Tropical Storm Helene Discussion Number 26
NWS National Hurricane Center Miami FL AL082018
500 PM AST Thu Sep 13 2018

Helene has maintained deep convection this afternoon though only
in the northern semicircle. Dvorak estimates from TAFB and SAB are
continuing to drop, and a blend of these with the CIMSS SATCON
supports lowering the intensity to 55 kt.

Strong tropospheric vertical shear along with only lukewarm waters
are likely contributing toward Helene’s weakening. The shear
should further increase during the next two days, though the sea
surface temperatures will rebound up to 27C along with an increase
in low-level moisture. Helene is expected to be either slowly
weakening or steady state during this time. Beginning in about
three days, Helene will commence baroclinic transition, and it is
expected to become an extratropical cyclone by 96 hours. The
baroclinic forcing should preclude any additional weakening through
the remainder of the forecast period, and the official intensity
forecast is nearly the same as the last advisory. This prediction
is based on a blend of the stronger HWRF/COAMPS dynamical models and
the weaker SHIPS/LGEM statistical schemes for the next couple of
days, and is based on a blend of the GFS/ECMWF/UKMET global models
at the extended lead times.

Helene is moving northward at about 12 kt between the subtropical
ridge to its east and a deep-layer trough to its west. The system
should accelerate and turn toward the northeast over the next few
days as it rounds the subtropical ridge and starts getting picked
up by the mid-latitude westerlies. The official track forecast is
based upon the tightly clustered variable consensus technique (TVCN)
and is nearly the same as the previous forecast.

FORECAST POSITIONS AND MAX WINDS

INIT 13/2100Z 26.3N 37.0W 55 KT 65 MPH
12H 14/0600Z 28.8N 36.7W 50 KT 60 MPH
24H 14/1800Z 32.3N 35.8W 50 KT 60 MPH
36H 15/0600Z 35.5N 34.2W 50 KT 60 MPH
48H 15/1800Z 37.9N 31.7W 50 KT 60 MPH
72H 16/1800Z 40.6N 24.4W 50 KT 60 MPH
96H 17/1800Z 44.0N 17.5W 50 KT 60 MPH…POST-TROP/EXTRATROP
120H 18/1800Z 51.0N 7.5W 50 KT 60 MPH…POST-TROP/EXTRATROP

$$
Forecaster Landsea

ALL TROPICAL STORM WATCHES AND WARNINGS FOR ISAAC HAVE ENDED… …HEAVY RAIN THREAT REMAINS

Tropical Storm Isaac Discussion Number 25
NWS National Hurricane Center Miami FL AL092018
500 PM AST Thu Sep 13 2018

The center of Isaac has been mostly exposed during the past several
hours as temporary bursts of convection fire just east of the
center. Assuming some spin down of the circulation after the plane
left, the initial wind speed is set to 35 kt. Since there haven’t
been any reports of sustained tropical-storm-force winds in the
Lesser Antilles and the center is moving away, all watches and
warning have been lowered. However, there remains a threat of flash
flooding due to a large area of rain that has yet to move through
the Lesser Antilles.

Strong shear and mid-level dry air is forecast to persist for the
next day or so, which should cause Isaac to drop back to tropical
depression status on Friday. However, the shear is forecast to
weaken by the weekend, and moisture values are expected to rise as
the system gets deeper into the Caribbean Sea. These conditions
would normally allow for some restregthening, but the initial
character of the system is pretty weak, and it is uncertain whether
there will be much left of Isaac to take advantage of the more
conducive conditions. Model guidance has been inconsistent on the
future of Isaac, which speaks to the lack of predictability of this
situation. The bulk of the guidance show Isaac staying weak or
degenerating into a tropical wave over the next few days, so the
official forecast follows that consensus. At this point it is
just best to keep an eye on the long-range forecast on each
advisory to see if there are any changes.

Isaac is moving westward at about 15 kt. A mid-level ridge is
forecast to persist over the southwestern Atlantic, steering Isaac
westward for the next few days, with a turn to the west-northwest
possible at long range. The future track is very uncertain at long
range because it is dependent on the intensity. If Isaac
degenerates into a wave, the remnants will likely move south and
west of the guidance mean, and if the cyclone is stronger, it will
probably move slower and more to the right. The new NHC forecast
assumes that Isaac will stay weak, and therefore is on the left and
faster side of the guidance.

FORECAST POSITIONS AND MAX WINDS

INIT 13/2100Z 14.9N 63.2W 35 KT 40 MPH
12H 14/0600Z 14.9N 65.4W 35 KT 40 MPH
24H 14/1800Z 14.9N 68.2W 30 KT 35 MPH
36H 15/0600Z 14.9N 70.8W 30 KT 35 MPH
48H 15/1800Z 15.0N 73.0W 30 KT 35 MPH
72H 16/1800Z 16.2N 77.4W 30 KT 35 MPH
96H 17/1800Z 17.5N 81.5W 30 KT 35 MPH
120H 18/1800Z…DISSIPATED

600 PM POSITION UPDATE… …DANGEROUS STORM SURGE FLOODING OCCURRING ON THE WESTERN SIDE OF PAMLICO SOUND

Hurricane Florence Discussion Number 58
NWS National Hurricane Center Miami FL AL062018
500 PM EDT Thu Sep 13 2018

The satellite and radar presentations of Florence have changed
little this afternoon with a 20-25 nmi wide eye waxing and waning
as intrusions of dry air have occasionally eroded the southern and
eastern eyewall. Despite the occasional ragged appearance of
Florence’s eye, reconnaissance aircraft data indicate that the
central pressure has remained steady at 955 mb. The last reports
from the Air Force Reserve hurricane hunters support an intensity
of 85 kt, and this is corroborated by average Doppler velocity
values of near 105 kt at 2500-3000 ft ASL. A report of a 10-minute
average wind of 59 kt and a gust to 74 kt was recently received from
the Cape Lookout C-MAN station (CLKN7). The 59-kt 10-minute wind
speed is roughly equivalent to a 65-kt 1-minute wind.

Florence has continued to slow down, and radar fixes over the past
couple hours suggest that Florence has possibly stalled due to a
re-organization of the eye/eyewall. Smoothing through the fixes
over the past 6 h yields an initial motion estimate of 295/04 kt.
There is no change to the previous forecast reasoning. The ridge to
the north and east of Florence remains intact over the Atlantic
Ocean, but water vapor imagery and special upper-air observations
indicate that a shortwave trough has weakened that portion of the
ridge along the U.S. mid-Atlantic coast, resulting in a weakening
of the steering currents. The ridge is forecast by all of the
latest model guidance to remain intact, albeit weak, for the next 72
hours, which will nudge Florence on a slow westward to west-
southwest track into central South Carolina. On days 4 and 5,
Florence is expected to become an extratropical low as it interacts
with a front while moving northward and northeastward along the
Appalachian Mountains. The official forecast track is similar to,
but slightly south of the previous advisory track through 72 hours,
with little change indicated on days 4 and 5. This scenario closely
follows the simple consensus model TVCN/TVCA, which is north of the
corrected-consensus models HCCA and FSSE, which are heavily weighing
the southernmost model, ECMWF, which keeps Florence over or near the
Atlantic through about 48 hours.

Radar data indicate that Florence may be developing an outer
eyewall. If this trend continues, then little change to the
intensity is likely until landfall occurs in about 24 hours due in
part to the low vertical wind shear conditions and the warm, deep
waters of the Gulfstream current. Florence is expected to weaken
after landfall, but the rate of weakening may be tempered somewhat
due to much of the hurricane’s circulation remaining over the warm
waters of the Atlantic Ocean and Gulfstream. Once Florence moves
farther inland over central South Carolina, rapid weakening of
the inner-core wind field should occur due to land interaction and
the cyclone’s slow forward speed of 5 kt or less. However, intense
rainbands are expected to continue developing over the Atlantic
waters and keep moving along the coast and inland, likely producing
strong wind gusts through Saturday night.

Aircraft and satellite wind data show that Florence remains a large
hurricane. Life-threatening storm surge, heavy rainfall, and
damaging wind will cover a large area regardless of exactly where
the center of Florence moves.

Key Messages:

1. A life-threatening storm surge is highly likely along portions of
the coastlines of South Carolina and North Carolina, and a Storm
Surge Warning is in effect for a portion of this area. The greatest
storm surge inundation is expected between Cape Fear and Cape
Hatteras, including the Neuse and Pamlico Rivers and western Pamlico
Sound.

2. Life-threatening, catastrophic flash flooding and prolonged
significant river flooding are likely over portions of the Carolinas
and the southern and central Appalachians through early next week,
as Florence is expected to slow down as it approaches the coast and
moves inland.

3. Damaging hurricane-force winds are likely along portions of the
coasts of South Carolina and North Carolina beginning this evening,
and a Hurricane Warning is in effect. Strong winds could also spread
inland into portions of the Carolinas.

4. Large swells affecting Bermuda, portions of the U.S. East Coast,
and the northwestern and central Bahamas will continue this week,
resulting in life-threatening surf and rip currents.

FORECAST POSITIONS AND MAX WINDS

INIT 13/2100Z 33.7N 76.2W 85 KT 100 MPH
12H 14/0600Z 34.1N 77.2W 85 KT 100 MPH
24H 14/1800Z 34.2N 78.2W 75 KT 85 MPH…INLAND
36H 15/0600Z 33.9N 79.0W 50 KT 60 MPH…INLAND
48H 15/1800Z 33.8N 79.9W 35 KT 40 MPH…INLAND
72H 16/1800Z 34.8N 82.3W 25 KT 30 MPH…INLAND
96H 17/1800Z 37.9N 82.6W 20 KT 25 MPH…POST-TROP/INLAND
120H 18/1800Z 42.7N 76.3W 20 KT 25 MPH…POST-TROP/INLAND

$$
Forecaster Stewart

JOYCE DRIFTING SOUTHWESTWARD WITH NO CHANGE IN STRENGTH

Subtropical Storm Joyce Discussion Number 3
NWS National Hurricane Center Miami FL AL102018
500 AM AST Thu Sep 13 2018

Joyce is still co-located with an upper-level low, and the most
recent TAFB intensity classification was still a subtropical type.
Furthermore, there has been no discernible increase of convection
near the center of the cyclone, and cloud tops have actually warmed
over the past few hours. Therefore, Joyce is still classified as
subtropical for this advisory. Since the cyclone hasn’t changed
much in organization, the initial intensity has been held at 40 kt
based on the TAFB subtropical intensity estimate, but this is still
possibly generous.

Joyce’s struggle to maintain deep convection bodes poorly for its
intensification prospects, considering that the shear is forecast
by the global models to increase considerably over the next few
days. SHIPS, LGEM, and the global models continue to show Joyce
changing little in strength over the next few days until the
system dissipates by early next week. The HWRF and HMON models are
still outliers in showing substantial intensification, but even
these models aren’t quite as aggressive in strengthening Joyce as
they were previously. The NHC forecast continues to favor the
statistical guidance and is just below the intensity consensus at
most forecast hours because it is frankly hard to believe that Joyce
will become a hurricane in 48 h based on its current appearance.

Joyce is still moving slowly southwestward with an initial motion
of 220/5 kt. The global models remain in fairly good agreement
that Joyce will move slowly southwestward for the next day or
so as the storm remains in light steering flow. A turn toward the
south will likely occur as Helene approaches from the southeast,
followed by a quick turn northeastward once the larger tropical
cyclone passes by. Joyce should then accelerate quickly
northeastward ahead of a mid-latitude trough before dissipating
southwest of the Azores. Little change was made to the official
track forecast which is near the middle of the various consensus
aids.

FORECAST POSITIONS AND MAX WINDS

INIT 13/0900Z 33.8N 43.1W 40 KT 45 MPH
12H 13/1800Z 33.3N 43.8W 40 KT 45 MPH
24H 14/0600Z 32.6N 44.2W 45 KT 50 MPH
36H 14/1800Z 32.0N 44.0W 45 KT 50 MPH
48H 15/0600Z 32.1N 43.2W 45 KT 50 MPH
72H 16/0600Z 34.6N 39.4W 40 KT 45 MPH
96H 17/0600Z 37.0N 32.5W 40 KT 45 MPH
120H 18/0600Z…DISSIPATED

$$
Forecaster Zelinsky

There are no tropical cyclones in the Eastern North Pacific at this time