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Hurricane Helene


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413 
WTNT43 KNHC 072046
TCDAT3

Tropical Depression Eight Discussion Number   2
NWS National Hurricane Center Miami FL       AL082018
500 PM AST Fri Sep 07 2018

Satellite data and surface observations indicate that the low
pressure system located just west of the coast of Africa has
developed into a tropical depression.  Conventional satellite
imagery shows a well-defined convective band has formed near the
center, and microwave satellite imagery has hinted at the formation
of an inner ring of convection.  The initial intensity of 30 kt
and the central pressure of 1002 mb are based on surface
observations from ships and the west coast of Africa. The depression
currently has good cirrus outflow in all directions.

The initial motion is 275/9.  During the next 3-4 days, the cyclone
should move generally west-northwestward with an increase in forward
speed on the south side of the subtropical ridge over the eastern
Atlantic.  Near the end of the forecast period, a developing mid- to
upper-level trough over the central Atlantic is expected to weaken
the ridge and allow the cyclone to turn northwestward with a
decrease in forward speed.  There is some spread in the guidance
late in the period, with the UKMET and the Canadian models showing
an earlier turn than the other models.   The new forecast track,
which is similar to the previous track, is in best agreement with
the TVCN and HCCA consensus models.

The depression is in an environment of light to moderate easterly
vertical shear and over sea surface temperatures of 27-28C.  This
should allow at least steady strengthening, and rapid strengthening
is possible based on the hints of the inner core in microwave
imagery.  This portion of the intensity forecast has been increased
to the upper edge of the intensity guidance, with the cyclone
forecast to become a tropical storm in 12 h or less and a hurricane
between 48-72 h.  After 72 h, the system should be over sea surface
temperatures near 26C and encountering southwesterly vertical shear
due to the aforementioned trough.  This should cause at least a
gradual weakening.

FORECAST POSITIONS AND MAX WINDS

INIT  07/2100Z 13.2N  18.6W   30 KT  35 MPH
 12H  08/0600Z 13.5N  19.9W   35 KT  40 MPH
 24H  08/1800Z 13.9N  21.9W   45 KT  50 MPH
 36H  09/0600Z 14.4N  24.5W   55 KT  65 MPH
 48H  09/1800Z 14.9N  27.5W   60 KT  70 MPH
 72H  10/1800Z 16.5N  33.5W   70 KT  80 MPH
 96H  11/1800Z 18.0N  38.5W   65 KT  75 MPH
120H  12/1800Z 20.5N  42.0W   60 KT  70 MPH

$$
Forecaster Beven

 

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We have TS Helene

BULLETIN
Tropical Storm Helene Advisory Number   3
NWS National Hurricane Center Miami FL       AL082018
1100 PM AST Fri Sep 07 2018

...TROPICAL STORM HELENE FORMS BETWEEN THE COAST OF AFRICA AND THE
CABO VERDE ISLANDS...
...EXPECTED TO BRING TROPICAL STORM CONDITIONS TO PORTIONS OF THE
CABO VERDE ISLANDS SATURDAY NIGHT AND SUNDAY.


SUMMARY OF 1100 PM AST...0300 UTC...INFORMATION
-----------------------------------------------
LOCATION...13.6N 18.5W
ABOUT 405 MI...650 KM E OF THE SOUTHERNMOST CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...W OR 280 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...1002 MB...29.59 INCHES
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Tropical Storm Helene Discussion Number   3
NWS National Hurricane Center Miami FL       AL082018
1100 PM AST Fri Sep 07 2018

A very timely ASCAT pass indicated that the winds associated with
the depression have increased to 35 kt, and also that the center was
a little east of the location previously indicated. This is very
common in systems during the formative stage.  Based on the ASCAT
data, the system has been upgraded to Tropical Storm Helene, the
eighth named storm of the season. The satellite presentation has
also improved during the past several hours, and now the cyclone has
large cyclonically curved convective bands to the south of the
center. The outflow is fair in all quadrants.

Helene will be moving over warm waters and within an environment
of light shear through the next 3 to 4 days, and most of the
guidance responds to that environment by gradually strengthening the
cyclone. The NHC forecast follows the trend of the intensity
consensus, and brings Helene to hurricane intensity in about 3 days.

Currently, Helene is embedded within a southwest monsoon-type flow,
and is moving toward the west at about 9 or 10 knots. However, as
the cyclone moves away from the African coast, it will become
steered by the easterly flow around the subtropical ridge and should
then increase in forward speed. Most of the track models are in
extremely good agreement, at least for the next 3 days when the
confidence in the forecast is high. At the long range, a mid-level
trough is expected to develop over the central Atlantic, forcing the
cyclone to turn more to the northwest and even north later on. The
NHC forecast is in the middle of the guidance envelope and basically
on top of the corrected consensus HCCA.


FORECAST POSITIONS AND MAX WINDS

INIT  08/0300Z 13.6N  18.5W   35 KT  40 MPH
 12H  08/1200Z 13.8N  19.9W   35 KT  40 MPH
 24H  09/0000Z 14.3N  22.3W   40 KT  45 MPH
 36H  09/1200Z 14.8N  25.0W   50 KT  60 MPH
 48H  10/0000Z 15.4N  28.0W   55 KT  65 MPH
 72H  11/0000Z 17.0N  33.9W   65 KT  75 MPH
 96H  12/0000Z 18.5N  39.0W   65 KT  75 MPH
120H  13/0000Z 20.5N  42.5W   60 KT  70 MPH

$$
Forecaster Avila
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Helene expected to become a major hurricane before lifting north into east-central Atlantic and weakening.

274 
WTNT43 KNHC 101436
TCDAT3

Hurricane Helene Discussion Number  13
NWS National Hurricane Center Miami FL       AL082018
1100 AM AST Mon Sep 10 2018

Helene's cloud pattern has become much better organized during the
past few hours, with a clear 20 n mi wide eye seen in Meteosat
infrared imagery, and a clear eye noted in a 1135 UTC AMSU overpass.
A Dvorak classifications from TAFB and SAB were both T5.0/90 knots
at 1200 UTC, and that is the initial intensity for this advisory.
Helene will remain in an environment that should support some
additional strengthening for the next 12 hours or so, with very low
shear and SSTs of 27C or above. After that time, the SSTs cool
quickly and the shear increases dramatically by 36 hours, which
should induce steady weakening. Late in the period, the shear
continues, and while SSTs increase the atmosphere dries out, and the
intensity is held steady at day 5. The new NHC intensity forecast is
above most of the guidance through 24 hours to account for the
recent intensification of the storm, but then is close to or
slightly above HCCA and IVCN.

The initial motion estimate is 285/14. The subtropical ridge north
of Helene will gradually weaken as an upper-level trough digs
southward around 40W. This pattern will cause Helene to slow down
and gradually recurve during the forecast period, with a faster
northward motion expected at days 4 and 5 as Helene interacts with
the aforementioned upper-level trough. While there is significant
variability between the various global models in the exact
evolution of the upper-level pattern, the track model guidance is
in generally good agreement on this evolution, although the UKMET,
its ensemble mean and the GEFS mean are notable outliers to the
right. The new NHC track was not changed much from the previous one
and lies near the middle of the guidance envelope and is close to
HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT  10/1500Z 14.6N  30.0W   90 KT 105 MPH
 12H  11/0000Z 15.2N  32.0W  100 KT 115 MPH
 24H  11/1200Z 15.9N  34.3W  100 KT 115 MPH
 36H  12/0000Z 16.8N  36.1W   90 KT 105 MPH
 48H  12/1200Z 18.0N  37.7W   75 KT  85 MPH
 72H  13/1200Z 21.5N  39.5W   60 KT  70 MPH
 96H  14/1200Z 27.0N  40.5W   50 KT  60 MPH
120H  15/1200Z 32.5N  40.5W   50 KT  60 MPH

$$
Forecaster Brennan

 

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Hurricane Helene Discussion Number  14
NWS National Hurricane Center Miami FL       AL082018
500 PM AST Mon Sep 10 2018

Helene's cloud pattern continues to be well organized with a large
eye of about 20 nm in diameter, surrounded by a ring of deep
convection.  The cyclone's circulation is large with numerous
cyclonically curved convective bands, and the outflow is fair in all
quadrants. Dvorak classifications have not changed much, and support
an initial intensity of 90 kt.

Helene has the opportunity to strengthen some during the next 24
hours or so, while the shear is low and the ocean is still
relatively warm. After that time, both shear and SSTs will become
less favorable and gradual weakening should then begin. The NHC
intensity forecast is similar to the previous one, and follows
closely the intensity consensus aids.

Helene is moving toward the west-northwest or 290 degrees at 15 kt.
Soon, the hurricane will encounter a mid-level trough which the most
reliable global models are forming in the eastern Atlantic.
This new flow pattern will force Helene to turn to the northwest
and north ahead of the developing trough.  Track models are quite
consistent with this solution and the the spread of the guidance
envelope is small through the forecast period.  This increases the
confidence in the Helene's northward turn, followed by recurvature
over the eastern Atlantic.

FORECAST POSITIONS AND MAX WINDS

INIT  10/2100Z 14.9N  31.5W   90 KT 105 MPH
 12H  11/0600Z 15.5N  33.3W  100 KT 115 MPH
 24H  11/1800Z 16.4N  35.3W  100 KT 115 MPH
 36H  12/0600Z 17.5N  37.0W   85 KT 100 MPH
 48H  12/1800Z 19.0N  38.0W   75 KT  85 MPH
 72H  13/1800Z 23.0N  39.5W   60 KT  70 MPH
 96H  14/1800Z 28.5N  40.0W   50 KT  60 MPH
120H  15/1800Z 34.0N  37.0W   50 KT  60 MPH

$$
Forecaster Avila

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Hurricane Helene Discussion Number  15
NWS National Hurricane Center Miami FL       AL082018
1100 PM AST Mon Sep 10 2018

Helene remains a well organized hurricane with a fairly large 25 n
mi diameter eye and ring of cold cloud tops surrounding that
feature.  There are a few dry slots beyond the inner core, however,
which have prevented some of the Dvorak estimates from increasing
this cycle.  A blend of the latest subjective and objective
satellite intensity estimates supports raising the initial intensity
slightly to 95 kt.

The hurricane has a limited amount of time to strengthen as the
current environment of low wind shear and relatively warm SSTs will
be changing soon.  Helene is expected to cross the 26 deg C isotherm
within 24 hours and it will enter an environment of increasing
south-southwesterly shear just beyond that time period.  These
conditions should promote a steady weakening trend beginning late
Tuesday.  By the end of the forecast period, however, SSTs increase
along the forecast track, and there could be some baroclinic
enhancements to end the weakening trend and perhaps even cause some
strengthening.  This scenario is supported by the HWRF/HMON regional
models and the GFS/ECMWF global models.  The NHC intensity forecast
is largely an update of the previous one, except it is slightly
higher at the end of the period.

The hurricane is gradually turning to the right, with the latest
initial motion estimate being 295/13 kt.  A large mid- to
upper-level trough over the north Atlantic is expected to dig
southward causing a significant weakness in the subtropical ridge.
In response to this change in the steering pattern, Helene is
forecast to gradually turn northward and then north-northeastward
during the next several days.  The track models are in fairly good
agreement, and the NHC track forecast lies near the middle of the
guidance envelope.  This forecast is only a little to the right of
the previous track.

FORECAST POSITIONS AND MAX WINDS

INIT  11/0300Z 15.5N  32.4W   95 KT 110 MPH
 12H  11/1200Z 16.2N  34.0W   95 KT 110 MPH
 24H  12/0000Z 17.2N  35.7W   85 KT 100 MPH
 36H  12/1200Z 18.6N  37.2W   75 KT  85 MPH
 48H  13/0000Z 20.1N  38.0W   65 KT  75 MPH
 72H  14/0000Z 24.6N  39.0W   60 KT  70 MPH
 96H  15/0000Z 30.0N  38.0W   50 KT  60 MPH
120H  16/0000Z 36.2N  34.0W   55 KT  65 MPH

$$
Forecaster Cangialosi
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