ATL: HELENE - Post-Tropical - Discussion

[cycloneye]

Tropical Storm Helene Discussion Number 25
NWS National Hurricane Center Miami FL AL082018
1100 AM AST Thu Sep 13 2018

Helene continues with very limited deep convection, only observed
this morning in the northwestern quadrant. Dvorak estimates from
TAFB and SAB are dropping, and a blend of these with the CIMSS
SATCON indicates an intensity of 60 kt. Thus Helene has weakened to
a tropical storm.

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. A slow weakening is thus expected.
Beginning in about three days, Helene will commence baroclinic
transition, and it is expected to become an extratropical cyclone by
96 hours. This forcing should preclude any additional weakening
through the remainder of the forecast period. The official
intensity forecast is similar to the previous advisory. The
prediction is based on a blend of the stronger HWRF dynamical model
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 variable consensus technique (TVCN) and is just
north of the previous forecast through three days. The
guidance suite remains tightly clustered and indicates that Helene
will pass near the Azores in 2 or 3 days. Interests in
those islands should closely monitor the progress of Helene.


FORECAST POSITIONS AND MAX WINDS

INIT 13/1500Z 24.8N 37.3W 60 KT 70 MPH
12H 14/0000Z 27.2N 37.1W 55 KT 65 MPH
24H 14/1200Z 30.8N 36.6W 50 KT 60 MPH
36H 15/0000Z 34.2N 35.5W 50 KT 60 MPH
48H 15/1200Z 37.2N 33.3W 50 KT 60 MPH
72H 16/1200Z 40.5N 26.0W 50 KT 60 MPH
96H 17/1200Z 44.0N 18.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 18/1200Z 51.5N 9.0W 50 KT 60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Landsea

[cycloneye]

Tropical Storm Helene Discussion Number 27
NWS National Hurricane Center Miami FL AL082018
1100 PM AST Thu Sep 13 2018

Helene's cloud pattern is beginning to have the appearance of a
cyclone in extratropical transition with the rain shield limited to
the northwest quadrant. The Dvorak numbers have decreased, but very
recent scatterometer data show winds of around 55 kt associated with
Helene's circulation, and this is the intensity value used in this
advisory. Most of the guidance suggests that Helene's strength
should remain steady through the forecast period, perhaps due to
baroclinic forcing. On this basis, the NHC forecast keeps Helene
with the same intensity through the forecast period, but as a
post-tropical cyclone beyond 3 days.

Helene has increased its forward speed and is now moving toward the
north or 010 degrees at about 18 kt. The flow ahead of a sharp
mid-level trough is well established, and this pattern will continue
to steer Helene toward the north and then northeast during the next
few days. This is consistent with most of the track guidance which
is indeed tightly clustered. The NHC forecast is not very different
from the previous one, and is in between the corrected consensus
HCCA and the multi-model consensus aids.

Given the increase in forward speed and the expansion of the wind
field, a tropical storm warning will likely be required for a
portion of the Azores early Friday.

FORECAST POSITIONS AND MAX WINDS

INIT 14/0300Z 28.6N 36.5W 55 KT 65 MPH
12H 14/1200Z 31.4N 35.9W 55 KT 65 MPH
24H 15/0000Z 34.8N 34.7W 50 KT 60 MPH
36H 15/1200Z 37.8N 32.8W 50 KT 60 MPH
48H 16/0000Z 40.0N 29.0W 50 KT 60 MPH
72H 17/0000Z 43.0N 21.5W 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 18/0000Z 46.5N 15.0W 50 KT 60 MPH...POST-TROP/EXTRATROP
120H 19/0000Z 53.0N 5.0W 50 KT 60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Avila

[EquusStorm]

Pretty convenient that Helene and Joyce can be watched now from pretty much the same satellite image.

[AnnularCane]

They look like best friends.

[EquusStorm]

Back up to 70 with a lot more deep convection and banding on west side despite, well, everything; prolly gonna hang on to tropical designation a while longer

[Sciencerocks]

[EquusStorm]

The neat thing is, they both look better today. Which is reflected already in advisories (65 to 70; 40 to 45) but still. You don't get many storms closer than that without one eating or smothering the other.

[cycloneye]

Tropical Storm Helene Discussion Number 34
NWS National Hurricane Center Miami FL AL082018
500 PM AST Sat Sep 15 2018

The cloud pattern of Helene is gradually decaying and transitioning
to a shear pattern, with the low-level center occasionally visible
through breaks in the cirrus overcast. The initial intensity is
reduced to 55 kt in best agreement with the CIMSS satellite
consensus. Helene is moving toward colder water and into increasing
shear associated with the mid-latitude westerlies. This should
cause continued weakening and, as Helene merges with a frontal
system, extratropical transition in about 36 h. The forecast will
continue to call for Helene or its remnants to be absorbed into a
large non-tropical low over the North Atlantic between 72-96 h, and
the latest global model guidance suggests this could happen earlier
than forecast.

The initial motion is now 050/19. A mostly northeastward motion at
a faster forward speed should occur during the remainder of the
cyclone's life as it becomes more embedded in the westerlies. On
the forecast track, Helene should pass near or over the western
Azores during the next several hours, and head toward Ireland and
the United Kingdom Sunday night and Monday. There are again no
significant changes to either the track guidance or the official
forecast track since the last advisory.

Tropical storm conditions are spreading over portions of the Azores
and should continue through tonight.

Helene is also expected to be a storm-force post-tropical cyclone
when it approaches Ireland and the United Kingdom in two to three
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 15/2100Z 40.2N 32.1W 55 KT 65 MPH
12H 16/0600Z 41.9N 29.2W 55 KT 65 MPH
24H 16/1800Z 44.4N 24.1W 55 KT 65 MPH
36H 17/0600Z 47.2N 17.9W 55 KT 65 MPH...POST-TROP/EXTRATROP
48H 17/1800Z 51.2N 11.1W 50 KT 60 MPH...POST-TROP/EXTRATROP
72H 18/1800Z 61.0N 1.0E 50 KT 60 MPH...POST-TROP/EXTRATROP
96H 19/1800Z...ABSORBED BY EXTRATROPICAL LOW

$$
Forecaster Beven

[HurricaneRyan]

Looks like Helene will be extratropical tomorrow.

[cycloneye]

Post-Tropical Cyclone Helene Discussion Number 37
NWS National Hurricane Center Miami FL AL082018
1100 AM AST Sun Sep 16 2018

Helene's cloud pattern has transformed into that of an
extratropical cyclone, with a large area of cold cloud tops
located well to the north and northwest of the center. An ASCAT
pass that caught the eastern portion of the circulation revealed
maximum winds of 40 to 45 kt, therefore the initial wind speed
remains 45 kt for this advisory. The global models suggest that
the system will strengthen slightly today due to baroclinic
processes, and this is reflected in the official forecast.
Weakening is anticipated by late Monday, and the system is forecast
to be absorbed by a larger low pressure to the northwest of the
United Kingdom by Tuesday morning.

The cyclone is moving quickly northeastward or 055/25 kt. Now that
the system is embedded within the mid-latitude westerlies, a
northeastward motion with some additional increase in forward speed
is expected over the next day or so. The global models remain in
good agreement on the track of the post-tropical low, and the new
official forecast is an update of the previous advisory.

This is the last NHC advisory on this system. Interests in Ireland
and the United Kingdom should consult products from their local
meteorological service for information about potential impacts from
the post-tropical cyclone. 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 16/1500Z 43.8N 25.7W 45 KT 50 MPH...POST-TROPICAL
12H 17/0000Z 45.8N 21.2W 50 KT 60 MPH...POST-TROP/EXTRATROP
24H 17/1200Z 49.0N 14.8W 50 KT 60 MPH...POST-TROP/EXTRATROP
36H 18/0000Z 52.8N 6.2W 40 KT 45 MPH...POST-TROP/EXTRATROP
48H 18/1200Z...ABSORBED

$$
Forecaster Brown

[galaxy401]

Poor Helene never got any attention due to Florence and Isaac. Couldn't quite reach Category 3.

[bob rulz]

I think there's a chance it could be upgraded to category 3 briefly in post-analysis. Didn't some satellite estimates put it right around 100kt at peak?

[EquusStorm]

There were certainly a couple of 100kt estimates at its peak (and the NHC even forecast a 100kt peak) so it's possible, though I imagine it would have been very brief