1. A non-tropical area of low pressure is located about 700 miles
southwest of the Azores. This system is beginning to acquire
subtropical characteristics, and environmental conditions are
expected to be conducive for the development of a tropical or
subtropical cyclone during the next day or so while the low drifts
toward the southwest. Thereafter, the atmosphere is expected to
become hostile for further development.
* Formation chance through 48 hours...high...70 percent.
* Formation chance through 5 days...high...70 percent.
Tropical Depression Seventeen Discussion Number 1
NWS National Hurricane Center Miami FL AL172017
500 AM AST Mon Oct 09 2017
The well-defined low pressure system over the eastern Atlantic
several hundred miles southwest of the Azores has maintained an
area of deep convection near and to the east of the center for the
past several hours. Therefore, the system now meets the criteria
for a tropical depression. The initial intensity is estimated to be
30 kt based on ASCAT data from around 0000 UTC and a 2.0/30 kt
Dvorak classification from TAFB.
The depression is moving slowly northward at 3 kt, and a continued
slow north to north-northeast motion is expected today. A turn to
the east and then southeast is forecast to begin tonight and
continue through Wednesday while the cyclone is steered by the flow
on the east side of a subtropical ridge. Thereafter, a mid- to
upper-level trough is expected to approach the depression, and that
should cause the system to move a little faster to the east and
east-northeast. The NHC track forecast lies near the middle of the
guidance envelope, in agreement with various consensus aids, and
keeps the system far from any land areas.
The depression is currently over marginally warm 27 deg C SSTs and
in an environment of moderate to strong westerly shear. The
models suggest that the shear will likely let up a little during the
next couple of days, which could allow the depression to gradually
strengthen while it remains over relatively warm waters. After that
time, the shear could increase again, but the amount of shear
depends on the exact track of the tropical cyclone. There is a fair
amount of spread in the models at the longer range with the regional
models (HWRF, HMON, and COAMPS-TC) bringing the system to hurricane
strength while the statistical SHIPS and LGEM models show a much
weaker system. The NHC intensity forecast leans toward the more
conservative side of the guidance for now, but it should be noted
that confidence in the intensity forecast is low.
Tropical Storm Ophelia Discussion Number 2
NWS National Hurricane Center Miami FL AL172017
1100 AM AST Mon Oct 09 2017
The convective pattern of the small cyclone has continued to improve
since the previous advisory, with some thunderstorm activity having
developed near or over the center, along with an increase in curved
banding features in the eastern semicircle. The Dvorak intensity
estimate at 1200Z from TAFB was T2.0/30 kt, but given the
aforementioned improvement in the curved-band structure since that
time, the initial intensity is increased to 35 kt. This makes
Ophelia the fifteenth named storm of the 2017 Atlantic hurricane
season.
The initial motion estimate is 030/04 kt. The steering flow
surrounding Ophelia is expected to continue to be weak for the next
day or so while the cyclone remains entangled with a weak
upper-level low located just to its north and northwest. As a
result, only a slow drift toward the northeast and and east is
forecast. By 36 h, increased mid-level ridging to the northwest of
Ophelia should induce a motion toward the east-southeast and
southeast through 72 h, after which a broad mid-latitude trough is
expected to gradually accelerate the cyclone toward the northeast at
a forward speed of near 10 kt. The official forecast remains down
the middle of the guidance envelope, in close agreement with the
previous forecast track, and the HCCA and TVCX consensus models.
Ophelia should remain far away from land for the next 5 days.
Although Ophelia is forecast to remain over marginally warm SSTs of
26.5-27 deg C, much colder-than-normal temperatures aloft should
offset the relatively cool ocean temperatures and produce sufficient
instability to generate moderate to strong convection for the next
120 h. The GFS-based SHIPS model is currently assessing 25 kt of
westerly vertical wind shear, which obviously isn't occuring based
on the recent development of anticyclonic outflow over the low-level
center and across most of Ophelia's circulation. In contrast, the
UW-CIMSS shear analyses indicate lower shear values near 15 kt.
However, both shear assessments are likely too high given the large
domains that they use to calculate environmental wind shear. Based
on recent trends noted in water vapor imagery showing the shear
decreasing, along with the overall favorable upper-level wind fields
forecast by the GFS and ECMWF, especially after 36 hours, slow but
steady strengthening is expected for the next 5 days, with only
occasional intrusions of dry mid-level air being the primary
inhibiting factor to the intensification process. The official
intensity forecast is close to the NOAA HCCA consensus model, is
about midway between the more aggressive IVCN consensus model that
makes Ophelia a hurricane in about 72 h, and the more conservative
statistical SHIPS and LGEM models. However, even the SHIPS and LGEM
models have increased their peak intensity forecasts by at least 15
kt since the previous advisory.
Hurricane Ophelia Discussion Number 15
NWS National Hurricane Center Miami FL AL172017
500 PM AST Thu Oct 12 2017
Ophelia's structure has continued to improve during the afternoon.
The eye of the hurricane has cleared and is surrounded by a ring of
cloud tops colder than -55 deg C. Dvorak classifications from TAFB
and the UW-CIMSS ADT have increased accordingly, and on that basis
the initial intensity has been increased to 85 kt. Ophelia is
nearly stationary, which could cause the hurricane to stop
strengthening, or even weaken slightly during the next 12 to 24
hours due to upwelling effects. That said, the intensity guidance
is in good agreement that Ophelia will remain at hurricane strength
for the next 48 h while it remains in a fairly unstable, low-shear
environment. Beyond that time, extratropical transition will begin,
though baroclinic forcing will likely keep post-tropical Ophelia
near hurricane strength as it approaches Ireland and the UK. The
new intensity forecast is very similar to the previous advisory, and
is generally close to the multi-model intensity consensus.
Although Ophelia is currently stationary, a mid-latitude trough
should cause it to begin moving toward the east-northeast within
about 24 hours. The cyclone will then accelerate on that heading in
the faster mid-latitude westerly flow ahead of the trough, before
turning toward the northeast around day 3 as extratropical
transition occurs and the hurricane becomes entangled with the
southern extent of the trough. Confidence in the track forecast is
fairly high for the first 72 h, and all of the dynamical guidance is
tightly clustered through this period. The model spread increases
substantially at 96 h and beyond. As a post-tropical cyclone,
Ophelia will continue to interact with the southern extent of the
trough, and should turn toward the northeast as a result of this
interaction. However, the details of this turn vary greatly from
model to model. The new NHC forecast has been nudged slightly
toward the east at this time range, closer to the UKMET and ECMWF
models, as well as the corrected consensus aids HCCA and FSSE.
While the track guidance keeps the center of Ophelia south and east
of the Azores, tropical-storm-force winds are possible throughout
the Azores by Sunday due to an approaching front. In addition, the
wind field of Ophelia will likely expand as the cyclone begins
extratropical transition, and any deviation to the left of the
forecast track could bring stronger winds to the islands. Interests
in the Azores should refer to products issued by the Azores Weather
Forecast and Watch Center.
KEY MESSAGES:
1. Ophelia is expected to transition to a hurricane-force post-
tropical cyclone by Monday when it moves near Ireland and the
United Kingdom. While post-tropical Ophelia will likely bring some
direct impacts from wind and heavy rain to portions of these areas,
as well as dangerous marine conditions, given the forecast
uncertainty at these time ranges it is too soon to determine the
exact magnitude, timing and location of the impacts. Residents in
Ireland and the United Kingdom should monitor the progress of
Ophelia for the next several days. For more information on local
impacts, residents of Ireland should refer to products issued by Met
Eireann and residents in the United Kingdom should refer to products
issued by the Met Office.
Hurricane Ophelia Discussion Number 16
NWS National Hurricane Center Miami FL AL172017
1100 PM AST Thu Oct 12 2017
Remarkably, the hurricane has continued to strengthen this evening.
Satellite images indicate that cloud tops in the eyewall have cooled
in the past several hours, with a warm eye remaining. Dvorak
estimates from TAFB, SAB and CIMSS range between 90 to 95 kt, so the
initial intensity is raised to 90 kt.
It seems that the marginal SSTs that Ophelia has been moving over
have been offset by the cold upper-level temperatures and low shear
environment. SSTs only slightly cool in the next 24 hours with
similar shear conditions, so a minor decrease in strength is in the
forecast. After that time, while the hurricane should move over
colder waters, it will likely be accelerating to the northeast and
experiencing favorable mid-latitude jet dynamics, which will help to
maintain the cyclone's intensity. All of the guidance show
extratropical transition by 3 days with the cyclone keeping
hurricane-force winds, as indicated in the new forecast. Little
change was made to the previous prediction, except to account for
the higher initial wind speed.
Ophelia is finally moving, estimated at 6 kt to the east-northeast.
This general motion with an increase in forward speed is expected
for the next couple of days as the hurricane gets picked up by a
large mid-latitude trough. Confidence in the track forecast remains
fairly high for the first 72 h, although the spread increases after
that time. The GFS-based guidance generally then show a more
northward track to the west of Ireland then over the far North
Atlantic, while the UKMET/ECMWF show a track over Ireland and Great
Britain then eastward and dissipating over northern Europe.
The forecast is close to the consensus at long range, but some large
changes could be required for later forecasts.
While the NHC track keeps the center of Ophelia south and east
of the Azores, tropical-storm-force winds are possible throughout
the Azores by Saturday night due to an approaching front. In
addition, the wind field of Ophelia will likely expand as the
cyclone begins extratropical transition, and any deviation to the
left of the forecast track could bring stronger winds to the
islands. Interests in the Azores should refer to products issued by
the Azores Weather Forecast and Watch Center.
KEY MESSAGES:
1. Ophelia is expected to transition to a hurricane-force post-
tropical cyclone by Monday when it moves near Ireland and the
United Kingdom. While post-tropical Ophelia will likely bring some
direct impacts from wind and heavy rain to portions of these areas,
as well as dangerous marine conditions, given the forecast
uncertainty at these time ranges it is too soon to determine the
exact magnitude, timing and location of the impacts. Residents in
Ireland and the United Kingdom should monitor the progress of
Ophelia for the next several days. For more information on local
impacts, residents of Ireland should refer to products issued by Met
Eireann and residents in the United Kingdom should refer to products
issued by the Met Office.
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