FOUS30 KWBC 090813
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Wed Oct 9 2024

Day 1
Valid 12Z Wed Oct 09 2024 - 12Z Thu Oct 10 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL FLORIDA...

...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING ARE
EXPECTED WITH LIFE-THREATENING AND CATASTROPHIC FLASH FLOODING PROBABLE...

Generally the ERO risk areas have been trimmed from both the north
and south now that the full life of Milton moving over the Florida
Peninsula is within the CAMs range, as well as good agreement
among them. Forecast rainfall quantity and location within the
High Risk area is largely the same, so no major changes were made.

Major Hurricane Milton will approach the central Florida Gulf Coast
today, make landfall overnight tonight, and track part-way across
the Florida Peninsula during this Day 1/Wednesday period.
Convection out ahead of the main circulation is already beginning
to impact the southwest Florida Gulf coast. This convection is
likely being predominantly forced by a stalled out front over
central Florida, which will act to steer Milton across the
Peninsula. This rainfall will make up a significant fraction of the
total rainfall seen in central Florida, as Milton's eye and eyewall
only begin impacting the coast late this afternoon at the earliest.
The thunderstorms that have developed well out ahead of Milton are
a function of the warm air advection with the southerly flow
associated with Milton's broader wind field.

Based on the latest NHC advisory, Milton should make landfall
south of Tampa Bay near Bradenton/Sarasota sometime between
midnight and 4am tonight. It almost goes without saying that the
heaviest, most persistent, and impactful rainfall will be
associated with the eyewall. As regards broader impacts, one should
not focus on the exact track. However, as regards rainfall, the
exact track will matter a great deal. The guidance has been
incredibly consistent that Milton will have at least begun
extratropical transition as it encounters the aforementioned front
and much stronger upper level winds...impacting shear on the
cyclone. Thus, the rain shield associated with Milton should
largely focus along and north of the track of Milton, as dry air
typical of an extratropical cyclone effectively erodes any rain
south of the center.

With the center of Milton expected to track just south of Tampa,
that will put the Tampa Metro squarely in the core of the heaviest
and most persistent rainfall. The latest WPC forecast rainfall has
increased to between 12 and 16 inches around the Tampa Metro. In
addition to the Category 3-4 winds, this amount of rain is
expected to overwhelm any soils and rivers very quickly and thus
convert to runoff. Thus, expect widespread and catastrophic
flooding in the Tampa area tonight...exacerbated by expected power
outages. Again the heaviest and most persistent rainfall in Tampa
will be with the eyewall...generally between 9pm tonight and 5am
Thursday morning.

The storm will then track generally parallel to but a bit south of
the I-4 corridor. This will then put metro Orlando in the same core
of heaviest rainfall as Tampa. With Orlando further inland and
therefore the storm somewhat weaker as it moves south of the city,
rainfall amounts in Orlando will be just a bit lower, generally
between 10 and 14 inches. Nonetheless, since most of this will fall
with the eyewall, expect similar impacts from inland flash flooding
as in Tampa. Also similar to Tampa, widespread power outages will
likely greatly increase the impacts.

To the south of Milton's circulation, the aforementioned dry air
entrainment will make Milton's satellite presentation look more
comma-like. This will greatly reduce the impacts from rainfall,
albeit offset by this area being on the stronger south side of the
circulation for winds. At the Gulf coast in the Ft. Myers metro,
storm surge flooding will be the much greater threat, though any
inland flooding from off-and-on training convection will worsen
flooding where the storm surge and freshwater rainfall meet.

To the north of Milton's immediate circulation, plentiful dry air
is in place with northeasterly flow off the continent reinforcing
the dry air. This will work to limit the northern extent of the
rain shield. With CAMs support, the northern extent of the ERO risk
areas remains very tight, with only about 50 miles or so separating
the Marginal and High Risk lines. For Jacksonville, rainfall
forecasts continue to decrease. However the "reverse storm surge"
if you will with onshore northeasterly flow may still complicate
drainage of what rainfall is received over and south of the city.
Thus, as a precaution the city remains in a Slight Risk.

Small wobbles of Milton's eye could vary the worst impacts of
Milton near the track. A northward jog would greatly reduce the
rainfall in Tampa and possibly Orlando, whereas continued southward
shifts will likely not make too much difference since the plume of
the heaviest rainfall associated with Milton is wide enough to make
up for those small changes.

Finally, for far south Florida, inflow bands of training and severe thunderstorms required keeping the Glades and the Gold Coast in a
Marginal risk in case a band becomes stationary over an urban or
other flood sensitive area.

Wegman


Day 2
Valid 12Z Thu Oct 10 2024 - 12Z Fri Oct 11 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR THE ORLANDO METRO
AND PORTIONS OF THE FIRST AND SPACE COASTS...

...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH LIFE-
THREATENING AND CATASTROPHIC FLASH FLOODING WILL LIKELY BE ONGOING
THURSDAY MORNING...

A High Risk area was introduced with this update in coordination
with MLB/Melbourne, FL forecast office. Almost no changes were made
to the other ERO risk areas.

Unfortunately, guidance continues to very gradually slow the
forward speed of Milton as it completes its track across the
Florida Peninsula and exits out into the Atlantic on Thursday. As a
result there was a small increase in the forecast rainfall after
12Z/8am Thursday from Orlando east up I-4 to the Atlantic coast.
The introduction of the High Risk for this area was much more the
result of expected continued catastrophic impacts from widespread
power outages and around a foot of storm total rain which will
continue into Thursday morning and perhaps a portion of Thursday
afternoon. Along the immediate coast, onshore flow at the time of
high tide may also hamper any drainage into the ocean by the local
streams, creeks, and rivers. Overall, with 3-5 inches of rain
expected after 8am Thursday in the High Risk area, which will be on
top of previous rainfall and likely the heaviest intensity of the
entire storm, it's likely the widespread impacts indicative of a
High risk will continue into the day Thursday.

The rain will quickly end from west to east Thursday morning for
west Florida and Thursday afternoon for east Florida. Thus, all of
the ERO risk areas are likely to be downgraded long before the 12Z
Friday end of the period. Since there remains some uncertainty,
especially around Tampa, as to what the nature of any lingering
rainfall will look like, the area remains in a Marginal Risk.

Wegman


Day 3
Valid 12Z Fri Oct 11 2024 - 12Z Sat Oct 12 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
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