286
FXUS62 KMLB 260701
AFDMLB

Area Forecast Discussion
National Weather Service Melbourne FL
301 AM EDT Thu Sep 26 2024

...New DISCUSSION, MARINE...

.DISCUSSION...
Issued at 301 AM EDT Thu Sep 26 2024

KEY MESSAGES:

- Hurricane Helene is expected to move toward the Florida
  Panhandle today. For East Central Florida, numerous downpours,
  strong wind gusts, and possible tornadoes are the primary
  threats. Rough, battering surf will produce dangerous conditions
  at the beaches.

- A Tropical Storm Warning is in effec.t Preparations for Helene
  should be completed. Conditions will quickly deteriorate this
  morning.


- Hot and humid Friday into the weekend behind Helene as a plume
  of deep moisture returns. Heavy rain is also possible over the
  weekend.

              ----------Synoptic Overview----------

Hurricane Helene is beginning to accelerate northward over the
eastern Gulf of Mexico this morning, located around 400 miles
southwest of Tampa as of 2 AM ET. Meanwhile, a cut-off upper low
over the Ohio-Mississippi River confluence is slowly meandering
southward. On the eastern flank of this cut-off low, Helene is
forecast to quickly be drawn poleward. Landfall is expected over the
eastern Florida Panhandle this evening as a major hurricane.
Thereafter, Helene will penetrate well inland, eventually phasing
with the cut-off low over the Tennessee Valley by Friday after a
rare occurrence of the Fujiwhara Effect over the Southeastern United
States. There is high confidence in this solution among the
available model guidance.

This weekend and into early next week, the guidance suggests the
upper low will slowly weaken north of a building H5 ridge axis over
Florida. This ridge is then forecast to slowly retreat into the
Atlantic by the middle of next week but with increasing spread in
the upper-air pattern noted over the Eastern U.S.

Boundary-layer flow should remain southwesterly behind Helene
through the weekend before becoming light early next week. A tail of
deep tropical moisture will be funneled from the Caribbean across
Florida through the weekend before total moisture values dip closer
to seasonal norms as we push deeper into next week.

             -----Short Term Mesoscale Features-----

As Helene moves into the NE Gulf today, the Florida Peninsula will
reside on its eastern flank. The wind field will expand,
characterized by 3K to 5K FT AGL winds reaching 50-75 KT over our
district by this afternoon. Such a remarkable mass response will
advect rich tropical moisture (PWs >2.5"), intervals of
convergence, and at least moderate destabilization (MLCAPE ~ 1000
J/kg). In addition, winds will back SSE at the surface, generating
200-400 m^2/s^2 of ESRH. This environment will be capable of
producing periodic bands of rain and storms along with the threat
of tornadoes and enhanced wind gusts. 26/00Z HREF proximity
soundings also indicate some potential breaks in the thick
overcast which would promote deeper mechanical mixing of the
strong boundary layer winds to the surface. Momentum transfer
algorithms agree with this assessment.

             -------Sensible Weather & Impacts-------

Through Today/Tonight...

Wind: After sunrise, wind speeds will start to ramp up and gusts
will begin to take place more frequently. By midday, a few gusts
exceeding 40 mph will begin to occur, particularly on the coast and
near/south of Orlando. These gusts will become frequent and ramp up
well into tropical storm force this afternoon and into the evening.
The timeframe for the highest potential gusts looks to reside
between 2 PM and 9 PM. In that window, there is a 60% or greater
probability of wind gusts exceeding 50 MPH roughly from Melbourne to
Okeechobee and points northwest, as well as along the immediate
Treasure Coast. West of Orlando, including Lake County, there is a
40-60% chance for wind gusts to near or just over 60 MPH. The
frequency of these gusts will be greater if more breaks in the
overcast occur and within bands of rain/storms. Wind gusts of this
magnitude can turn lightweight objects into flying debris, cause
sporadic power interruptions, and perhaps result in some tree or
minor structural damage. Please remain indoors if at all possible.
Tropical storm force gusts will diminish after midnight.

Tornadoes: The Storm Prediction Center has highlighted almost all of
the district in a Slight Risk (level 2 of 5) for severe storms, the
primary threat being a few tornadoes. A Marginal Risk exists for St
Lucie & Martin Counties. We will be monitoring fast-moving
rain/storm bands for this potential. Have multiple ways of receiving
warnings today!

Flooding Rain: A Flood Watch remains in effect for East Central
Florida. On the whole, additional rainfall totals will average 1-3".
However, locations that see repeated rainbands (on an isolated
basis) would be able to exceed those values which could lead to some
minor flooding issues. Max-member HREF rainfall shows a non-zero
threat of a few locations reaching 4"+ of rain. If you encounter a
flooded roadway, turn around, don`t drown.

Beaches/Marine: Here`s the takeaway... stay away from the water
today! A High Surf Advisory is in effect for battering surf which
may run up to the dune line. This could cause minor beach erosion.
Additionally, numerous life-threatening rip currents and a powerful
longshore current will exist. Seas will rapidly build and become
dangerous.

With breaks in the rain and perhaps even a few hints of sun, high
temps should still reach the upper 80s to low 90s. Combined with the
high humidity, heat indices may reach 105F this afternoon in a few
spots.

Friday-Weekend...

A tail of deep tropical moisture will oscillate from South-Central
FL on Friday to Central Florida on Saturday and Sunday. Beneath it,
scattered-numerous (50-70%) shower and storm coverage is forecast
particularly in the afternoon/evening hours. While brief gusty winds
can`t be ruled out, our main concern is additional rounds of heavy
rain over already saturated ground. We will monitor for the risk of
flooding in a few locations once we better pin down the coverage and
extent of the expected rain/storms.

Without question, high humidity persists with dew points well into
the mid/upper 70s. Combined with the offshore flow, low to even a
few mid 90s are forecast. Heat index readings from 102-107F are
currently expected each afternoon. This could get us very close to
Heat Advisory territory.

Next Week...

As the upper ridge sits nearby, the deep tropical moisture overhead
will trend closer to normal. Some members show bouts of even drier
air infiltrating the area from time to time, but confidence lessens
overall. This yields continued 30-50% shower/storm coverage each day
with highs in the upper 80s to lower 90s. Heat indices will subtly
"dip" closer to 98-103F as surface moisture trends toward normal
values.

&&

.MARINE...
Issued at 301 AM EDT Thu Sep 26 2024

BOTTOM LINE: Dangerous Marine Conditions From Helene through
early Friday...

Today/Tonight... Hurricane Helene will produce extremely dangerous
marine conditions. All operators should remain in port through
tonight. Nearshore seas are forecast to peak from 9-11 FT by this
evening, increasing to 10-15 FT in the Gulf Stream. Sustained
tropical-storm-force winds are likely along with southerly gusts as
high as 50-55 KT. Winds will start to decrease overnight but
tropical storm force gusts will persist through the overnight.
Numerous showers and storms with waterspouts are possible!

Friday-Sunday... Seas quickly subside through the period: 5-7 FT
nearshore / 6-10 FT on Friday, 2-3 FT nearshore / 3-5 FT offshore
this weekend. However, fresh southerly winds continue at 20-25 KT
Friday before becoming SW 8-15 KT this weekend. Scattered-numerous
showers and storms with offshore-moving activity expected.

&&

.AVIATION...
(06Z TAFs)
Issued at 225 AM EDT Thu Sep 26 2024

Increasing SE to S winds as Hurricane Helene strengthens over the
eastern Gulf and accelerates N/NE next 12-18 hours. Prevailing VFR
to start with VCSH at all terminals. Will be watching for some
increase in SHRA/isold TSRA developing over the terminals toward
daybreak as rainbands lift northward across the southern and
western FL peninsula. "Helene" will bring significant aviation
impacts to all terminals on Thursday as S/SE winds increase 20-30
knots with gusts 45-55 knots possible esp in fast-moving
rainbands. AMDs likely needed but have inserted a TEMPO from
18Z-22Z for best chance for IFR/MVFR reductions. Have maintained
mention of non- convective LLWS for MCO btwn 00Z-06Z when wind
fields maximize just above the sfc.

&&

.PRELIMINARY POINT TEMPS/POPS...
DAB  90  78  92  76 /  80  60  40  30
MCO  90  79  91  77 /  80  50  50  30
MLB  90  79  92  77 /  70  50  70  50
VRB  90  78  92  77 /  70  50  70  50
LEE  88  78  90  77 /  90  70  30  20
SFB  88  79  91  77 /  80  50  50  30
ORL  89  80  91  78 /  80  60  50  30
FPR  90  79  91  77 /  70  50  70  50

&&

.MLB WATCHES/WARNINGS/ADVISORIES...
FL...Tropical Storm Warning for FLZ041-044>046-053-058-141-144-154-
     159-164-247-254-259-264-347-447-547-647-747.

     Flood Watch through Friday morning for FLZ041-044>046-053-058-
     141-144-154-159-164-247-254-259-264-347-447-547-647-747.

     High Surf Advisory until 11 AM EDT Friday for FLZ141-154-159-164-
     347-447-647-747.

AM...Tropical Storm Warning for AMZ550-552-555-570-572-575.

&&

$$

DISCUSSION...Heil
AVIATION...Kelly