FOUS30 KWBC 080816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Tue Oct 8 2024

Day 1
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...

No significant changes were made to the ERO risk areas for this period.

"The calm before the storm" will be in place over much of Florida
today. This isn't to say it won't rain anywhere...but the risk of
heavy rain causing flooding on most of the Peninsula today will
most certainly be the lowest of the next few days. A stationary
front in the same tropical moisture plume as Milton is draped
across southern Florida. This is providing a focus for heavier
showers and storms south of the front from Miami south through the
Keys. Since the storms are tracking parallel to the front, training
is a significant concern this morning. As any mesolow tracking
along the front moves off, the heavy rain threat in the Keys should
diminish by this afternoon due to increasing influence of Milton.
For the rest of the Florida Peninsula, most of today should be dry,
especially the further north you go. This is because drier air
associated with a jet across the Panhandle is sinking southward
with a front and will ultimately catch Milton and force
extratropical transition as it approaches Florida. For today
however, it will keep things largely dry along and north of the
eventual track of the storm.

The rainfall threat (and the basis for the ongoing Slight) is
largely tied to any showers and storms that may form...most likely
from mesoscale effects like differential heating and sea breezes.
Due to an excess of atmospheric moisture over most of Florida with
PWATs of 2.25 to 2.75 inches today, any storms that form, even if
disorganized and not associated with any discernible forcing...will
have a superabundance of moisture to work with to convert to
rainfall. Much of south Florida has seen multiple inches of rain
over the past few days. While not prolific enough to cause
widespread flooding concerns, they have kept soils close to
saturated. This will play a role in the eventual flooding concerns
with Milton. 00Z HREF guidance highlights portions of the Treasure
Coast with the highest chances of 3 inches or more of rain today
not directly associated with Milton.

A higher end Slight remains in place for the Gulf Coast from Tampa
south through Naples. This is largely tied to the initial rain
bands from Milton which will begin to impact the coast in the
predawn hours Wednesday before the start of the Day 2 period.

Wegman


Day 2
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...

With the ingestion of some of the longer range CAMs into the model
suite, the forecasted rainfall directly associated with Hurricane 
Milton's crossing of the Florida Peninsula has increased markedly
from previous forecasts. The latest storm total rainfall has over
11 inches of rain for Tampa, 8 inches near and north of Orlando,
and 5.5 inches for Jacksonville. The corridor of the High risk
following the entire length of I-4 has seen the most impactful
increases in forecasted rainfall. Obviously there has been a nearly
historic increase in Milton's strength in the last 24 hours, and
all of this added energy and Gulf moisture will mean more rainfall
to come along its track, despite its fast forward speed.

The heaviest rainfall amounts along I-4 and north will generally 
will be immediately along and north of the most probable track of 
Milton's center. As mentioned in the NHC discussions, Milton is
expected to be well into the process of extratropical transition by
the time the storm crosses the Peninsula. This process should
greatly limit the amount of rainfall expected south of the center,
as dry air entrainment into the southwest quadrant of the storm is
an essential aspect of the extratropical transition process. Thus,
nearly all of the impressive tropical moisture associated with
Milton will be focused along and north of the center, and therefore
represents the greatest threats for flash flooding. The ERO risk
categories drop off a bit slower on the south side since the
predecessor rainfall event (PRE) of the past couple days and today
was largely focused on south Florida, as the stationary front that
has been there has persisted. Inflow into Milton's circulation and
the eventual development of a cold front south of the center 
should still lead to bands of rain criss-crossing south Florida 
from west to east. Since the PRE saturated this area the most...the
lesser amounts of rain expected here may still cause flooding
impacts...so the ERO risk categories south of the track may be a
bit generous.

Meanwhile on the north side of the circulation, abundant dry air
associated with a separate cold front and jet streak over the
Panhandle and far north Florida should act as a very effective sink
for the portion of Milton's rainfall that drifts too far away from
the center. Since this area has been quite dry in recent days, the
threat for flooding will be much more constricted, and therefore
the ERO risk categories are much more compact.

The additional rainfall now in the forecast will be one factor for
flooding...while another one will be storm surge and tidal
flooding, especially south of the center where the flow will be
onshore and to a slightly lesser but still impactful extent, to 
the north of the center on the Atlantic side where there will also 
be onshore flow. At the time of high tide and the storm surge, the 
added water from the Gulf/Atlantic will effectively block effective
drainage from the rainfall falling over the interior. The 6-12 
inches (with locally higher amounts of rainfall) having nowhere to 
drain due to the high tide and storm surge flooding will also work 
to exacerbate the flooding impacts from Milton since that rain 
water will have nowhere to drain. 

Given all of the above, and despite the small to medium size of the
storm and the acceleration of the storm in the forecast, a High
Risk was introduced in coordination with TBW/Tampa, FL;
MLB/Melbourne, FL, and JAX/Jacksonville, FL forecast offices. It's
probable that the greatest flooding impacts will be with storm
surge primarily, but also the portions of the nearshore rivers,
streams, and creeks (especially near landfall) where the inland 
flood waters from heavy rainfall are unable to drain to the Gulf.

Wegman


Day 3
Valid 12Z Thu Oct 10 2024 - 12Z Fri Oct 11 2024

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

A rare double upgrade was introduced with this update, with
potential for further adjustments with future updates. The latest
guidance suggests that the wraparound rain to the north and
northwest of the center of Milton will persist well into the day on
Thursday. Thus, associated forecast rainfall has increased 
markedly from the previous forecasts. For the Moderate Risk area,
expect an additional 2 to 4 inches of rain after the 12Z/8am
Thursday start of the day 3 period. With a High Risk on Day 2,
further increases in the forecast rainfall may require a High Risk
in this general area to continue into Thursday. Obviously, 
adjustments in the speed and track of Milton as it races off the 
coast will result in additional big changes to this ERO forecast. 

Essentially, the Day 3 ERO is just a continuation of the Day 2
rainfall with Milton. The vast majority of the rainfall for the
period will fall during the day Thursday, and expect trimming and
downgrades through the day as the rain ends from west to east.
Expect widespread and catastrophic flooding from the Day 2 period
to continue through the Day 3 period as the rain ends.

Wegman
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)