589
AGUS74 KWCO 071522
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT MON OCT 7 2024

.Synopsis...
Hurricane Milton brings the potential for considerable flooding impacts to
the Florida Peninsula and Coastal Georgia... Flooding possible in Puerto
Rico and the USVI...

.Discussion...

Florida Peninsula and Coastal Georgia...
Hurricane Milton will bring the potential for considerable flash, urban,
and areal flooding as heavy rainfall impacts the area mid-week.
Considerable flooding impacts are expected along the track of the storm,
and significant impacts are possible elsewhere throughout Florida and
portions of far southeast GA. Ahead of Milton, locally heavy rainfall will
impact portions of FL through today, priming soils. Antecedent conditions
have been steadily becoming more wet due to past rainfall, with more to
come before Milton, leaving the region hydrologically vulnerable. As of
now, soil conditions are nearly saturated throughout the FL Peninsula (0 -
10 cm RSM, 70 - 90%, NASA SPoRT) and mid-layer soils are wet and
approaching saturation. Streamflows are much above normal (USGS) suggesting
limited relevant channel capacity available, and an increased potential for
out of bank river rises. Soils have some capacity to absorb rainfall, at
least initially however, as rains persist today, soil capacity will likely
become overwhelmed, leading to rapid rises in small streams. The most
likely impacts ahead of Milton include standing water on roads, overflowing
of storm drains and culverts, as well as flooding in poorly drained and
low-lying areas. Mid-week, impacts are expected to ramp up as Milton
approaches and crosses the FL Peninsula. The risk of flooding for this
event has continuously increased and extends across the Peninsula and to
portions of southeast GA. With expected rainfall amounts of 5 - 10 inches
forecast for the Florida Peninsula and even higher local totals of 15
inches, confidence is high in the occurrence of widespread flash, urban,
and areal river flooding.

Widespread minor to moderate river flooding is likely with major flooding
possible near maximum rainfall totals. The latest RFC forecasts show minor
to moderate and locally major river flooding forecast for areas surrounding
Tampa and with the latest run of the river ensemble forecasts (HEFS)
indicating the potential for moderate to locally major river flooding in
the Peace-Tampa Bay River Basin, south of Tampa, and widespread minor to
moderate elsewhere.

The National Water Model (NWM) Medium-Range Forecast (MRF) signals from the
GFS 5-day rapid-onset flooding (ROF) probability forecast service have been
steadily increasing, with basin-wide probabilities in the 25 - 75% range
from southeast GA southward into central FL. These signals will only
increase as the event draws nearer, giving confidence in the placement of
potential flooding impacts. Low annual-exceedance probabilities are present
throughout FL and widely range from 50 - 2% depending on the location of
where the heaviest QPF is found. Overall, flooding impacts are possible
anywhere persistent heavy rain sets up and disruptions to daily life can be
expected.

Puerto Rico and the USVI...
Moderate to locally heavy rainfall bringing the potential for urban and
small stream flooding is forecast through day 4 (Thu). Heavy rainfall may
overwhelm drainage and lead to standing water on roadways and pluvial
ponding in low-lying and poorly drained areas.

//Kirkpatrick



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