situated across the Southeast FL
metro corridor. An axis of favorable surface based instability
coupled with anomalous deep layer moisture centered across South
Florida will create an environment conducive for locally enhanced
rainfall rates, especially within the confines of any boundaries
advecting from the Atlantic and Northern Straits. Probabilities of
>2"/hr rainfall rates within the latest HREF are running upwards of
35-45% across the Southeastern FL coast tomorrow morning through
the afternoon hours with the general proxy in-of the Miami metro.
This area has notoriously been more prone to flash flooding due to
high run off capabilities within the urbanized setting. HREF
blended mean QPF also depicts a footprint of 3+" over the same area
as the higher hourly rate probs, a positive correlation that allows
for better consensus within the ensemble suite. Considering the
evolving circumstance of what occurred this morning, plus what is
coming down the line in later periods, this is a precursor what
will be an active week for convectively driven flash flood
scenarios. The SLGT risk across the urban metro from Miami to West
Palm Beach was deemed appropriate given the above variables and was
agreed upon in coordination with the local WFO.

Kleebauer

..Previous Forecast..

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 the and the 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. With an
eventual propagation of Tropical Cyclone Milton out of the Gulf
towards the Florida Peninsula mid to late week, this will be the
first day of the predecessor rain event, or PRE that will end up
priming regional soils before Milton`s impact.

Wegman/Kleebauer


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...

20Z Update: Only minor adjustments were made to the previous
forecast with a general consensus of the bimodal QPF distribution
within the coastal sections of the Peninsula and a further north
expansion of the overall precip field. Weak surface reflection
riding along a quasi-stationary front is still forecast to maneuver
onshore of Southwest FL with increasing convergence signature
located along and south of I-4. Current NBM the and the
percentiles indicate a wide swath of 2-4" with locally upwards of
6" within either coast and that`s with a general loss of CAMs
integrated into the blend. So long as their is limited adjustment
in the synoptic setup, would expect an uptick in the mean and upper
quartile of the blends spatial output. Total QPF through 72 hrs
will likely be between 4-8" with locally as high as 10" along
portions of either coast within the Peninsula south of I-4. This
allowed for a continuation of the SLGT risk with some outside
prospects of an upgrade in later cycles, pending observed precip
and FFG adjustments prior to the forecast period.

Kleebauer

..Previous Forecast..

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. This means the PRE will remain in full force across the
Peninsula through much of the period. 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

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt