Tropical Report

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 PM EDT Fri Sep 14 2018

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Hurricane
Florence, near the North Carolina coast, on Tropical Storm Helene,
located over the northeastern Atlantic Ocean, on Tropical Depression
Isaac, located in the eastern Caribbean Sea, and on Tropical Storm
Joyce, located about 1000 miles west-southwest of the Azores.

1. Numerous showers and thunderstorms continue in association with an
elongated low pressure area located near the coasts of Texas and
northeastern Mexico. While development into a tropical depression
is not expected since the system is almost onshore, heavy rainfall
and gusty winds are likely across portions of northeastern Mexico
and Texas through Saturday. Interests there should monitor the
progress of this system, and refer to products from their local
weather office.
* Formation chance through 48 hours…low…10 percent.
* Formation chance through 5 days…low…10 percent.

2. A broad area of low pressure could form near Bermuda late this
weekend or early next week. Additional development, if any, is
expected to be slow while the system moves little.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…low…10 percent.

Forecaster Blake

ISAAC BECOMES A TROPICAL STORM AGAIN BUT ITS FUTURE IS UNCLEAR

Tropical Storm Isaac Discussion Number 29
NWS National Hurricane Center Miami FL AL092018
500 PM AST Fri Sep 14 2018

It has been a very interesting flight of the NOAA Hurricane Hunter
aircraft this afternoon. Dropsonde and SFMR data indicate that the
maximum winds are around 35 kt again, and the pressure has fallen
to about 1002 mb. However, the system is tilted southward with
vertical height due to northerly shear, and scatterometer and
Hurricane Hunter tail-Doppler radar data indicate the system is
elongated from northeast to southwest, albeit closed. Note that the
plane was unable to sample the southwestern quadrant well because of
strong convection and intense lightning, so the flight-level winds
showing no circulation were unrepresentative.

The intensity forecast hasn’t gotten any easier with Isaac.
Vertical shear is expected to decrease a bit more, which might
diminish the effects of dry air seen on water vapor images around
the cyclone. However, the GFS and ECMWF continue to insist that
Isaac will degenerate into a wave within the next 2 or 3 days, which
is almost unanimously supported by their respective ensemble
members. This is the solution that the official forecast follows.
It should be noted that none of the global models show pressures
this low now with Isaac, except the UKMET, and that model is the one
that holds onto the circulation the longest. The reality of the
situation is that the long-term future of Isaac is cloudy, and the
best advice is just to follow NHC advisories every 6 hours to see
if we gain any confidence.

Isaac is moving westward at about 12 kt. A ridge over the
southwestern Atlantic should steer the cyclone westward over the
weekend then west-northwestward early next week as Isaac reaches a
weaker portion of the ridge. Just like yesterday, a stronger system
would likely turn more to the west-northwest earlier, moving
closer to Jamaica. Since the system is forecast to be weaker, the
forecast lies on the westward and faster side of the guidance, and
should also be considered low confidence since it depends on the
intensity. Interests in Jamaica should monitor the progress of the
Isaac.

FORECAST POSITIONS AND MAX WINDS

INIT 14/2100Z 15.3N 68.2W 35 KT 40 MPH
12H 15/0600Z 15.4N 70.1W 35 KT 40 MPH
24H 15/1800Z 15.4N 72.2W 35 KT 40 MPH
36H 16/0600Z 15.7N 74.2W 35 KT 40 MPH
48H 16/1800Z 16.2N 76.3W 30 KT 35 MPH
72H 17/1800Z 17.5N 80.0W 25 KT 30 MPH
96H 18/1800Z…DISSIPATED

$$
Forecaster Blake

HELENE EXPECTED TO TURN TOWARD THE AZORES SATURDAY… …EXPECTED TO BRING GUSTY WINDS AND HEAVY RAINS TO THE AZORES THIS WEEKEND

Tropical Storm Helene Discussion Number 30
NWS National Hurricane Center Miami FL AL082018
500 PM AST Fri Sep 14 2018

The 40-45 kt of southwesterly vertical shear has really taken its
toll on Helene’s cloud pattern. A severe tilt toward the northeast
is evident in conventional and microwave imagery. Despite the
sheared structure of the cyclone, the initial intensity is held at
60 kt due to the earlier scatterometer pass showing that value in
the northeast quadrant, and a 1603 UTC SATCON analysis that yields
an intensity estimate of 58 kt.

Little change in strength is forecast during the next 12 hours as
Helene remains over 26 to 27C sea surface temperatures. Beyond that
brief period, the cyclone should weaken a bit as it completes
extratropical transition while passing just to the northwest of the
Azores Islands and traversing much cooler waters. The European
models and the Canadian still show Helene being absorbed by a larger
baroclinic system in 4 days, as the cyclone quickly approaches
Ireland and the United Kingdom. The GFS, which earlier indicated a
different solution resulting in Helene taking a more north-
northeastward to northwest track around the periphery of a
larger, complex non-tropical low, now shows Helene merging with a
front, similar to the European and Canadian scenario.

A series of recent microwave passes reveal that Helene’s surface
circulation has been tracking quite a bit to the left of the
previous forecasts. Consequently, an adjustment to the best track
was made to conform more with the microwave overpasses. The initial
motion is estimated to be northward, or 010/16 kt. It is also quite
possible that there maybe some binary involvement between Helene and
Tropical Storm Joyce that could be influencing a motion just to the
left of track. In any event, a north-northeast to northeast turn is
expected over the weekend as the cyclone accelerates within the
mid-latitude westerlies. The NHC forecast is again adjusted to the
left through 36 hours, due to the much welcomed microwave images,
and is in best agreement with TVCN multi-model consensus.

Helene is expected to be a post-tropical cyclone when it approaches
Ireland and the United Kingdom in a few days. Interests in those
locations should consult products from their local meteorological
service for information about potential impacts from Helene. Local
forecasts and warnings for the UK can be found on the website of the
UK Met Office at https://www.metoffice.gov.uk/. Local forecasts
and warnings for Ireland can be found on the website of Met Eireann
at https://www.met.ie/.

FORECAST POSITIONS AND MAX WINDS

INIT 14/2100Z 34.1N 36.8W 60 KT 70 MPH
12H 15/0600Z 36.4N 36.0W 60 KT 70 MPH
24H 15/1800Z 38.9N 33.7W 55 KT 65 MPH
36H 16/0600Z 41.0N 30.0W 50 KT 60 MPH
48H 16/1800Z 43.3N 25.2W 50 KT 60 MPH…POST-TROP/EXTRATROP
72H 17/1800Z 49.5N 12.9W 50 KT 60 MPH…POST-TROP/EXTRATROP
96H 18/1800Z…DISSIPATED

$$
Forecaster Roberts

LIFE-THREATENING STORM SURGES TO CONTINUE TONIGHT… …CATASTROPHIC FRESHWATER FLOODING EXPECTED OVER PORTIONS OF NORTH AND SOUTH CAROLINA

Tropical Storm Florence Discussion Number 62
NWS National Hurricane Center Miami FL AL062018
500 PM EDT Fri Sep 14 2018

Florence’s satellite appearance continues to be quite impressive
with well-established outflow and a nearly symmetrical cloud
pattern. In radar imagery, however, the inner-core convection has
continued to weaken and the echoes are now more stratiform in
nature, while outer banding remains rather vigorous, especially
south of Cape Lookout and Morehead City, North Carolina. An eye is
no longer evident, and the pressure has continued to rise to a
now estimated to be 972 mb based on nearby surface observations. Air
Force Reserve aircraft data, NOAA Doppler weather radar velocity
data from Wilmington, and nearby surface observations indicate that
Florence’s intensity has decreased to 60 kt, tropical storm status.

Florence has turned westward and the motion estimate is now 270/03
kt. The new 1200Z global and regional model guidance is in good
agreement on Florence moving slowly in a general westward direction
for the next 48 hours or so, followed by a northward motion on day 3
as the system moves around the western periphery of a narrow
subtropical ridge. On days 4 and 5, the cyclone is forecast to turn
northeastward and accelerate ahead of an approaching shortwave
trough and frontal system, and emerge off the northeast U.S. coast
as an extratropical low. The new official forecast track is very
similar to the previous advisory, and is lies near the northern
and eastern edge of model guidance envelope, is closer to the
TVCA/TVCN consensus models.

Florence is expected to only slowly weaken overnight due to its
proximity to the warm Atlantic where convective bands are expected
to continue to develop and propagate inland in the eastern and
southern portion of the circulation, which will act to bring down
some of the stronger winds aloft. It is worth noting that the last
reconnaissance pass indicated 700-mb flight-level winds of 77 kt
just east of Charleston, South Carolina, so it won’t take much
convection to bring down some of those stronger winds to the
surface as gusts. More rapid weakening is forecast over the weekend
as Florence moves westward across the higher terrain of central
and northwestern South Carolina. The official intensity forecast
follows the weakening trend of the previous advisory, and is similar
to an average of the GFS- and ECMWF-based Decay SHIPS models.

Although coastal storm surge flooding will gradually subside
tonight and Saturday, it cannot be emphasized enough that another
serious hazard associated with slow-moving Florence will continue
to be extremely heavy rainfall. More than 16 inches of rain has
already fallen in many areas across southeastern North Carolina, and
more rain is still to come, which will cause disastrous flooding
that will spread inland through the weekend.

Key Messages:

1. Life-threatening storm surge will continue along portions of
the North Carolina coast through tonight, and also along the Neuse
and Pamlico Rivers in western Pamlico Sound, where rainfall and
freshwater flooding will also contribute to high water levels.
Dangerous storm surge could also affect portions of the northeast
coast of South Carolina coast tonight.

2. Life-threatening, catastrophic flash floods and prolonged
significant river flooding are likely over portions of the
Carolinas and the southern to central Appalachians from western
North Carolina into southwest Virginia through early next week, as
Florence moves slowly inland. In addition to the flash flood and
flooding threat, mudslides are also possible in the higher terrain
of the southern and central Appalachians across western North
Carolina into southwest Virginia.

3. Tropical storm conditions will continue along the coast within
the tropical storm warning area and also well inland across portions
of South Carolina and North Carolina.

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 14/2100Z 34.0N 78.6W 60 KT 70 MPH…INLAND
12H 15/0600Z 33.9N 79.3W 50 KT 60 MPH…INLAND
24H 15/1800Z 33.9N 80.3W 40 KT 45 MPH…INLAND
36H 16/0600Z 34.4N 81.4W 25 KT 30 MPH…INLAND
48H 16/1800Z 35.5N 82.6W 25 KT 30 MPH…INLAND
72H 17/1800Z 38.5N 82.6W 20 KT 25 MPH…POST-TROP/INLAND
96H 18/1800Z 41.5N 76.0W 20 KT 25 MPH…POST-TROP/INLAND
120H 19/1800Z 44.0N 64.5W 30 KT 35 MPH…POST-TROP/EXTRATROP

$$
Forecaster Stewart

JOYCE STRENGTHENS SLIGHTLY AS IT TURNS SOUTHEASTWARD.

Tropical Storm Joyce Discussion Number 9
NWS National Hurricane Center Miami FL AL102018
500 PM AST Fri Sep 14 2018

Convection associated with Joyce has increased since the last
advisory, with a vigorous band now near the center in the northern
semicircle. In response, satellite intensity estimates have edged
upward, and the initial intensity is increased to 40 kt as a blend
between the TAFB subjective Dvorak estimates and the CIMSS
satellite consensus. The cyclone now has good cirrus outflow in
the northern semicircle as an upper-level low seen in water vapor
imagery passes south of the cyclone.

The storm is starting its forecast turn toward the east and the
initial motion is now 125/5. As Tropical Storm Helene to the
east-northeast of Joyce moves away, Joyce is expected to turn
northeastward with an increase in forward speed during the next
24 h, followed by an eastward turn after 48 h. The global models
continue to trend faster, and after 48 h there has been a southward
shift compared to the previous guidance. Therefore, the new
forecast track will be notably faster than the previous track and
the 72 h point will be shifted southward. The new track is still a
little to the north of the various consensus models.

Southwesterly upper-level winds are still forecast to increase over
the cyclone during the next day or two, and the forecast track takes
Joyce over decreasing sea surface temperatures. This combination
should lead to a weakening trend, especially after 24 h. The new
intensity forecast is adjusted upward a little from the previous
one based on the initial intensity, but it still calls for Joyce to
dissipate by 96 h in agreement with most of the global models. It
is possible that Joyce could strengthen a little more during the
first 12 h while the upper-level low helps keep the stronger shear
away from the storm.

FORECAST POSITIONS AND MAX WINDS

INIT 14/2100Z 31.4N 44.1W 40 KT 45 MPH
12H 15/0600Z 31.6N 43.1W 40 KT 45 MPH
24H 15/1800Z 32.6N 41.1W 40 KT 45 MPH
36H 16/0600Z 33.9N 38.7W 35 KT 40 MPH
48H 16/1800Z 35.0N 35.6W 30 KT 35 MPH
72H 17/1800Z 35.5N 28.5W 30 KT 35 MPH
96H 18/1800Z…DISSIPATED

$$
Forecaster Beven

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